Tesi sul tema "Air-sea interactions"

Thesis (M.S. in Meteorology and M.S. in Physical Oceanography) Naval Postgraduate School, December 1997.
"December 1997." Thesis advisor(s): Peter Guest, Roland Garwood. Includes bibliographical references (p. 73-74). Also available online.

The ocean carries more heat poleward than the atmosphere at low latitudes, whilst the reverse occurs at high latitudes. In the Northern Hemisphere, the largest ocean-atmosphere heat fluxes occur over the Gulf Stream, suggesting that an ocean-atmosphere 'relay' is active at mid-latitudes. This thesis is concerned with the significance of the extremes in air-sea heat fluxes over the Gulf Stream. In the first research chapter, the direct interaction between the ocean and the atmosphere is examined in the ERA-Interim dataset. Based on Lagrangian trajectory calculations, the most extreme air-sea heat flux events are found to be associated entirely with air of continental origin. The subsequent heat gain in the overlying air is caused almost completely by surface heat fluxes. For average air-sea heat fluxes, the associated air is both continental and maritime in origin, with a noticeable contribution to the heat content of the air parcels from entrainment at the top of the boundary layer. The second research chapter determines the causes for variations in surface heat flux in the ERA-Interim dataset. Roughly 90% of the time, one observes a baroclinic waveguide of varying strength over the Gulf Stream, setting the intensity of the air-sea heat exchange and the mean state in precipitation and tropospheric wind divergence. A potential mechanism whereby a change in sea-surface temperature gradient could cause an alteration of these mean patterns is discussed. Finally, the link between sea-surface temperature gradients and atmospheric fronts is explored in model simulations. A smoothing in the sea-surface temperature gradient is found to broadly reduce front intensity over the Gulf Stream. Increases in front intensity are shown to be consistent with a thermal damping mechanism. A significant effect is also observed on the regional precipitation and tropospheric vertical velocity, as well as on the direction of frontal propagation.

Thesis (M.S. in Physical Oceanography) Naval Postgraduate School, March 1998.
"March 1998." Thesis advisor(s): Peter C. Chu, Robert H. Bourke. Includes bibliographical references (p. 61-62). Also available online.

Le interazioni oceano-atmosfera sono di primaria importanza sia in ambito climatico che meteorologico. Sono importanti sia su scale temporali orarie, come nell'intensificazione di cicloni tropicali, che su scale interannuali o interdecadali, come nel modo di variabilità climatica ENSO. Questa tesi si focalizza sui transferimenti di energia e quantità di moto all'interfaccia aria-mare in processi su scale temporali brevi caratterizzati da condizioni estreme. Sono presi in considerazione sia la risposta dinamica dell'oceano ad una forzante atmosferica estrema che l'effetto dello stato del mare sullo sviluppo di un evento meteorologico estremo. I sistemi studiati sono il campo di onde interne oceaniche nella scia di un ciclone tropicale e il ruolo dello stato termico dell'oceano superficiale nello sviluppo di piogge intense. In particolare, gli scambi di energia tra onde interne oceaniche nella scia di un ciclone tropicale idealizzato sono studiati con un approccio teorico supportato da appropriate simulazioni numeriche alle equazioni primitive. Si vuole capire come i cicloni tropicali possano contribuire al mescolamento oceanico interno in luoghi lontani dalla loro scia. Infatti, nonostante siano intermittenti nel tempo e nello spazio, sono caratterizzati da venti molto intensi, che eccitano onde interne oceaniche. Esse contribuiscono al mescolamento lontano dal luogo in cui sono state generate, attraverso la loro rottura. Dato che la propagazione di energia è legata alle loro caratteristiche spettrali, una descrizione dettagliata di come l'energia è divisa tra modi verticali e frequenze aiuta a quantificare l'estensione e la velocità di tale propagazione. Una nuova descrizione analitica degli scambi energetici che portano alla formazione del picco doppio-inerziale viene introdotta sulla base della teoria sviluppata. Rispetto a lavori precedenti, si considera una stratificazione oceanica realistica e viene sottolineata una possibile cascata energetica dalla larga scala della forzante atmosferica alla piccola scala del mescolamento. L'altra categoria di eventi estremi considerata è quella del sistemi convettivi a mesoscala (MCS). Essi sono fenomeni comuni lungo le coste del Mediterraneo e rilasciano abbondanti volumi di pioggia in poche ore e su aree dell'ordine di 100 km2. Si sa che un mare mediamente più caldo in prossimità di un MCS produce più pioggia, ma prima di questa tesi non c'erano informazioni circa l'influenza che un pattern spaziale di temperatura marina superficiale (SST) a scala chilometrica potesse avere sull'evento precipitativo. Opportune simulazioni atmosferiche, eseguite con un modello numerico non-idrostatico alle equazioni primitive, fanno luce sui meccanismi attraverso cui le strutture alla sotto-mesoscala di SST possono influenzare la struttura del vento superficiale e, di conseguenza, possono influenzare l'evoluzione della pioggia intensa. Si trova che, attraverso un maggiore mescolamento verticale di quantità di moto su aree di SST più calda, la presenza di fronti di temperatura nel mare può significativamente influenzare la convergenza superficiale, che è spesso l'elemento scatenante della convezione nei MCS, su scale temporali orarie. Questo potrebbe anche far spostare le linee di pioggia. Viene introdotta, poi, la possibilità di un fenomeno di retroazione oceanico legato al profilo verticale di temperatura. Con simulazioni accoppiate oceano-atmosfera, si trova che, in condizioni particolari, i venti intensi in cui il MCS è inglobato possono mischiare l'oceano superficiale a tal punto che la stabilità atmosferica è aumentata e la convezione è soppressa. Tali condizioni, tipiche della tarda estate, sono caratterizzate da uno strato mescolato sottile e una forte stratificazione. Questo potrebbe essere il motivo per cui i MCS sono generalmente osservati più avanti nell'anno, quando lo strato mescolato è più profondo e tale effetto oceanico di mitigazione è assente.
Ocean-atmosphere interactions are of paramount importance in both climatic and meteorological contexts. They are known to play important roles from hourly time scales, such as in the intensification of tropical cyclones, to interannual and even longer time scales, such as in El Niño Southern Oscillation mode of variability of the climate system. The focus of this thesis has been on the energy and momentum transfers at the air-sea interface in short time scales processes characterized by extreme conditions. Both the oceanic dynamical response to an extreme atmospheric forcing and the effects of the sea state on the development of a meteorological extreme event are considered. The systems under study are the ocean internal wave field in the wake of a tropical cyclone and the role of the upper ocean thermal state on the development of heavy rainfalls. In particular, the energy exchanges among oceanic internal waves in the wake of an idealized tropical cyclone are studied with a theoretical approach supported by relevant primitve equation numerical simulations. The goal of this analysis is to understand how tropical cyclones might contribute to the internal ocean mixing in locations far from their track. In fact, despite their intermittency in space and time, they are characterized by very intense winds, which are known to excite oceanic internal waves. These are thought to contribute to ocean mixing far from their generation site through their breaking. Since the energy propagation is linked to the spectral features of the waves, a detailed description of the energy partitioning in different vertical modes and frequencies helps to better constrain the extent and the velocity of such energy propagation. A new detailed analytical description of the exchanges leading to the formation of the first superinertial peak is introduced on the basis of the theory developed. Compared to previous works, a realistic oceanic stratification is included and a path for the energy cascade from the large scales of the atmospheric forcing to the small scales of the mixing is highlighted. The second category of extreme events considered are the heavy-rain-producing mesoscale convective systems (MCSs). They are common phenomena along the coasts of the Mediterranean sea and they release large amounts of rain in few hours and over relatively small areas, O(100 km2). It is known that an average warmer sea in the vicinity of their location produces a larger volume of rain, but before this thesis work no information was available on the influence that a spatial pattern of sea surface temperature (SST), with structures on the kilometric scale, might have on the precipitation event. Appropriate atmospheric numerical simulations, run with a non-hydrostatic primitive equation model, shed light on the mechanisms through which submesoscale SST oceanic features can influence the surface wind structure and, in turns, can affect the evolution of the heavy rainfall. It is found that through enhanced vertical momentum mixing in the atmosphere over warmer SST areas, the presence of temperature fronts in the sea can significantly affect the surface wind convergence, which is often the trigger for deep convection in MCSs, over hourly time scales. This might also lead to significant displacement of the rain bands. The possibility of an ocean dynamical feedback related to the vertical temperature profile is then introduced. By means of atmosphere-ocean coupled numerical simulations, it is found that in particular conditions the intense winds in which the MCS is embedded can mix the upper ocean strongly enough to enhance the stability of the atmospheric boundary layer and suppress deep convection. Such conditions, characterized by a shallow mixed layer and strong stratification, are typical of the late summer. This could be the reason why MCSs are generally observed later during the year, when the mixed layer is deeper and this oceanic mitigating effect is absent.

Tropical cyclone (TC) intensity change is governed by internal dynamics (e.g. eyewall contraction, eyewall replacement cycles, interactions of the inner-core with the rainbands) and environmental conditions (e.g. vertical wind shear, moisture distribution, and surface properties). This study aims to gain a better understanding of the physical mechanisms responsible for TC intensity changes with a particular focus to those related to the vertical wind shear and surface properties by using high resolution, full physics numerical simulations. First, the effects of the vertical wind shear on a rapidly intensifying storm and its subsequent weakening are examined. Second, a fully coupled atmosphere-wave-ocean model with a sea spray parameterization is used to study the impact of sea spray on the hurricane boundary layer. The coupled model consists of three components: the high resolution, non-hydrostatic, fifth generation Pennsylvania State University-NCAR mesoscale model (MM5), the NOAA/NCEPWAVEWATCH III (WW3) ocean surface wave model, and theWHOI threedimensional upper ocean circulation model (3DPWP). Sea spray parameterizations were developed at NOAA/ESRL and modified by the author to be introduced in uncoupled and coupled simulations. The model simulations are conducted in both uncoupled and coupled modes to isolate various physical processes influencing TC intensity. The very high-resolutionMM5 simulation of Hurricane Lili (at 0.5 km grid resolution) showed a rapid intensification associated with a contracting eyewall. Changes in both the magnitude and the direction of the vertical wind shear associated with an approaching upper-tropospheric trough were responsible for the weakening of the storm before landfall. Hurricane Lili weakened in a 5-10 m/s vertical wind shear environment. The simulated storm experienced wind shear direction normal to the storm motion, which produced a strong wavenumber one rainfall asymmetry in the downshear-left quadrant of the storm. The rainfall asymmetry was confirmed by various observations from the TRMM satellite and the WSR-88D ground radar in the coastal region. The increasing vertical wind shear induced a vertical tilt of the vortex with a time lag of about 5-6 hours after the wavenumber one rainfall asymmetry was first observed in the model simulation. Other key factors controlling intensity and intensity change in tropical cyclones are the air-sea fluxes. Accurate measurement and parameterization of air-sea fluxes under hurricane conditions are challenging. Although recent studies have shown that the momentum exchange coefficient levels off at high wind speed, little is known about the high wind behavior of the exchange coefficient for enthalpy flux. One of the largest uncertainties is the potential impact of sea spray. The current sea spray parameterizations are closely tied to wind speed and tend to overestimate the mediated heat fluxes by sea spray in the hurricane boundary layer. The sea spray generation depends not only on the wind speed but also on the variable wave state. A new spray parameterization based on the surface wave energy dissipation is introduced in the coupled model. In the coupled simulations, the wave energy dissipation is used to quantify the amount of wave breaking related to the generation of sea spray. The spray parameterization coupled to the waves may be an improvement compared to sea spray parameterizations that depends on wind speed only.

Thesis (Ph. D.)--Joint Program in Marine Chemistry and Geochemistry, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution, 1998.
Includes bibliographical references (p. 251-266).
by Eda Maria Hood.
Ph.D.

Summer monsoon rainfall accounts for at least 80% of the annual-total precipitation in many Indian states. Intraseasonal variations (ISV) in rainfall produce floods and droughts that can devastate agriculture. ISVs are dominated by a 30-50 day northward-propagating oscillation (NPISO) between the eastern equatorial Indian Ocean (EEqIO) and India. This thesis evaluates the hypothesis that atmosphere-ocean interactions are critical to the NPISO's period, intensity, and propagation. Two simple NPISO indices are created from lag correlations in outgoing longwave radiation between the oscillation's two poles. The index in which India leads the EEqIO better captures the NPISO, implying that convection in the EEqIO cannot predict Indian rainfall. An idealized NPISO lifecycle suggests that air-sea interactions occur via atmospheric thermodynamic forcing and a feedback from SSTs on low-level atmospheric stability.

Includes bibliographical references (leaves 94-111).
The mesoscale structure of the low-level wind field associated with a strong upwelling event was investigated. During July 2005 when a strong upwelling event occurred, the Somali jet was found to have oscillated at lower frequency of 3-7 weeks than the normal bi-weekly mode observed in several studies and the mesoscale winds exhibited high covariability with the prevailing SSTs. Strong values of alongshore winds were deduced from late June to mid-July. These winds weakened significantly in the third and fourth week of July. A large off-shore pressure gradient due to differential thermal properties of land and sea was also observed.

Les embruns sont des aérosols en phase aqueuse générés à la surface de l’eau. Au large, ils sont générés par des mécanismes tel que le déferlement et l’écrêtage. Aujourd’hui, la connaissance portant sur les embruns excédant 20 µm de rayon reste limitée. Cette thèse vise à améliorer la compréhension des processus de génération, de transport, et les impacts sur les propriétés de la couche limite atmosphérique marine (CLAM). La campagne MATE2019 est ainsi menée à l’installation air-mer de Luminy (Marseille, France) afin d’étudier le rôle des interactions vague-vent sur la génération. Une analyse d’échelle révèle que la génération d’embruns corrèle le mieux avec la variance de pente de vagues pour les plus grosses gouttelettes ‘spume’ générées par écrêtage. Pour les plus petites gouttelettes ‘jet’ générées par éclatement de bulles, la meilleure corrélation est obtenue avec un nombre adimensionnel combinant la variance de pentes de vagues, l’age de vague, et un nombre de Reynolds adapté aux mers de vent. Il en résulte la formulation de deux fonctions de génération d’embruns dépendantes sur l’état de mer, valides pour des vents de 12–20 m s-1 et des rayons de 3–35 µm. Extrapolées aux conditions in situ, les fonctions de génération issues du laboratoire sont paramétrées dans les modèles numériques MACMod et MESO-NH, à leurs tours validés à l’aide de mesures terrain, dont une nouvelle campagne de mesure effectuée pendant la thèse dans le Golfe de Gascogne. Les meilleures performances de modélisation sont obtenues avec les fonctions de génération issues du laboratoire. Ces résultats permettent de mieux appréhender l’impact des embruns sur la CLAM
Sea spray droplets are aqueous phase aerosols generated from the water surface. In the open ocean, they are generated as a result of wind-forced wave breaking and surface-tearing mechanisms. To this day, knowledge of sea spray particles larger than 20 µm radius is sparse. The present thesis aims to improve knowledge of the sea spray generation flux, as well as transport and impacts on the properties of the marine atmospheric boundary layer (MABL). To this end, the effects of wind–wave interactions on the surface sea spray generation flux are investigated during the MATE2019 experiment, conducted at the large wave–wind facility in Luminy (Marseille, France). Scaling analysis shows that the sea spray generation is best correlated with the wave-slope variance for thelarger spume droplets generated by surface tearing. For the smaller jet droplets generated by bubble bursting, the highest correlation is found with a nondimensional number combining the wave-slope variance, the wave age, and a windsea Reynolds number. This resulted in the formulation of two wave-state-dependent sea spray generation functions, each valid for wind speeds 12–20 m s-1 and radii 3–35 µm. Upscaled to the field, the laboratory-derived generation functions are parameterized in the MACMod and MESO-NH numerical models, and validated using field data collected during the thesis in the Bay of Biscay for this purpose. Best model performance is found with the laboratory generation functions. Such results are encouraging for the study of sea spray impacts on the properties of the MABL

La Southern Annular Mode (SAM) è il modo di variabilità dominante dell'Emisfero Australe extratropicale, definito come la prima funzione ortogonale empirica del geopotenziale a 500 hPa e caratterizzato da un pattern di anomalie del campo di pressione al livello del mare di polarità opposta sull'Antartide e alle medie latitudini. I meccanismi dietro a questo modo di variabilità sono un argomento di crescente interesse per via del trend positivo che la SAM ha mostrato negli ultimi decenni, ricondotto all'aumento di concentrazioni di gas serra e alla riduzione di ozono stratosferico. Comprendere in che misura le variazioni nella SAM, predetta sempre più verso la sua fase positiva, influenzeranno il clima nel futuro richiede una solida comprensione degli impatti che la SAM ha sul clima presente. A questo scopo, è essenziale identificare i modelli climatici capaci di rappresentare lo stato corrente della SAM con grande accuratezza, condizione necessaria affinchè essi siano affidabili per le proiezioni future. Partendo da questo contesto, la tesi ha l'obiettivo di indagare la relazione tra la SAM e la temperatura superficiale del mare (SST) basandosi sulla rianalisi ERA5 e di valutare l'accuratezza della sua rappresentazione da parte dei modelli che partecipano alla sesta fase del Coupled Model Intercomparison Project (CMIP6). Nella prima parte del lavoro, è analizzata la rianalisi ERA5 per identificare la fenomenologia della SAM ed i processi fisici che conducono al pattern delle anomalie di SST in risposta alla SAM. La seconda parte del lavoro consiste nell'identificare le caratteristiche che i modelli climatici allo stato dell'arte riescono a riprodurre con realismo e quelle che sono invece più difficili da rappresentare adeguatamente. Risultato della tesi è una diagnosi dell'affidabilità dei modelli CMIP6 nella caratterizzazione della relazione tra SAM e SST allo stato presente.

Ocean wind waves represent the atmosphere-ocean boundary, playing a central role in the air-sea exchanging processes. Heat, mass and momentum are transferred across this boundary, with waves mediating the exchange of principally the momentum between the winds and the ocean surface. During the generation process waves are called wind sea. When they leave their generation area or outrun their generating wind they are called swell. The wave field can be said to be dominated either by wind sea or swell. Depending on the wave regime the momentum and energy exchanging processes and the degree of coupling between the waves and the wind is different. During the growing process, waves act as a drag on the surface wind and the momentum flux is directed downward. When swell dominates the wave field a reverse momentum flux mechanism occurs triggered by swell waves traveling considerably faster than the surface winds. The momentum transfer is now directed from the waves to the atmosphere, and takes place because swell waves perform work on the atmosphere as part of their attenuation process. This upward momentum transfer has an impact on the lower atmosphere dynamics, and on the overall turbulence structure of the boundary layer. A detailed qualitative climatology of the global wind sea and swell fields from wave reanalysis data, is presented, revealing a very strong swell dominance of the World Ocean. The areas of larger potential impact of swell on the atmosphere, from a climatological point of view, are also studied. A model that reproduces the swell impact on the lower atmosphere dynamics, conceptually based on the energy transfer from the waves to the atmosphere, is presented – a  new parameterization for the wave-induced stress is also proposed. The model results are compared with field observations. A modeling simulation, using a coupled wave-atmosphere model system, is used to study the impact of swell in a regional climate model, by using different formulations on how to introduce the wave state effect in the modeling system.
Gränsen mellan hav och atmosfär beskrivs av vågor, dessa spelar en central roll i utbytesprocesser mellan hav och atmosfär. Värme, massa och rörelsemängd överförs vid ytan och utbytet av rörelsemängd mellan vind och havsyta styrs i stor utsträckning av vågorna. Då vågor skapas kallas de för vinddrivna vågor. När vågorna sedan lämnar området där de genererats eller rör sig fortare än den vind som genererat dem kallas de dyning. Ett vågfält kan sägas vara dominerat av antingen vinddrivna vågor eller dyningsvågor. Beroende på vilken vågregim som råder så är kopplingen mellan vågor och vind olika och därmed också utbytesprocesserna för rörelsemängd och energi. Då vågorna genereras fungerar de som en bromsande kraft för vinden och impulsutbytet är nedåtriktat. När dyning dominerar vågfältet inträffar en mekanism för omvänt impulsutbyte som sätts igång av dyningsvågor som färdas avsevärt snabbare än vinden. Rörelsemängd överförs då från vågorna till atmosfären, eftersom dyningsvågorna utför arbete på atmosfären då de dämpas. Den uppåtriktade transporten av rörelsemängd har en stor effekt på dynamiken och turbulensstrukturen i lägre delen av atmosfären. En detaljerad kvalitativ klimatologi av globala vågfält (vinddrivna och dyning) från återanalysdata presenteras och visar att dyning dominerar vågfältet på världshaven. Områden där man kan förvänta sig störst effekt av dyning på atmosfären har identifierats. En konceptuellt baserad modell som reproducerar effekten av dyning på dynamiken i lägre delen av atmosfären presenteras. Modellen styrs av överföring av energi från vågor till atmosfären. I modellen föreslås även en ny parameterisering för våginducerad kraft på havsytan. Modellresultaten är utvärderade mot fältmätningar. En regional klimatmodell, med ett kopplat våg-atmosfärssystem, har använts för att studera den långtida effekten av dyning vid klimatsimulering. Olika formuleringar för beskrivningen av vågornas effekt på atmosfären har använts, beroende på om vinddrivna vågor eller dyning dominerar vågfältet.

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Variability of the MABL and its effect on the electromagnetic (EM) refractive structure around the Greenland Sea marginal ice zone were examined. Rawinsonde profiles and surface observations collected from 3 ships during MIZEX-87(20 March-11 April) served as the data set. A program, developed to calculate the refractivity at each vertical level of the rawinsonde profiles, also identified the levels at which trapping, superrefraction and subrefraction occurred. Temporal studies showed that a higher incidence of anomalous refractive layers occurred during periods when the region was under the influence of high pressure. More than 50% of the time, trapping and super-refractive layers were attributed to development of a capping inversion just above the MABL during these periods. Spatial studies showed that the refractive structure varied relative to distance from the ice edge as did the depth of the MABL. An upward slope in refractive layer heights was observed from the ice toward the open water. Significant spatial inhomogeneity was observed over horizontal ranges of less than 100 km. This was attributed to both the large-scale synoptic forcing affecting the region and to variations in the surface fluxes of heat and moisture over the ice and over the water. A range-dependent ray trace model developed at the Naval Ocean Systems Center was used to show how the ray paths of EM waves vary with a changing refractive structures. Keywords: Air water interactions, Greenland Sea, Atmospheric refraction, Electromagnetic wave propagation, Heat flux, Sea ice. Theses. (EDC)
http://archive.org/details/temporalspatialv00grot
Lieutenant, United States Navy

By collocating 10 years (1999-2009) of remotely sensed surface turbulent heat fluxes with satellite altimetry data, we investigate the impact of ocean mesoscale eddies on the latent and sensible heat fluxes in the South Atlantic ocean. Eddies were identified using the method proposed by Chaigneau et al. (2009), which is based on closed contours of sea level anomaly. Most of the identified eddies had a radius of ~70 km and amplitude of ~5 cm. On average, in the South Atlantic, eddies play a minor role on the ocean-atmosphere heat exchange. However, in strongly energetic regions such as the Brazil-Malvinas confluence or Agulhas Current retroflection regions, eddies can account up to 20-30% of the total variance of the surface turbulent heat fluxes with averaged anomalies of ±10-20 W/m2 for both heat flux components. Cyclonic (anticyclonic) eddies, associated with negative (positive) heat fluxes anomalies tend to cool (warm) the overlying atmosphere. A composite analysis of the turbulent heat fluxes anomalies within the eddies reveals a direct relationship between the eddy amplitude and the intensity of the latent and sensible fluxes anomalies, such that large-amplitude eddies have a stronger signature in the turbulent surface heat fluxes. Heat fluxes anomalies are also much stronger near the eddy centers and decay radially to reach minimum values outside the eddies.
Uma combinação de 10 anos (1999-2009) de fluxos turbulentos de calor pela superfície, medidos a partir de satélites, e dados altimétricos de anomalia da altura da superfície do mar, foram o utilizados com objetivo de investigar o impacto de vórtices de meso-escala nos fluxos de calor sensível e latente na bacia do Atlântico Sul. Para a detecção dos vórtices foi aplicado o método proposto por Chaigneau et al. (2009), que baseia-se em contornos fechados de anomalia da altura da superfície do mar. A maior parte dos vórtices identificados possui raio de ~70 km e amplitude de ~5 cm. Em média, no Atlântico Sul, o impacto dos vórtices para as trocas de calor entre oceano e atmosfera é relativamente fraco. Entretanto, em regiões de alta variabilidade energética como na Confluência Brasil- Malvinas e na retroflecção da Corrente das Agulhas, vórtices de meso-escala podem contribuir com anomalias médias de até ±10-20 W/m2 nos fluxos turbulentos. Vórtices ciclônicos (anti-ciclônicos), associados com anomalias negativas (positivas) de fluxos de calor, tendem a esfriar (esquentar) a atmosfera adjacente. Mapas composite foram analisados para milhares de vórtices, mostrando um relação direta entre a magnitude das anomalias dos fluxos e a amplitude dos vórtices, de tal modo que vórtices de maior amplitude contribuem com maiores anomalias de calor latente e sensível. Além disso, os padrões espaciais dos composites médios revelam que as anomalias são significativamente maiores próximo ao centro dos vórtices e decaem radialmente até atingirem valores absolutos mínimos fora dos contornos dos vórtices.

Chaque automne, des événements de précipitations intenses (HPEs) ont lieu en Méditerranée nord-occidentale. Cette thèse adopte une approche par la modélisation climatique régionale couplée atmosphère-océan pour traiter de la sensibilité de ces événements à des changements de température de surface de la mer (SST) résultant soit de biais dans le modèle couplé, soit de la réponse de la couche de mélange océanique à des forçages atmosphériques. Deux cas d’études mettent en évidence la sensibilité particulière des zones de convergence d’humidité aux changements de SST. L’élaboration d’indices synthétiques de changements dans les précipitations et de changements de SST en amont des zones précipitantes met en lumière dans plusieurs régions (Cévennes, région de Valence, Calabre) une relation linéaire entre ces deux quantités dans deux plateformes de modélisation différentes : MORCE et CNRM-RCSM4. Dans la région de Valence, en Espagne, nous montrons en outre que les événements de précipitations intenses sont souvent précédés d’un épisode de Mistral qui refroidit la zone amont des précipitations dans les jours précédant celles-ci, refroidissement qui tend ensuite à réduire l’intensité de l’événement précipitant
Every year in autumn, heavy precipitation events (HPEs) occur in the northwestern Mediterrranean. This thesis uses coupled atmosphere-ocean regional climate modeling to tackle the sensitivity of these events to sea surface temperature (SST) changes coming either from model biases or from the oceanic mixed layer response to atmospheric forcing. Two case studies show the particular sensitivity of moisture convergence zones to SST changes. The use of synthetic indexes of precipitation changes and SST changes in the upstream zones shows a linear relationship between the two indexes in several regions (Cévennes, the region of Valencia, Calabria) in the modeling platforms MORCE and CNRM-RCSM4. Furthermore, we show that the HPEs in the region of Valencia are often preceded by a Mistral event which cools the upstream zone whithin 5 days before the HPEs. In turn, this cooling tends to reduce the intensity of the HPE

Malgré les avancées dans la prédiction de la trajectoire des cyclones tropicaux et des vitesses de vent dans la région externe, la représentation numérique des vents les plus forts associés aux événements les plus intenses demeure une question ouverte, principalement en raison de la faible taille du cœur du cyclone et de la difficulté à comprendre et résoudre les échanges turbulents entre l’océan et l’atmosphère. Les limitations observationnelles ont longtemps entravé des mesures précises de la surface océanique près de la région centrale dans des conditions de vent extrême, tandis que les satellites géostationnaires aident à caractériser les motifs nuageux mais ne donnent pas d’information directe sur l’interface air-mer. Récemment, le radar à ouverture de synthèse (SAR) a émergé comme une technologie satellitaire prometteuse capable de produire des mesures bidimensionnelles haute résolution des vitesses du vent à la surface de l’océan, grâce à de nouveaux modes d’acquisition et à des développements algorithmiques. Compte tenu de ces nouvelles opportunités d’observation, nous explorons la contribution des caractéristiques structurelles près du cœur, exclusivement discernables à travers des instruments haute résolution, à la dynamique des cyclones. En utilisant un cadre théorique simple et examinant sa cohérence avec les mesures SAR, nous démontrons que les vents en surface près du cœur contrôlent l’évolution de la structure du vent du cyclone. Le cadre développé permet d’illustrer comment les futures mesures des caractéristiques de la couche limite océan-atmosphère pourraient bénéficier du suivi à court et à long terme des cyclones tropicaux
Despite advances in predicting the tropical cyclones (TCs) trajectory and outer-core wind speeds, the numerical representation of the strongest winds associated with the most intense events is still an open issue, essentially because of the small radial extent of the TC core and the difficulty in understanding and resolving turbulent air-sea exchanges. Observational limitations have for a long time hindered accurate measurements of the ocean surface near the core region in extreme wind conditions, while geostationary satellites help characterizing the cloud patterns but lack direct information on the air-sea interface. Recently, synthetic aperture radar (SAR) has emerged as a promising satellite technology capable of producing high-resolution two dimensional measurements of the ocean surface wind speeds, thanks to new acquisition modes and algorithmic developments. Given these new observational opportunities, we investigate the contribution of near-core structural features, exclusively discernible through high-resolution instruments, to the TC dynamics. Using a simple theoretical framework and examining its consistency with SAR measurements, we demonstrate that the near-core surface winds modulate the evolution of the TC wind structure. The developed framework allows to illustrate how future measurements of ocean-atmosphere boundary layer characteristics could benefit the short- and long-term monitoring of TCs

This study reports on a new formulation of the spectral dissipation source term Sds for wind-wave modelling applications. This new form of Sds features a nonlinear dependence on the local wave spectrum, expressed in terms of the azimuthally integrated saturation parameter B(k)=k^4 F(k). The basic form of this saturation-dependent Sds is based on a new framework for the onset of deep-water wave breaking due to the nonlinear modulation of wave groups. The new form of Sds is succesfully validated through numerical experiments that include exact nonlinear computations of fetch-limited wind-wave evolution and hindcasts of two-dimensional wave fields made with an operational wind-wave model. The newly-proposed form of Sds generates integral spectral parameters that agree more closely with observations when compared to other dissipation source terms used in state-of-the-art wind-wave models. It also provides more flexibility in controlling properties of the wave spectrum within the high wavenumber range. Tests using a variety of wind speeds, three commonly-used wind input source functions and two alternative full-development evolution limits further demonstrate the robustness and flexibility of the new saturation-dependent dissipation source term. Finally, improved wave hindcasts obtained with an implementation of the new form of Sds in a version of the WAM model demonstrate its potential usefulness in operational wind-wave forecasting applications.

Chaque automne, des événements de précipitations intenses (HPEs) ont lieu en Méditerranée nord-occidentale. Cette thèse adopte une approche par la modélisation climatique régionale couplée atmosphère-océan pour traiter de la sensibilité de ces événements à des changements de température de surface de la mer (SST) résultant soit de biais dans le modèle couplé, soit de la réponse de la couche de mélange océanique à des forçages atmosphériques. Deux cas d’études mettent en évidence la sensibilité particulière des zones de convergence d’humidité aux changements de SST. L’élaboration d’indices synthétiques de changements dans les précipitations et de changements de SST en amont des zones précipitantes met en lumière dans plusieurs régions (Cévennes, région de Valence, Calabre) une relation linéaire entre ces deux quantités dans deux plateformes de modélisation différentes : MORCE et CNRM-RCSM4. Dans la région de Valence, en Espagne, nous montrons en outre que les événements de précipitations intenses sont souvent précédés d’un épisode de Mistral qui refroidit la zone amont des précipitations dans les jours précédant celles-ci, refroidissement qui tend ensuite à réduire l’intensité de l’événement précipitant
Every year in autumn, heavy precipitation events (HPEs) occur in the northwestern Mediterrranean. This thesis uses coupled atmosphere-ocean regional climate modeling to tackle the sensitivity of these events to sea surface temperature (SST) changes coming either from model biases or from the oceanic mixed layer response to atmospheric forcing. Two case studies show the particular sensitivity of moisture convergence zones to SST changes. The use of synthetic indexes of precipitation changes and SST changes in the upstream zones shows a linear relationship between the two indexes in several regions (Cévennes, the region of Valencia, Calabria) in the modeling platforms MORCE and CNRM-RCSM4. Furthermore, we show that the HPEs in the region of Valencia are often preceded by a Mistral event which cools the upstream zone whithin 5 days before the HPEs. In turn, this cooling tends to reduce the intensity of the HPE

Cette thèse s’intéresse aux interactions entre l’océan et l’atmosphère dans le Pacifique Sud-Est, à des échelles comprises entre 10 et 300 km ("meso-échelle" océanique). Des observations satellites et un modèle couplé à haute résolution (1/12°) sont utilisés pour mettre en évidence et caractériser la relation entre la meso-échelle de température de surface de la mer (SST) et celle de l’intensité de la tension de vent (TV). Les observations montrent qu’environ un tiers de la meso-échelle de l’intensité de la TV est expliquée par les anomalies de la SST. L’intensité de la réponse de la TV aux anomalies de SST présente des variations spatiales et un cycle saisonnier marqué, également reproduits par le modèle. Une analyse de l’ajustement de la couche limite atmosphérique aux anomalies de meso-échelle de la SST dans les simulations permet d’expliquer ce cycle saisonnier et de comprendre l'origine des variations de la TV et de la vitesse du vent. Le modèle permet également d'étudier les conséquence de la modulation des flux à l’interface air-mer par la meso-échelle de SST et de courant de surface sur la dynamique océanique du Pacifique Sud-Est. D’une part, près de la côte, la réponse de la TV à la présence du front de SST diminue l’intensité de l’upwelling et la génération d’énergie cinétique turbulente (EKE) par instabilité barocline. La réponse de l’atmosphère à la meso-échelle de SST a également une rétroaction négative sur les anomalies de SST. D’autre part, la modulation de la TV par les courants de surface diminue la génération d’EKE par le travail des anomalies de TV, et créé un pompage d’Ekman qui atténue les anomalies de meso-échelle de la hauteur du niveau de la mer
This PhD thesis studies the air/sea interactions at the oceanic mesoscale (10-300 km) in the South-East Pacific and their consequences. Satellite observations and a high-resolution regional ocean-atmosphere coupled model are used to evidence and characterize the mesoscale Sea Surface Temperature (SST)-wind stress (WS) interactions. Offshore from 150km, observations show that one third of the WS mesoscale intensity is explained by the SST mesoscale anomalies. The intensity of the WS response intensity to the SST displays similar spatial and seasonal variability in both the model and the observations. The simulation is further analyzed to study this variations and to understand the boundary layer adjustment mechanisms. A momentum balance evidenced that the near surface wind anomalies are created by the anomalies of the turbulent mixing term. It is shown that WS intensity anomalies due to SST anomalies are are mainly forced by mixing coefficient anomalies and partially compensated by wind shear anomalies. The consequences on the oceanic dynamics of the air-sea momentum, heat and fresh water fluxes by mesoscale SST and surface current are investigated in the simulations. On one hand, near the coast, the WS response to the upwelling SST front decreases both the upwelling and the eddy kinetic energy (EKE) generation by baroclinic conversion. A negative feedback of the atmospheric response on the SST anomalies amplitude is also evidenced. On the other hand, the WS modulation by oceanic surface currents decreases the EKE generation by the mesoscale wind work. It also creates an Ekman pumping centered above the eddies and attenuating sea surface height anomalies

L'Asie du Sud-Est (SEA pour Southeast Asia) regroupe 10% de la population mondiale et est soumise à un large éventail de facteurs et aléas climatiques : typhons, mousson, Oscillation Australe El Niño (ENSO), changement climatique... A l'interface entre l'Océan Indien, l'Océan Pacifique et l'atmosphère, cette région, qui comprend le continent maritime, est de plus un élément clé du fonctionnement de la circulation océanique et atmosphérique globale. L'objectif général de cette thèse est de mieux comprendre le fonctionnement et l'impact des interactions air-mer dans le climat d'Asie du Sud-Est. Ceci est d'une importance primordiale pour une connaissance approfondie et une meilleure capacité de prédiction de la variabilité climatique à toutes les échelles dans la région, depuis les événements extrêmes et la variabilité interannuelle jusqu'aux projections futures, mais aussi pour mieux comprendre, modéliser et prévoir le climat global. Nous nous sommes concentrés sur deux processus qui jouent un rôle important dans le climat du d'Asie du Sud-Est : ENSO et les échanges de chaleur à l'interface air-mer. Tout d'abord, l'impact d'ENSO et de sa variante, ENSO Modoki, sur la variabilité des précipitations dans la région SEA a été étudié pour la période 1979-2019. La diminution (augmentation) observée des précipitations sur la SEA pendant les événements Modoki par rapport aux événements ENSO canoniques a été expliquée par une réduction (augmentation) du transport d'humidité dans la région et un affaiblissement (renforcement) de la branche ascendante de la circulation de Walker. Deuxièmement, nous avons analysé des jeux de données observationnelles et numériques disponibles et effectué des simulations de sensibilité afin d'explorer et d'évaluer la gamme d'estimations des flux de chaleur air-mer dans la région SEA. Cela a révélé une énorme incertitude dans les estimations provenant de divers ensembles de données, avec des valeurs de flux de chaleur net variant d'environ -30 à +40 W.m-2. Le modèle numérique SYMPHONIE a été utilisé avec deux méthodes de forçage des flux de chaleur de surface (formules bulk vs. flux prescrits à partir de jeux de données atmosphériques) pour étudier la sensibilité de la température de surface de la mer simulée à ces flux. Les résultats ont fourni une estimation de +12.5 W.m-2 du gain net de chaleur pour l'océan sur la période 2009-2018, et ont suggéré qu'ERA5, la cinquième génération de réanalyse de l'ECMWF ("European Centre for Medium-Range Weather Forecasts"), peut être utilisée comme référence bien qu'elle surestime légèrement le flux net de chaleur. Enfin, les flux air-mer produits par 30 modèles issus des simulations de la phase 6 du "Coupled Model Intercomparison Project" (CMIP6) ont été évalués par rapport à ERA5. Sur la période historique, la moyenne d'ensemble CMIP6 reproduit bien la variabilité spatiale des flux de chaleur, mais sous-estime de deux tiers le gain net de chaleur pour l'océan par rapport à ERA5. Les principaux facteurs contribuant à ce biais net sont le rayonnement en ondes courtes (SW) et le flux de chaleur latente (LH). Le gain de chaleur net devrait augmenter au cours du XXème siècle, en raison d'une augmentation du gain de SW et de la perte de LH et d'une diminution du rayonnement de grande longueur d'onde (LW) et des pertes de chaleur sensible (SH). Les modèles qui prévoient un réchauffement plus intense de la surface de la mer présentent des changements plus importants dans les flux de chaleur. Ceux-ci devraient changer le plus dans le cadre du scénario SSP5-8.5 (+3,7, +1,0, -8,4, +9,2, et +1,9 W.m-2, respectivement pour Qnet, SW, LH, LW, et SH), suivi par le scénario SSP2-4.5, et enfin le scénario SSP1-2.6
Southeast Asia (SEA) gathers 10% of the world population and is subject to a wide range of climate factors and hazards : typhoons, monsoon, El Niño Southern Oscillation (ENSO), climate change... At the interface between the Indian Ocean, the Pacific Ocean and the atmosphere, the SEA region, which includes the maritime continent, is moreover key to the functioning of global oceanic and atmospheric circulation. The general objective of this thesis is to better understand the functioning and impact of air-sea interactions in the SEA climate. This is of primary importance for an in-depth knowledge and a better prediction capacity of climate variability at all scales in the region, from extreme events and interannual variability to future projections, but also to better understand, model and forecast global climate. We focused on two processes that play an important role in SEA climate : El Niño Southern Oscillation (ENSO) and air-sea heat exchanges. First, the impact on SEA rainfall variability of ENSO and its variant, ENSO Modoki, were investigated for the period 1979-2019. The observed decrease (increase) in rainfall over SEA during Modoki events compared to the canonical ENSO events was explained by a reduced (enhanced) moisture transport into the region and a weakening (strengthening) of the ascending branch of the Walker circulation. Second, we analyzed available observational and numerical datasets and conducted sensitivity simulations to explore and assess the range of estimates of air-sea heat fluxes in the SEA region. This revealed a huge uncertainty in estimates from various datasets, with values of net heat flux varying from approximately -30 to +40 W.m-2. The SYMPHONIE numerical model was used with two methods of surface heat flux forcing (bulk formulae vs. prescribed fluxes from atmospheric datasets) to investigate the sensitivity of the model's sea surface temperature to those fluxes. Results provided a +12.5 W.m-2 estimate of net heat gain for the ocean over 2009-2018, and suggested that ERA5, the fifth generation of the European Centre for Medium-Range Weather Forecasts reanalysis, can be used as a reference though a slight overestimation of net heat flux. Last, air-sea fluxes produced by 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations were evaluated against ERA5. Over the historical period, the CMIP6 ensemble average reproduces well the spatial variability of heat fluxes, but underestimates the net heat gain for the ocean by two-third compared to ERA5. The primary contributors to the net bias are shortwave radiation (SW) and latent heat flux (LH). The net heat gain is projected to increase during the XXIst century, resulting from an increase of SW gain and LH loss and a decrease of longwave radiation (LW) and sensible heat (SH) losses. Models with higher projected sea surface warming exhibit larger changes in heat fluxes. Heat fluxes are predicted to change the most under the SSP5-8.5 scenario (+3.7, +1.0, -8.4, +9.2, and +1.9 W.m-2, respectively for Qnet, SW, LH, LW, and SH), followed by the SSP2-4.5 scenario, and finally the SSP1-2.6 scenario

Cette thèse s’intéresse aux interactions entre l’océan et l’atmosphère dans le Pacifique Sud-Est, à des échelles comprises entre 10 et 300 km ("meso-échelle" océanique). Des observations satellites et un modèle couplé à haute résolution (1/12°) sont utilisés pour mettre en évidence et caractériser la relation entre la meso-échelle de température de surface de la mer (SST) et celle de l’intensité de la tension de vent (TV). Les observations montrent qu’environ un tiers de la meso-échelle de l’intensité de la TV est expliquée par les anomalies de la SST. L’intensité de la réponse de la TV aux anomalies de SST présente des variations spatiales et un cycle saisonnier marqué, également reproduits par le modèle. Une analyse de l’ajustement de la couche limite atmosphérique aux anomalies de meso-échelle de la SST dans les simulations permet d’expliquer ce cycle saisonnier et de comprendre l'origine des variations de la TV et de la vitesse du vent. Le modèle permet également d'étudier les conséquence de la modulation des flux à l’interface air-mer par la meso-échelle de SST et de courant de surface sur la dynamique océanique du Pacifique Sud-Est. D’une part, près de la côte, la réponse de la TV à la présence du front de SST diminue l’intensité de l’upwelling et la génération d’énergie cinétique turbulente (EKE) par instabilité barocline. La réponse de l’atmosphère à la meso-échelle de SST a également une rétroaction négative sur les anomalies de SST. D’autre part, la modulation de la TV par les courants de surface diminue la génération d’EKE par le travail des anomalies de TV, et créé un pompage d’Ekman qui atténue les anomalies de meso-échelle de la hauteur du niveau de la mer
This PhD thesis studies the air/sea interactions at the oceanic mesoscale (10-300 km) in the South-East Pacific and their consequences. Satellite observations and a high-resolution regional ocean-atmosphere coupled model are used to evidence and characterize the mesoscale Sea Surface Temperature (SST)-wind stress (WS) interactions. Offshore from 150km, observations show that one third of the WS mesoscale intensity is explained by the SST mesoscale anomalies. The intensity of the WS response intensity to the SST displays similar spatial and seasonal variability in both the model and the observations. The simulation is further analyzed to study this variations and to understand the boundary layer adjustment mechanisms. A momentum balance evidenced that the near surface wind anomalies are created by the anomalies of the turbulent mixing term. It is shown that WS intensity anomalies due to SST anomalies are are mainly forced by mixing coefficient anomalies and partially compensated by wind shear anomalies. The consequences on the oceanic dynamics of the air-sea momentum, heat and fresh water fluxes by mesoscale SST and surface current are investigated in the simulations. On one hand, near the coast, the WS response to the upwelling SST front decreases both the upwelling and the eddy kinetic energy (EKE) generation by baroclinic conversion. A negative feedback of the atmospheric response on the SST anomalies amplitude is also evidenced. On the other hand, the WS modulation by oceanic surface currents decreases the EKE generation by the mesoscale wind work. It also creates an Ekman pumping centered above the eddies and attenuating sea surface height anomalies

Les évènements de convection profonde sont importants car ils forment les masses d'eau intermédiaires et profondes qui nourrissent la circulation globale. La mer du Labrador, qui forme la Labrador Sea Water (LSW), est le site le plus documenté de l'océan Atlantique Nord. La mer d'lrminger a également été citée mais n'est pas entièrement reconnue à cause du manque d'observations directes. Cette thèse fournit la première description de la convection profonde en mer d'lrminger à l'échelle du bassin grâce aux données Argo. Trois évènements de convection se sont produits en mer d'lrminger depuis 2010. Au cours de l'hiver 2O11-2O12, la convection atteint 1000 m et est expliquée par la séquence d'apparition des tip jets groenlandais.La convection de l'hiver 2O13-2O14, qui atteint 1300 m, est caractérisée par un préconditionnement particulièrement important et un forçage par les tip jets faible. La convection de l'hiver 2O14-2O15, qui atteint 1700 m, montre des tip jets très nombreux et persistants. L'advection de LSW provenant de la mer du Labrador explique les profondeurs exceptionnelles observées au cours de ces deux derniers hivers. Les résultats montrent que la convection n'est pas rare en mer d'lrminger et qu'elle joue un rôle non négligeable sur l'équilibre climatique
The deep convection events are important because they form the intermediate and deep water masses feeding the global circulation. The Labrador Sea is the main site of deep convection in the North Atlantic Ocean and produces the intermediate Labrador Sea Water (LSW). The lrminger Sea was also cited but was forgotten during decades because of the lack of direct observations. This thesis provides the first description of the lrminger Sea deep convection at basin scale, thanks to the Argo data. Three convective events occurred in the lrminger Sea since 2010. During the 2011-2012 winter, the convection reached 1000 m and is explained by the sequence of the Greenland tip jets. The event of the 2O13-2O14 winter, reaching 130O m, is characterized by a strong preconditioning and a weak forcings by the Greenland tip jets.The convection event of the 2O14-2015 winter, reaching 1700 m, shows many of persistant tip jets. The advection of LSW from the Labrador Sea explains the deepest mixed layers observed during the last two winters. The results show that deep convection in the lrminger Sea is not a rare isolated event and plays a significant role on the climate balance

Le but de cette thèse est de comprendre le rôle des fronts de température de surface de l'océan (SST) dans les interactions océan-atmosphère sur la région du Gulf Stream (GS). Nous étudions la réponse de l'atmosphère aux fronts de SST localement, au sein de la couche limite atmosphérique marine (MABL), mais également dans la troposphère libre. Par ailleurs, nous quantifions l'impact des fronts de SST sur la circulation atmosphérique en Atlantique Nord et en Europe. Nous nous focalisons sur la saison d'hiver (Décembre-Janvier-Février), les contrastes de température entre l'océan et l'atmosphère étant les plus marqués durant cette saison, ce qui induit de forts échanges en termes de flux de chaleur turbulents. Dans cette optique, nous réalisons et analysons des expériences numériques basées sur le modèle atmosphérique global ARPEGEv6, forcé en surface par des données de SSTs observées à haute résolution (1/4°). Dans la première partie de cette thèse, nous étudions les deux mécanismes principaux proposés dans la littérature pour expliquer la réponse de l'atmosphère aux fronts de SST dans la région du Gulf Stream. Ces mécanismes sont ceux du mélange vertical et de l'ajustement de pression. Dans cette étude, nous utilisons deux ensembles de simulations réalisées avec deux configurations du modèle d'atmosphère ARPEGEv6 : une configuration basse résolution (140 km) et une configuration haute résolution (50 km). Nous étudions spécifiquement la réponse de la divergence du vent en surface, un prédicteur important de l'influence des fronts de SST sur la MABL. Là où les études antérieures utilisaient majoritairement des moyennes mensuelles pour étudier la réponse de la divergence du vent aux fronts de SST, nos résultats montrent le rôle clé des perturbations synoptiques atmosphériques pour moduler la contribution relative de chacun de ces deux mécanismes de la MABL, et pour générer de la divergence du vent de surface en hiver. Nous montrons plus particulièrement qu’une grande partie de la divergence du vent simulée en moyenne sur l’hiver est due aux épisodes cycloniques qui ont lieu au cours de situations extrêmes au-dessus de la région du GS. La comparaison des résultats entre la version haute et la version basse résolution du modèle atmosphérique, montre que l'impact de l'augmentation de la résolution du modèle est faible comparée à la variabilité interne climatique et aux incertitudes observationnelles. Afin de mieux isoler l'influence des fronts de SST sur l’atmosphère, nous avons réalisé des expériences idéalisées dans lesquelles les SSTs de la région du GS sont spatialement filtrées. Cette expérience « lissée » ("smooth") est comparée à une expérience de contrôle dans laquelle le même modèle d'atmosphère est forcé à l'échelle globale par des SSTs à très haute résolution (1/12°). La comparaison entre ces deux expériences montre que la variabilité liée à la zone de fronts de SST influence localement les mécanismes de la MABL, mais également la troposphère libre dans son ensemble. En particulier, la bande de précipitations sur le GS est atténuée dans l'expérience smooth. Un gradient de SST mieux résolu dans la région du GS affecte également les routes dépressionnaires et les transports de chaleur et d'humidité par les tourbillons atmosphériques. En particulier, on montre que les fronts de SST provoquent un décalage vers le nord des transports de chaleur et d'humidité par les tourbillons atmosphériques. Ceci est en accord avec un élargissement du courant jet. La réponse du jet la plus intense est située sur l'Atlantique Nord, mais est également visible sur le bassin Pacifique. On montre ensuite qu'en accord avec la réponse du courant jet, les régimes de temps en Atlantique Nord sont également influencés par la présence des fronts de SST [...]
This thesis aims at understanding the role of sea surface temperature (SST) fronts on air-sea interactions in the Gulf Stream region. We study the local response of the atmosphere to the SST mesoscale variability, not only within the marine atmospheric boundary layer (MABL), but also in the free troposphere. We also evaluate the impact of SST fronts beyond the Gulf Stream region, in particular on the North Atlantic atmospheric circulation and climate over Europe. We focus on the winter season (December-January-February), when air-sea contrasts and hence air-sea exchanges are the strongest. On this purpose, we perform and analyze numerical experiments, using the global atmospheric model ARPEGEv6 forced by observed daily SSTs at high resolution (1/4°). In the first part of this thesis we investigate two major MABL mechanisms responsible for the atmospheric response to the SST fronts over the Gulf Stream region. These mechanisms are the vertical mixing and the pressure adjustment mechanisms. Two sets of atmospheric simulations performed with two ARPEGEv6 configurations are considered: a low-resolution version (140 km) and a high resolution version (50 km). We analyze the response of the divergence of the near surface wind, because this is one of the main imprint of the MABL response to the SST front. While in most of previous studies monthly averages were used to study the response of the wind divergence to the SST fronts, our results highlight the key role of synoptic atmospheric perturbations on modulating the contribution of these two MABL mechanisms and hence on shaping the time-mean divergence of near surface wind. We show in particular that most of the winter-mean wind divergence simulated above the Gulf Stream region can be explained by the cyclonic anomalous circulation that occurs during extreme conditions of heat flux exchanges. The comparison of the results obtained with the high and low resolution versions of the atmospheric model shows that the impact of model resolution is small compared to internal climate variability and observational uncertainties in the Gulf Stream region. In order to better isolate the influence of the SST fronts on the atmosphere, we then performed idealized numerical experiments in which the SSTs are spatially filtered only over the Gulf Stream region. This "smooth" experiment is compared with a control experiment in which the atmospheric model is globally forced by very high resolution observed SSTs (1/12°). The comparison between these two experiments shows that SST fronts variability locally influence not only the MABL mechanisms, but also the free troposphere. In particular, the precipitation band over the Gulf Stream is decreased in the smooth experiment. A better resolved SST gradient in the Gulf Stream also yields changes in the storm-tracks and in the associated heat and humidity eddy transports. In particular, we find that the Gulf Stream SST front induces a northward shift of the eddy heat and humidity transports. This shift is consistent with a poleward shift of the jet stream. The jet response is maximum over the North Atlantic, but it is also noticeable over the Pacific basin. We show that together with the jet stream changes, weather regimes in the North Atlantic are also influenced by the SST fronts. As a result, the response of the large scale atmospheric circulation yields changes in temperature and precipitation over Europe, suggesting a non negligible influence of the Gulf Stream SST fronts downstream

Cette thèse s'intéresse à la dynamique océanique induite par les échelles spatiales et temporelles de l'atmosphère, et du vent en particulier, dans les régions d'upwelling du Benguela et des Canaries. Ces régions sont sous l'influence d'un vent local ou régional, soufflant parallèlement à la côte. "Moteur" principal de la résurgence d'eau froide, ce vent est modulé par des processus physiques à des échelles spatio-temporelles variées. La nature des interactions avec l'atmosphère, l'océan et le continent environnants diffère selon les processus. Depuis deux décennies, des efforts remarquables portent sur la description par télédétection des champs atmosphériques à la surface de l’océan. Un nombre croissant de missions spatiales et des améliorations techniques majeures ont permis de raffiner la résolution horizontale et temporelle des produits disponibles à l'échelle globale. La disponibilité de multiples mesures diffusiométriques grillées, traitées et distribuées par le LOSCERSAT,nous amène dans un premier temps à comparer et analyser la richesse et la finesse des échelles retranscrites par différents produits. Ainsi, plusieurs gammes d'échelles de vent sont différenciées et leurs signatures sur l'upwelling côtier sont étudiées. L'intensité des anticyclones subtropicaux (Sainte Hélène et Açores) module la saisonnalité de l'upwelling le long des côtes Ouest africaines. Les régions centrales des upwellings de l’Atlantique, sous l'influence permanente de ces centres de haute pression, sont ainsi les cellules d'upwelling les plus intenses de chacun des systèmes en termes de pérennité et d'intensité (cellule de Lüderitz et cellule de Dakhla respectivement dans l'hémisphère Sud et l'hémisphère Nord). À l'échelle régionale, ou l'échelle des sous-bassins (O(1000 km)), la variabilité intrasaisonnière du vent est contrôlée par le renforcement ou l'atténuation des anticyclones entraînant à la côte l'activation ou la relaxation d'événements d'upwelling. À des échelles plus petites (O(100 km)), le front caractéristique de température de surface (SST) entre la côte et le large façonne la structure spatiale du vent par des processus de stabilisation/déstabilisation de la colonne d'air. Un vent soufflant en direction de l'équateur et parallèlement à un front de SST aura tendance à diminuer (augmenter) sur le flanc froid (chaud) de ce front. Le rotationnel (la divergence) du vent est directement impacté(e) et répond linéairement, au premier ordre, à la composante du gradient de SST normale (tangentielle) à la direction du vent. Ces rétroactions océaniques sont caractérisées par une échelle temporelle allant de l'hebdomadaire au mensuel. Enfin, de fines échelles du vent sont couramment observées dans les premiers kilomètres de l’océan au voisinage de la côte. L’interface entre le large et le continent est en effet associée à un affaiblissement significatif des vents. L'extension zonale de cette transition (O(10 km)) dépend notamment de l'orographie et de la rugosité de surface du continent adjacent. L'impact d'une telle réduction du vent sur la structure des upwellings côtiers, la dynamique sous-jacente et le transport côte-large de particules est appréhendé à l'aide d'analyses numériques eulériennes et lagrangiennes
This study focuses on the oceanic response to fine atmospheric spatial and temporal scales, and especially fine wind patterns in the Benguela and Canary upwelling systems. These regions are under the influence of local or regional wind, blowing parallel to the coast. Thewind is the main driver of the cold-water upwelling and is modulated by several physical processes at various scales. The nature of the interactions with the atmosphere, the ocean and the adjacent continent differs according to these processes. For the past 20 years, outstanding efforts have been made in the description and understanding of the atmospheric conditions at the sea surface. An increasing number of space missions and major technical improvements have allowed refinement of the horizontaland temporal resolution of the products available at global scale. The availability of multiple gridded scatterometer measurements,processed and distributed by the LOS-CERSAT, brings us first to compare and analyze the richness and fineness of the scales of a few products. We differentiate several wind scales and study their signatures on coastal upwelling dynamics. The intensity of the subtropical anticyclones (Saint Helena andAzores) modulates the seasonality of the upwelling along the Africanwest coast. The central regions of both upwelling systems are permanently under the influence of these atmospheric highs and,thus, are the most intense upwelling cells of each system, both interms of durability and intensity (Lüderitz and Dakhla cells for the southern and the northern hemisphere, respectively). On a regional scale, or basin scale (O(1000 km)), the intraseasonal wind variability is driven by the strengthening or weakening of these anticyclones, causing the activation or relaxation of upwelling events at the coast.At smaller scales (O(100 km)), the characteristic sea surface temperature (SST) front between the coastal and open ocean shapes the spatial structure of the wind by stabilization/destabilization of the air column. An equatorward-blowing wind parallel to an SST front tends to decrease (increase) on the cold side (warm) of this front. The curl (divergence) of the wind is directly impacted and the first order response varies linearly with the crosswind (downwind) SST gradient. This oceanic feedback is characterized by weekly to monthly temporal scales. Finally, small-scale wind structures are frequently observed in the first kilometers of the coastal ocean. Indeed, the interface between the open ocean and the continent is associated with a significant wind drop-off. The zonal extension of this transition (O(10 km) depends on the orography and on the surface roughness of the adjacent continent. The impact of such a wind reduction on the structure of the coastal upwelling, the underlying ocean dynamics and the cross-shore transport of particles is diagnosed with both Eulerian and Lagrangian numerical analyses

This thesis is a study of the air-sea fluxes of momentum, sensible heat and latent heat. Fluxes are estimated using the covariance, COARE2.6b bulk flux algorithm, and inertial dissipation methods. The bulk algorithm is validated against the covariance fluxes for the first time in a light-wind, shallow tropical sea, with strong atmospheric instability and low sea state conditions. The removal of ship motion contamination is investigated. This is the first study to quantify the errors associated with corrections for ship motion contamination, and the effects of motion contamination on the covariance calculated heat fluxes. Flow distortion is investigated. Bulk transfer coefficients and roughness lengths are computed and related to the sea state. Ship motion contamination is successfully removed in 86% of the runs. Error analysis of the motion removal algorithm indicates maximum uncertainties of 15% in the wind fluctuations, and 0.002 N/m/m for the wind stress. Motion correction changes the stress by more than 15% in half of the runs analysed. The ship is found to accelerate the mean air flow and deflect it above the horizontal. A correction is developed for the air flow acceleration. The scalar fluxes show good agreement on average for all the methods. As wind speed approaches zero, covariance wind stress is significantly larger than the bulk and inertial dissipation derived wind stress. The non-zero covariance wind stress is reflected in the drag coefficient, CdN10, and momentum roughness length, z0, which are much larger than the parameterisations used in the bulk algorithm. The MCTEX CdN10, wind speed (u10N) relation is 1000 x Cd10N = 1.03 + 7.88/(u10N)^2 0.8 < u10N < 7.5 m/s z0 is primarily a function of wind speed rather than sea state, with largest roughness lengths occurring as wind speed approaches zero. This relation is used in the bulk algorithm, yielding good agreement between covariance and bulk derived wind stress. A new parameterisation for the effects of gustiness, based on wind variance is developed. This brings the bulk wind stress into agreement with the covariance derived wind stress.

O objetivo deste trabalho é investigar a variabilidade das componentes do balanço de calor pela superfície no Atlântico Sul, entre 5oS e 30oS e para o período entre 2000 a 2004, através da combinação de dados provenientes de múltiplos satélites. Com isso, visamos verificar se os processos de troca de calor no oceano são dominados por variabilidade de larga escala e interanual. Os dados de temperatura da superfície do mar, vapor d\'água integrado e precipitação são provenientes do satélite de microondas do Tropical Rainfall Measuring Mission (TRMM). Os dados de vento são obtidos pelo escaterômetro QuikSCAT e estimativas de radiação de ondas curtas e ondas longas são distribuídas pelo projeto Surface Radiation Budget. Utilizamos o algoritmo desenvolvido por Liu et. al. (1979) para o cálculo do fluxo de calor latente e sensível. Para obtermos uma determinação mais precisa dos fluxos turbulentos, utilizamos o algoritmo de Fairall et. al. (1996), onde estimamos a correção de Webb para o calor latente e o calor sensível devido à chuva. Analisamos as variáveis medidas e estimadas em termos da média, anomalia e diagramas de espaço-tempo (Hovmöller). As estimativas de balanço de calor pela superfície mostram que o Atlântico Sul perde calor para a atmosfera, principalmente na forma de calor latente, ao sul de 7oS. Em média, o máximo de perda de calor de -100W/m2 ocorre entre 12oS e 17oS. O balanço de calor é notadamente marcado por um forte ciclo sazonal, onde a amplitude anual chega a 250W/m2. A anomalia do balanço de calor apresenta correlações significativas com fenômenos remotos em escalas interanuais, indicando a estabelecimento de teleconexões rápidas através da atmosfera.
The objective of this study is to investigate the variability of the surface heat budget components in the tropical South Atlantic, between 5oS and 30oS and for the period between 2000 and 2005, through a methodology based on a multi--satellite approach. We aim to verify if the heat exchange processes in the ocean are dominated by large scale and interannual variability. The sea surface temperature, integrated water vapor and precipitation data are obtained by the Tropical Rainfall Measuring Mission (TRMM) microwave satellite. Wind vectors are measured by the QuikSCAT scatterometer satellite and the estimates of the shortwave and longwave radiation are distributed by the Surface Radiation Budget projetct. We used the algorithm developed by Liu79 et. al. (1979) to estimate the latent and sensible heat fluxes. To obtain a more precise estimation of the turbulent fluxes, we used an algorithm developed by Fairall et. al. (1996), where the Webb correction for the latent heat flux and the sensible heat flux due to the rainfall were included in the calculations. The results were analized in terms of the mean, anomaly, and space--time (Hovmöller) diagrams. The estimates of the surface heat balance showed that the South Atlantic loses heat to the atmosphere, mostly in the form of latent heat fluxes, south of 7oS. On average, there is a maximum in heat loss of -100W/m2 between 12oS e 17oS. The net surface heat flux has a strong seasonal cycle, with an amplitude of about 250W/m2. The surface energy balance shows significant correlations with remote phenomena at interannual scales indicating the establishment of rapid teleconnections through the atmosphere.

Thesis (M.S. in Meteorology and M.S. in Physical Oceanography) Naval Postgraduate School, December 1993.
Thesis advisor(s): James Thomas Murphree ; Peter Chu. "December 1993." Bibliography: p. 88-89. Also available online.

Cette thèse est consacrée à l'étude de la variabilité des rails des dépressions des moyennes latitudes induite par les anomalies de température de surface (SST) associées aux tourbillons océaniques. Les effets locaux et de grande échelle de ces tourbillons sur la couche limite et la troposphère libre sont analysés à partir de simulations atmosphériques à haute résolution. Un canal barocline périodique représente de manière idéalisée le rail des dépressions sur une aquaplanète, avec un forçage par des SST décrivant un front océanique et ses tourbillons. La première partie de la thèse traite de la réponse des basses couches de l'atmosphère, en lien avec les mécanismes d'ajustement de pression et de mélange vertical turbulent. L'ajustement de pression domine dans les conditions de vent faible, en accord avec la théorie. En présence d'un front de grande échelle, la direction du vent module également la stabilité des basses couches : pour des vents forts venant du côté chaud du front, la réponse atmosphérique est aussi dominée par une réponse en ajustement de pression, à l'inverse des vents de direction opposée. Cela montre que les mécanismes de réponse sont déterminés par la variabilité rapide de l'atmosphère. La divergence de tension de surface est, comme dans les observations, linéairement reliée au gradient de SST. Pour certaines conditions, la divergence du vent horizontal peut cependant présenter une réponse proportionnelle au laplacien de température de la couche limite. Ceci concerne une gamme d'échelles allant de 100 à 500 km. La seconde partie de la thèse compare la réponse atmosphérique à un champ de tourbillons à une expérience témoin caractérisée par un front de SST zonalement symétrique. Des déplacements vers le pôle du rail des dépressions et du courant-jet apparaissent, tous deux faibles mais statistiquement robustes. Les forçages à la surface conduisent à une réponse locale dans la troposphère libre, qui est dominée par la variabilité synoptique. L'asymétrie de réponses aux tourbillons chauds et froids engendre aussi une augmentation des flux de surface moyens, qui permet d'interpréter une grande partie de la réponse de grande échelle. Les déplacements vers le pôle sont expliqués à partir des bilans de chaleur et d'énergie mécanique du rail des dépressions. Le dégagement additionnel de chaleur latente, qui se produit dans les tempêtes, joue un rôle primordial dans ces deux bilans
This thesis aims to understand how sea surface temperature (SST) anomalies associated with mesoscale oceanic eddies may contribute to mid-latitude storm tracks variability. Based on idealized high resolution atmospheric simulations, local and large scale remote influences of the eddies onto the boundary layer and the free troposphere are investigated. Basic configuration is a generic baroclinic channel above an eddying SST front. The first part of the manuscript deals with the response of the atmosphere in the boundary layer, based on the well-known mechanisms of pressure adjustment and vertical mixing of momentum. As expected from theoretical scaling arguments, the former is important for weak wind conditions. Due to the large scale SST front, direction of strong background wind also controls stability : a transition, from a forcing by mixing of momentum to a forcing by SST-driven pressure forces, occurs for equatorward winds as cold air is advected above warmer SST. It shows how sub-weekly atmospheric variability modulates the low level response. Wind stress divergence matches the linear relationship observed with downwind SST gradient. In specific conditions, horizontal divergence is however proportional to the laplacian of boundary layer temperature, within a large range of horizontal scales (500-100~km). In a second part, the experiment with oceanic eddies is compared to its zonally symmetric counterpart. It reveals weak but robust modifications of atmospheric large-scale circulation and synoptic variability : the Eulerian storm track and the jet stream are shifted poleward. Surface forcing is associated with a local response in the mid-troposphere, which is intermittent and dominated by the day-to-day variability. Because of an asymmetry in the response to cold and warm eddies, the oceanic eddies are also responsible for a net increase of both latent and sensible surface heat fluxes. It accounts for a large part of the differences obtained between eddying and zonally symmetric configurations. Based on heat and mechanical energy budgets, an interpretation of the poleward displacements of the storm track and the jet is given. The contribution of latent heat release to both budgets, occurring mainly within synoptic storms, plays a major role in the atmospheric response

Cette thèse concerne l'étude de l'interaction air-mer, due aux échanges de mouvements, avec un modèle idéalisé mais consistant. Les études sont réalisées à partir d'un modèle shallow-water bicouches (une pour l'océan et une pour l'atmosphère), avec une fine résolution spatiale et temporelle. L'interaction est uniquement due à la friction de surface entre les deux couches.Elle est implémentée par une loi de friction quadratique. La force appliquée à l'océan est calculée en utilisant la différence de vitesse entre les vents et les courants. Pour la force appliquée à l'atmosphère on distingue deux cas l'interaction ``1way'' et ``2way''. Pour la première, la friction appliquée à l'atmosphère néglige la dynamique de l'océan; elle est calculée en utilisant uniquement les vents. Pour l'interaction ``2W'', la friction appliquée à l'atmosphère est l'opposée de celle appliquée à l'océan.Trois configurations idéalisées sont explorées ici.La première configuration explique la génération d'une instabilité barotrope dans l'océan due à la force de friction quadratique et la dissipation visqueuse horizontale de l'atmosphère. Dans le cas 1W le cisaillement entraîne une instabilité barotrope dans l'océan. Dans le cas 2W, l'instabilité est amplifiée en amplitude et en dimension et est transférée à l'atmosphère. L'échelle principale de cette instabilité correspond à celle du rayon de Rossby dans l'océan. Elle est uniquement visible dans les modèles numériques, lorsque la dynamique est résolue à cette échelle à la fois dans l'océan mais aussi dans l'atmosphère.Dans la deuxième configuration, des expériences pour différentes valeurs du coefficient de traînée de surface sont réalisées. Le forçage diffère de la première configuration, et permet d'avoir une dynamique turbulente dans l'océan et l'atmosphère. L'énergie perdue par l'atmosphère et gagnée par l'océan par cisaillement à l'interface sont déterminées et comparées aux estimations basées sur les vitesses moyennes. La corrélations entre la vorticité océanique et atmosphérique est déterminée à l'échelle synoptique et méso-échelle de l'atmosphère. L'océan a un rôle passif, et absorbe l'énergie cinétique à quasiment tout les instants et tous les lieux. Les résultats différent des études réalisées à l'échelle du bassin. De par les faibles vitesses de l'océan, le transfert d'énergie dépend que faiblement des courants. La dynamique de l'océan laisse cependant son empreinte dans la dynamique de l'atmosphère conduisant à un état `quenched disorder' du système océan-atmosphère, pour le plus fort coefficient de friction utilisé.La dernière configuration, considère l'échange de mouvements entre l'océan et l'atmosphère autour d'une île circulaire. Dans les simulations actuelles de la dynamique océanique, le champs du forçage atmosphérique est généralement trop grossier pour inclure la présence de petites îles (<100km). Dans les calculs présentés ici, l'île est représenté dans la couche atmosphérique par un coefficient de traînée cent fois plus fort au dessus de l'île que l'océan. Cela engendre de la vorticité dans l'atmosphère , autour et près du sillage de l'île. L'influence de la vorticité atmosphérique sur la vorticité de l'océan, l'upwelling, la turbulence et le transfert d'énergieest considéré en utilisant des simulations couplées océan-atmosphère.Les résultats sont comparés avec des simulations ayant un forçage atmosphérique constant dans le temps et l'espace (pas de sillage) et des simulations "1W" (pour lesquelles les courants n'ont pas d'influence sur l'atmosphère).Les résultats des simulations sont en accords avec les travaux et les observations précédemment réalisés, et confirment que le sillage atmosphérique est le principal processus générant des tourbillons océanique dans le lit de l'île. Il est aussi montré que la vorticité est injectée directement par le rotationel du vent, mais aussi par la force du vent perpendiculaireau gradient d'épaisseur de la couche de surface océanique
This thesis considers air-sea interaction, due to momentum exchange, in an idealized but consistent model. Two superposed one-layer fine-resolution shallow-water models are numerically integrated. The upper layer represents the atmosphere and the lower layer the ocean. The interaction is only due to the shear between the two layers. The shear applied to the ocean is calculated using the velocity difference between the ocean and the atmosphere.The frictional force between the two-layers is implemented using the quadratic drag law. Three idealized configurations are explored.First, a new mechanism that induces barotropic instability in the ocean is discussed. It is due to air-sea interaction with a quadratic drag law and horizontal viscous dissipation in the atmosphere. I show that the instability spreads to the atmosphere. The preferred spatial scale of the instability is that of the oceanic baroclinic Rossby radius of deformation.It can only be represented in numerical models, when the dynamics at this scale is resolved in the atmosphere and the ocean.In one-way interaction the shear applied to the atmosphere neglectsthe ocean dynamics, it is calculated using the atmospheric wind, only. In two-way interaction it is opposite to the shear applied to the ocean.In the one-way interaction the atmospheric shear leads to a barotropic instability in the ocean. The instability in the ocean is amplified, in amplitude and scale, in two-way interaction and also triggers an instability in the atmosphere.Second, the air-sea interaction at the atmospheric synoptic and mesoscale due to momentum transfer, only, is considered. Experiments with different values of the surface friction drag coefficient are performed, with a different atmospheric forcing from the first configuration, that leads to a turbulent dynamics in the atmosphere and the ocean. The actual energy loss of the atmosphere and the energy gain by the ocean, due to the inter-facial shear,is determined and compared to the estimates based on average speeds.The correlation between the vorticity in the atmosphere and the ocean is determined. Results differ from previous investigations where the exchange of momentum was considered at basin scale. It is shown that the ocean has a passive role, absorbing kinetic energy at nearly all times and locations.Due to the feeble velocities in the ocean, the energy transfer depends only weakly on the ocean velocity. The ocean dynamics leaves nevertheless its imprint in the atmospheric dynamics leading to a quenched disordered state of the atmosphere-ocean system, for the highest value of the friction coefficient considered. This finding questions the ergodic hypothesis, which is at the basis of a large number of experimental, observational and numericalresults in ocean, atmosphere and climate dynamics.The last configuration considers the air-sea interaction, due to momentum exchange, around a circular island. In todays simulations of the ocean dynamics, the atmospheric forcing fields are usually too coarse to include the presence of smaller islands (typically $<$ 100km).In the calculations presented here, the island is represented in the atmospheric layer by a hundred fold increased drag coefficient above the island as compared to the ocean. It leads to an increased atmospheric vorticity in the vicinity and in the wake of the island. The influence of the atmospheric vorticity on the ocean vorticity, upwelling, turbulence and energy transfer is considered by performing fully coupled simulations of the atmosphere-oceandynamics. The results are compared to simulations with a constant, in space and time, atmospheric forcing (no wake) and simulations with one-waycoupling only (where the ocean velocity has no influence on the atmosphere).Results of our simulations agree with previous published work and observations, and confirm that the wind-wake is the main process leading to mesoscale oceanic eddies in the lee of an island

The world ocean is rich in mesoscale structures that locally affect the overlying atmosphere. The interaction of these mesoscale oceanic features with the overlying atmosphere is an interesting topic because of the ubiquity of mesoscale structures in the ocean and the lacking of a definitive representation of the interaction mechanisms. This thesis presents two case studies of air-sea interaction using data collected during the EUREC4A campaign in the tropical north-western Atlantic. The objective is to learn about mesoscale air-sea interaction by case study analysis, particularly for what concerns the effect of sea surface temperature structures. SST measurements from Merian’s thermosalinograph identify the case studies. Then, ship-borne observations from the two weather stations, wind lidar, cloud radar on the R/V Merian, characterize each case study. Radiosondes launched from the R/Vs Merian and Atalante complete the ground-based dataset. Satellite SST and cloud cover observations complement the dataset and allow for comparison. Also, wind data from the ICON-LEM model output are exploited to provide field information to the ship-based point measurements. For the first case study, no effects of the SST on the overlaying atmosphere can be detected. The absence of a detectable SST effect in the first case study is attributed to the limited temporal and spatial extent of the SST anomaly. In the second case study a connection between an SST cold front, vertical velocity, and cloud cover is found. It is proposed that the colder SST dampen the vertical motion on the overlaying atmosphere reducing cloud formation.

Dissertation (Ph.D. in Meteorology)--Naval Postgraduate School, June 2010.
Dissertation supervisor: Montgomery, Michael. "June 2010." Description based on title screen as viewed on July 14, 2010. Author(s) subject terms: Air-sea interaction, tropical cyclones, surface fluxes, drag coefficient, CBLAST. Includes bibliographical references (p. 125-131). Also available in print.

Air-sea interactions are a key process in the forcing of the ocean circulation and the climate. Water Mass Formation is a phenomenon related to extreme air-sea exchanges and heavy heat losses by the water column, being capable to transfer water properties from the surface to great depth and constituting a fundamental component of the thermohaline circulation of the ocean. Wind-driven Coastal Upwelling, on the other hand, is capable to induce intense heat gain in the water column, making this phenomenon important for climate change; further, it can have a noticeable influence on many biological pelagic ecosystems mechanisms. To study some of the fundamental characteristics of Water Mass Formation and Coastal Upwelling phenomena in the Mediterranean Sea, physical reanalysis obtained from the Mediterranean Forecating System model have been used for the period ranging from 1987 to 2012. The first chapter of this dissertation gives the basic description of the Mediterranean Sea circulation, the MFS model implementation, and the air-sea interaction physics. In the second chapter, the problem of Water Mass Formation in the Mediterranean Sea is approached, also performing ad-hoc numerical simulations to study heat balance components. The third chapter considers the study of Mediterranean Coastal Upwelling in some particular areas (Sicily, Gulf of Lion, Aegean Sea) of the Mediterranean Basin, together with the introduction of a new Upwelling Index to characterize and predict upwelling features using only surface estimates of air-sea fluxes. Our conclusions are that latent heat flux is the driving air-sea heat balance component in the Water Mass Formation phenomenon, while sensible heat exchanges are fundamental in Coastal Upwelling process. It is shown that our upwelling index is capable to reproduce the vertical velocity patterns in Coastal Upwelling areas. Nondimensional Marshall numbers evaluations for the open-ocean convection process in the Gulf of Lion show that it is a fully turbulent, three-dimensional phenomenon.

The feedback between clouds and sea ice got more importance in the last years, because of the declining Arctic sea ice extent. Previous observations show the formation of low clouds over newly formed open water. These low clouds are very important for the Arctic Energy Budget, because they warm the surface. This leads to increasing temperatures and stronger sea ice loss. To assess the relationship between sea ice cover and cloudiness, satellite observations by DARDAR were compared with both global climate reanalyses ERA–Interim and MACC. The analysis focuses on 2007 – 2010 and the relationship between different parameters from the different datasets. It is found that the reanalyses only poorly approximate the cloud cover in the Arctic. Consequently no strong correlation was found for the time period 2007 – 2010.
Das Wolken–Albedo–Feedback in der Arktis gewann in den letzten Jahren immer mehr an Bedeutung aufgrund des Rückganges der Meereisfläche. Vorhergehende Arbeiten zeigten die Bildung von tiefer Bewölkung über kürzlich aufgebrochenen Meereisstellen. Diese tiefen Wolken sind sehr wichtig für das arktische Energiebudget, wegen des Erwärmens der Oberfläche. Daraus folgt ein Anstieg in der bodennahen Temperatur und ein verstärkter Rückgang des Meereises. Um den Einfluss der Meereiskonzentration auf die Wolkenbildung zu untersuchen, werden in dieser Arbeit Satellitendaten von DARDAR mit den beiden globalen Klimareanalysen Era–interim und MACC verglichen. Analysiert werden Daten aus den Jahren 2007 bis 2010 und für verschiedene Oberflächenbedingungen werden Korrelationen der einzelnen Datensätze erstellt. Es hat sich gezeigt, dass die Darstellung der Wolkenbedeckung in der Arktis durch die Reanalyse Daten nicht geeignet ist. Aus diesem Grund wurden keine signifikanten Korrelationen in der Zeitspanne von 2007 bis 2010 gefunden.

This dissertation investigates wind-wave-current interaction, wave breaking detection and the analysis of breaking characteristics at the air-sea interface. In-situ data measured during the Shoaling Waves Experiment (SHOWEX) and Baltic Sea Swell Experiment (BASE) are applied in the studies and analysis. Wind, wind stress and wave data were obtained from several Air Sea Interaction Spar (ASIS) buoys. Surface currents were measured by a High-Frequency Ocean Surface Current Radar. Two distinct types of wave-current-wind interaction were observed in the presence of a strong along-coast current. First, the horizontal current shear resulted in wind-sea waves shifting away from the wind direction. This motion resulted in a steering of the stress away from the mean wind direction. Second, short wind waves on a uniform current are shifted to the current direction, and the wind stress is steered toward the current direction by the short waves. The wind stress veering has been confirmed by data from the SeaWind scatterometer on board the QuikSCAT satellite. This finding is in agreement with the results from some recent studies. The present study also describes an experimental investigation of breaking wave detection by ASIS buoys. A method, developed from the laboratory, and using local wave parameters to provide a detailed description of breaking, is applied to wave data from ASIS buoys. One the basis of these data, the relation between breaking probability and wind speed shows characteristics similar to those from several field experiments with different conditions. Furthermore, additional parameters, wave age and wave steepness, are also shown to affect the breaking probability during our in-situ measurements. Upper ocean shear, which can modulate wave breaking as predicted by both theory and laboratory work, are also observed to change the breaking properties. This characteristic is rarely reported by in-situ experiment.

Les systèmes de courant de bords est (EBUS) sont les régions océaniques des latitudes tropicales à moyennes le long des côtes ouest des continents. Ils abritent des écosystèmes marins très productifs en raison de la circulation atmosphérique de surface dirigée vers l'équateur qui font remonter des eaux profondes froides (upwelling) enrichies en éléments nutritifs à l'origine de la vie marine le longde la côte. Si les processus océaniques fondamentaux de l'upwelling côtier sont bien connus (transport et pompage d'Ekman), la modélisation océanique des EBUS reste problématique en raison des difficultés pour prendre en compte de manière réaliste des phénomènes à fine échelle spatiale dans la zone de transition entre le littoral et l'océan du large. Dans cette thèse, nous nous sommes concentrés sur le système d'upwelling dit de Humboldt (côtes du Pérou et du Chili) et sur l'influence des caractéristiques méso-échelles des vents près de la côte, en particulier la décroissance vers la cotes du vent (appelé "drop-off") qui détermine l'importance relative des processus d'Ekman, et donc, la structure spatiale de la zone d'upwelling. Une approche combinée basée sur l'analyse de données satellitaires et sur la modélisation régionale, océanique et atmosphérique, est utilisée pour étudier la sensibilité de la circulation océanique le long de la côte Chili central aux caractéristiques du drop-off. Dans un premier temps, la circulation atmosphérique de surface moyenne à saisonnière le long du littoral du Pérou et du Chili est documentée pour la première fois à partir des données altimétriques de quatre missions satellites (ENVISAT, JASON1, JASON2 et SARAL). L'analyse révèle l'existence d'une réduction marquée de la vitesse du vent le long de la côte, bien que le taux de réduction varie en fonction de la latitude. Malgré la répétitivité relativement faible des satellites, nous montrons que les données altimétriques permettent néanmoins d'échantillonner le cycle saisonnier du drop-off. L'estimation de l'upwelling côtier à partir de ces données suggère que le pompage d'Ekman tend en moyenne à dominer par rapport au transport d'Ekman le long de la côte péruvienne, alors que le long de la côte chilienne, le transport d'Ekman est le processus dominant. Dans un second temps, un modèle atmosphérique régional (WRF) à différentes résolutions horizontales (36 km, 12 km et 4 km) dans une configuration imbriquée zoomée sur la région centrale du Chili a été développé afin de produire des champs atmosphériques présentant des caractéristiques différentes du drop-off. Les solutions du modèle atmosphérique sont d'abord évaluées par rapport aux observations, indiquant un plus grand réalisme près de la côte que les réanalyses atmosphériques. Le rotationnel du vent cyclonique simulé le long de la côte associé au drop-off présente des échelles transversales comprises entre 8 et 45 km avec une variabilité latitudinale significative, en accord avec les vents altimétriques. Lorsque la résolution du modèle est augmentée, le drop-off est généralement d'autant plus confiné à la côte et le modèle indique une saisonnalité marquée avec un maximum d'intensité au printemps-automne. La contribution relative de la divergence côtière et du pompage d'Ekman présente une modulation latitudinale liée aux détails de l'orographie et de la ligne de côte
Eastern Boundary Upwelling Systems (EBUS) are the tropical to mid-latitudes oceanic regions along the west coast of the continents. They host very productive marine ecosystems owing to the mean equatorward low-level atmospheric circulation that uplifts cool subsurface nutrient-enriched waters that trigger marine life along the coast. While the fundamental oceanic processes behind such process are well known (i.e. Ekman transport and pumping), the oceanic modeling of the EBUS has remained problematic owing to difficulties in accounting realistically for phenomena at fine spatial scales in the transition zone between the littoral and the off-shore ocean. In this thesis we have focused on the Peru-Chile Upwelling System (so-called Humboldt system) and on the influence of the cross-shore mesoscale features of the winds near the coast, particularly the shoreward wind drop-off, which determinate the relative importance of the Ekman processes, and thus, the spatial and temporal structure of the upwelling. A combined approach based on satellite data analysis and regional modeling, both oceanic and atmospheric, is used to investigate the sensitivity of the oceanic circulation along the coast of central Chile to the characteristics of the wind drop-off. As a first step, the mean to seasonal near-shore surface atmospheric circulation along the coast of Peru and Chile is documented for the first time based on the altimeter data from four satellite missions (ENVISAT, JASON1, JASON2 and SARAL). The analysis reveals the existence of a marked shoreward reduction in the wind speed all along the coast, although the reduction rate is latitudinally dependent. Despite the relatively weak repetitivity of the satellites, it is shown that the altimetric data are able to sample the seasonal cycle of the wind drop-off at some locations. The estimate of coastal upwelling from these data suggests that Ekman pumping tends on average to dominate with respect to Ekman transport over the Peruvian coast, whereas over the central-Chilean coast, the Ekman transport is the dominant process. In a second step, a regional atmospheric model (WRF) at different horizontal resolutions (36km, 12km and 4km) in a nested configuration zoomed over the central-Chile region was developed in order to produce atmospheric fields with different characteristics of the wind-stress curl (drop-off) along the coast. The atmospheric model solutions are first evaluated against the satellite observations, showing a much larger realism than atmospheric Reanalyses near the coast. In particular, the simulated cyclonic wind curl along the coast related to the wind drop-off exhibit length scales between 8 and 45 km with a significant latitudinal variability, which is in agreement with the altimetric winds. The higher model resolution, the more confined to the coast the wind drop-off, with the latter evidencing a marked seasonality with a maximum intensity in spring-fall and minimum in winter. The relative contribution of the coastal divergence and Ekman pumping exhibits a latitudinal modulation linked to details in the orography and coastlines

Les surcotes de tempête sont souvent sous-estimées dans les modèles hydrodynamiques, ainsi que les grandes vagues dans les modèles de vagues. Les causes possibles sont une sous-estimation des vents dans les modèles atmosphériques et/ou une formulation incorrecte de la tension de vent. Les objectifs de cette thèse sont (1) d’estimer les biais par vents forts dans les modèles atmosphériques (2) de développer une nouvelle paramétrisation du coefficient de traı̂née permettant de réduire ce biais (3) d’étudier l’impact des vagues sur la tension de vent. La méthode consiste à étudier la réponse de l’atmosphère et de l’océan à la tension de vent. Dans une première partie, nous utilisons le modèle couplé vagues-atmosphère d’ECMWF. Nous montrons que les vents forts sont sous-estimés, avec un biais de l’ordre de -7 m/s à 30 m/s. Des écarts significatifs existent aussi entre les observations, les bouées et les vents issus de ASCAT-KNMI étant généralement inférieurs à ceux des plateformes et des autres données satellites utilisées dans cette étude (AMSR2, ASCAT-RSS, WindSat, SMOS et JASON-2). La nouvelle paramétrisation développée permet d’obtenir des vents plus forts qu’avec celle d’ECMWF par défaut. Dans une deuxième partie (réponse de l’océan), nous utilisons le modèle global océanique TUGO du LEGOS forcé par le modèle couplé vagues-atmosphère d’ECMWF. Nous montrons qu’une paramétrisation de la tension de vent dépendant des vagues plutôt que du vent est plus appropriée quand l’état de mer est jeune. Elle conduit à des surcotes plus proches des observations (marégraphes et traces altimétriques de JASON-2). L’impact des vagues sur la surcote est significatif, et peut atteindre 20 cm
Storm surges may be underestimated in hydrodynamic models, as well as large wave heights in wave models. This could come from an underestimation of strong winds in atmospheric models and/or an inappropriate wind stress formulation. The objectives of the present work are (1) to estimate how strong are the biases for high winds in atmospheric models (2) to develop a new drag parameterization that could reduce this bias (3) to investigate the impact of the waves on the wind stress. The method consists of studying the response of the atmosphere and the ocean to the wind stress.In a first part, we use the coupled wave-atmosphere model from ECMWF. We show that strong winds may be underestimated, as much as -7 m/s at 30 m/s.Significant differences also exist between observations, with buoys and ASCAT-KNMI generally showing lower wind speeds than the platforms and other remote-sensing data used in this study(AMSR2, ASCAT-RSS, WindSat, SMOS and JASON-2).The newly empirically adjusted Charnock parameterization leads to higher winds compared to the default ECMWF parameterization. In a second part, we use the global ocean model TUGO fromLEGOS forced with ECMWF coupled wave-atmopshere model. We show that a wave-dependent rather than wind-dependent stress formulation is more appropriate, when the sea state is young and the sea rougher. It yields to simulated surges closer to observations (i.e. tide gauges and JASON-2 altimeter tracks). The wave impact on the surges is significant, and may reach 20 cm

During an upwelling event the cold bottom-water is brought to the sea surface. This cools the atmosphere from below and the stratification becomes more stable. When the atmosphere is more stable the turbulence is reduced and, as a consequence, so are the turbulent fluxes. This study is investigating four periods of upwelling from the Östergarnsholm-site, in the Baltic Sea east of Gotland, during the summer of 2005. The air measurements are taken at a tower at the southernmost tip of Östergarnsholm while the measurements in the water are from a buoy moored 1 km south-southeast of the tower. During all the upwelling events the wind is south-westerly, along the coast of Gotland. This means that the buoy is not within the flux footprint area and is perhaps not always representative of what happens there. All the periods show a stabilization of the atmosphere as the SST (Sea Surface Temperature) decreases. The heat fluxes, especially the latent heat flux, decreases as the SST decreases. The amount of CO2 in the atmosphere, in the summer, is usually higher than the amount in the surface water of the seas because the oceans are a net sink of CO2. The air-sea flux of CO2 is to a large extent controlled by this difference. Therefore the flux of CO2 is usually directed to the sea. The deep-water contains more CO2 than the surface water because the phytoplankton near the surface removes CO2 through photosynthesis. The deep-water is also colder and can solve more CO2. During an upwelling event this CO2-rich water is brought to the surface. As an upwelling event progresses the difference in CO2-concentration between the air and the sea is reduced, sometimes reversed, and the flux decreases. This is what happens in three of the investigated periods in this study. During the fourth period a counter gradient flux is observed.
När en uppvällning inträffar förs kallt djupvatten upp till havsytan. Det kalla vattnet kyler atmosfären nedifrån, något som leder till mer stabil skiktning. När atmosfären blir mer stabilt skiktad dämpas turbulensen och det medför att de turbulenta flödena också avtar. I den här studien analyseras fyra perioder med uppvällning. Mätningarna kommer från Östergarnsholm, öster om Gotland, under sommaren 2005. Mätningarna i luften är tagna från en mast vid Östergarnsholms södra udde. Mätningarna i vattnet kommer från en boj som är förankrad 1 km sydsydöst om masten. Vid samtliga uppvällnings-perioder i den här studien är vinden sydvästlig (längs Gotlandskusten). Det betyder att bojen inte befinner sig inom flödenas footprint-area och dess mätningar är kanske inte hela tiden representativa för vad som händer i footprint-arean. Samtliga undersökta perioder visar på en stabilisering av atmosfären då havsytans temperatur avtar. Värmeflödena, i synnerhet det latenta värmeflödet, avtar i samband med att temperaturen i havsytan sjunker. Halten av CO2 i atmosfären är vanligtvis högre än halten i havens ytvatten (under sommaren) eftersom de är en nettosänka för CO2 globalt sett. CO2-flödet mellan havsytan och atmosfären styr till en stor del av denna skillnaden i CO2-halt. Det innebär att CO2-flödet är riktat neråt, mot havet. Havens djupvatten innehåller mer CO2 därför att växtplankton nära ytan reducerar CO2-halten genom fotosyntesen. Djupvattnet är också kallare och kan därför lösa mer CO2. Under en uppvällning förs detta CO2-rika vatten upp till ytan. När en uppvällning fortskrider minskar skillnaden i CO2-halt mellan hav och atmosfär (ibland kan CO2-halten i ytvattnet även komma att överstiga atmosfärens halt) och flödet avtar. Tre av perioderna i den här studien visar på ett avtagande flöde. Den fjärde perioden uppvisar ett flöde motriktat CO2-gradienten.

The investigation of climate variability in different timescales such as daily, monthly, seasonal and inter-annual has utmost importance for managing the socio-economic processes on regional to global scale. Indeed, the variation in the climate has a crucial impact on agriculture, water, health, tourism, economy and transportation. Therefore the development of climate forecasting tools is necessary which helps to manage these sectors more efficiently. However, there are limitations on producing accurate climate forecast for more than two weeks in advance due to the chaotic nature of the climate system, especially for the region like the Mediterranean, which is characterized by high interannual variability. Due to its importance and challenging nature, a collective effort is being done to improve the skill of models and climate forecasting in the Mediterranean. The contribution of this thesis is a overall effort, which consists of developing a high resolution model application in the Mediterranean, which can provide reliable estimate of Sea Surface Temperature (SST) and Mixed Layer Depth (MLD). This approach is based on the fact that the atmospheric predictability in seasonal to interannual time scale is significantly dependent on slowly varying lower boundary conditions (e.g. Charney and Shukla 1981) i.e. Mediterranean SSTs. The spatial resolution of model is increased for taking into account the mesoscale processes in the Mediterranean. Since the first internal Rossby radius of deformation in the Mediterranean sea is of the order of 10-15 kms, the spatial resolution of an eddy resolving model should have at least a resolution one half of the Rossby radius. Based on this assumption, the spatial resolution is explored to the order of -5 km (1/16°). The regional ocean modeling system (ROMS) adopted from the Rutgers University is used in the current study. The objective is to validate certain fields (such as SST and MLD) obtained from model simulations and study air-sea interactions. The validation is done by performing two experiments namely, climatological and interannual simulations. The model simulated results are validated with observations as well as intercompared to evaluate the skill of model. The monthly mean SST climatology is obtained from ten years of model run forced with climatological air-sea fluxes is well captured by the model configuration and follows the annual cycle. Model simulated summer SST climatology shows biases of the order of 0.8-1.0 °C with observation (MedAtlas) and 1.0-1.2 °C with other datasets (intercomparison). The vertical structure of temperature climatology is found to be well simulated by model in which upper layer shows a difference of 1.0 °C and it further decreased at intermediate layers. The simulated sea surface height and surface currents is validated with Aviso altimetry data. On the large scales the surface currents generated by model captures general structures of surface circulation. The monthly mean mixed layer depth (MLD) climatology computed from model is validated with observed monthly MLD climatology and found that the winter MLD is overestimated by model. In second experiment, model is forced with six hourly air-sea interaction fluxes from ERA-Interim and interannual simulations are obtained for the period 1998-2007. The monthly mean SST climatology obtained from above interannual simulation follows climatological annual cycle with cold biases in summer season. The weak SSTs (bias of the order of 1.0 to 1.5 °C) are observed in the summer for the period 2002-2007 in the model simulations. The monthly mean SST anomalies are well simulated by model except for the year 2006. The time evolution of monthly mean SST anomalies area averaged over different sub-basins are exhibits interannual variability. The comparison with satellite derived SSTs reveals that our model is able to capture both, the seasonal and inter-annual variability, although it still has a bias of the order of 1 to 1.2 °C. The model is able to reproduce the temperature at subsurface layer having the signatures of existence of intermediate water masses. The monthly mean mixed layer climatology derived from interannual simulations is quite well reproduced by model. In the Gulf of Lions, MLD values are reached upto 1500 meter deep in winter whereas it shows 50 meter in summer season. The time evolution of monthly mean mixed layer climatology derived from the model is able to reproduce annual variability. The interannual variability of monthly mean mixed layer depth is simulated quite well by model for the year 2004-2007. The timeseries of climatological, monthly and daily mixed layer depth which is area averaged over various sub-basins follows seasonal cycle. The high resolution regional model application developed in the current study is thus able to reproduce certain fields. The surface currents and eddy kinetic energy in the model shows small scale structures and strong variability. The model is also capable to generate mesoscale eddies in the western Mediterranean although model overestimated surface fields.
La investigación de la variabilidad climática en diferentes escalas de tiempo, como diario, mensual, estacional e interanual tiene suma importancia para la gestión de los procesos socio-económica en la región a escala global. De hecho, la variación en el clima tiene un impacto crucial en la agricultura, el agua, la salud, el turismo, la economía y el transporte. Por lo tanto el desarrollo de herramientas de predicción del clima es necesario que ayuda a gestionar estos sectores de manera más eficiente. Sin embargo, existen limitaciones en la producción de pronóstico climático precisa durante más de dos semanas de antelación debido a la naturaleza caótica del sistema climático, especialmente para la región como el Mediterráneo, que se caracteriza por una alta variabilidad interanual. Debido a su importancia y naturaleza desafiante, se está haciendo un esfuerzo colectivo para mejorar la habilidad de los modelos y la predicción del clima en el Mediterráneo. La contribución de esta tesis es un esfuerzo global, que consiste en el desarrollo de una aplicación de modelo de alta resolución en el Mediterráneo, que puede proporcionar una estimación fiable de la temperatura superficial del mar (TSM) y la profundidad de la capa mixta (MLD). Este enfoque se basa en el hecho de que la previsibilidad atmosférica en estacional a interanual escala de tiempo es significativamente dependiente de variación lenta condiciones límite inferior (por ejemplo, Charney y Shukla 1.981), es decir TSM mediterráneos. La resolución espacial de modelo se incrementa por tomar en cuenta los procesos de mesoescala en el Mediterráneo. Desde la primera radio interno de deformación de Rossby en el mar Mediterráneo es del orden de 10-15 kms, la resolución espacial de un remolino resolución de modelo debe tener al menos una resolución de la mitad del radio de Rossby. Basándose en esta suposición, la resolución espacial se explora a la orden de — 5 km (1/16°). El sistema de modelado regional de los océanos (ROMS), aprobada por la Universidad de Rutgers se utiliza en el estudio actual. El objetivo es validar ciertos campos (como SST y MLD) obtenidos a partir de simulaciones de modelos y estudiar las interacciones aire-mar. La validación se realiza mediante la realización de simulaciones de dos experimentos saber, climatológicos e interanuales. Los resultados del modelo simulado se validan con las observaciones, así como intercomparados para evaluar la habilidad del modelo. La media mensual climatología SST se obtiene a partir de diez años de ejecución del modelo forzada con climatológicas flujos aire-mar es bien capturados por la configuración del modelo y sigue el ciclo anual. Simulado Modelo SST climatología verano muestra sesgos del orden de 0,8 a 1,0 °C con observación (MEDATLAS) y 1,0-1,2 °C con otros conjuntos de datos (de intercomparación). La estructura vertical de la climatología de temperatura se encuentra para ser bien simulado por modelo en el que la capa superior muestra una diferencia de 1,0 °C y disminuyó aún más en las capas intermedias. Las corrientes simulados altura y la superficie de la superficie del mar se valida con los datos de altimetría aviso. En las grandes escalas de las corrientes superficiales generadas por el modelo capta las estructuras generales de la circulación superficial. La climatología media mensual profundidad de la capa mixta (MLD) calculada a partir del modelo se valida con la observada climatología mensual MLD y encontró que el invierno MLD se sobreestima el modelo. En segundo experimento, el modelo se ve obligado a seis por hora aire-mar flujos de interacción a partir de simulaciones ERA-Interim e interanuales se obtienen para el período 1998-2007. La climatología TSM mensual media obtenida desde arriba simulación interanual sigue el ciclo anual climatológica con sesgos fríos en la temporada de verano. Los TSM débiles (sesgo del orden de 1,0 a 1,5 °C) se observan en el verano para el período 2002-2007 en las simulaciones del modelo. Las anomalías medias mensuales de la TSM son bien simuladas por el modelo, excepto para el año 2006. La evolución en el tiempo de media área de anomalías de TSM mensual promedio durante diferentes subcuencas son exposiciones variabilidad interanual. La comparación con TSM satélite derivada revela que nuestro modelo es capaz de capturar tanto, la variabilidad estacional e interanual, a pesar de que todavía tiene un sesgo del orden de 1 a 1,2 °C. El modelo es capaz de reproducir la temperatura a la capa subsuperficial tener las firmas de la existencia de masas de agua intermedias. La media climatología capa de mezcla mensual derivado de simulaciones interanuales está bastante bien reproducido por modelo. En el Golfo de León, los valores de MLD se alcanzan hasta 1.500 metros de profundidad en invierno mientras que muestra 50 metros en temporada de verano. La evolución en el tiempo de la media climatología capa de mezcla mensual derivada del modelo es capaz de reproducir la variabilidad anual. La variabilidad interanual de media profundidad de la capa mixta mensual se simula bastante bien por el modelo para el año 2004-2007. Las series de tiempo de profundidad de la capa mixta climatológica, mensual y diaria, que es el área de media sobre varias subcuencas del siguiente ciclo estacional. La aplicación modelo regional de alta resolución desarrollado en el presente estudio es, pues, capaz de reproducir ciertos campos. Las corrientes superficiales y la energía cinética de Foucault en el modelo muestra las estructuras de pequeña escala y fuerte variabilidad. El modelo también es capaz de generar remolinos de mesoescala en el Mediterráneo occidental, aunque el modelo sobreestima campos superficiales.

The enthalpy (sensible and latent heat) exchange processes within the surface layers at an air-water interface have been examined in 15-m wind-wave tunnel at the University of Miami. Measurements yielded 72 mean values of fluxes and bulk variables in the wind speed (referred to 10 m) range form 0.6 to 39 m/s, covering a full range of aerodynamic conditions from smooth to fully rough. Meteorological variables and bulk enthalpy transfer coefficients, measured at 0.2-m height, were adjusted to neutral stratification and 10-m height following the Monin-Obukhov similarity approach. The ratio of the bulk coefficients of enthalpy and momentum was estimated to evaluate Emanuel's (1995) hypothesis. Indirect "Calorimetric" measurements gave reliable estimates of enthalpy flux from the air-water interface, but the moisture gained in the lower air from evaporation of spray over the rough water remained uncertain, stressing the need for flux measurements along with simultaneous spray data to quantify spray's contribution to the turbulent air-water enthalpy fluxes.

O papel dos fluxos de superfície de calor sensível e latente (FCSL) em dois ciclones extratropicais com desenvolvimento distinto na costa do sul do Brasil foi avaliado a partir de simulações numéricas utilizando o modelo de área limitada WRF versão 2.2. Em um dos ciclones, a circulação se originou em baixos níveis e propagou-se para a média troposfera (ciclone 1). No outro ciclone (ciclone 2) a circulação originou-se em níveis médios e propagou-se até a superfície. Foram realizadas simulações numéricas com e sem FCSL para cada um dos ciclones. A trajetória do ciclone 1 foi fortemente alterada na ausência de FCSL, exibindo deslocamento incorreto (para nordeste) e menor tempo de vida. Este comportamento esteve associado às mudanças no padrão de advecção de temperatura em baixos níveis e à diminuição da convergência de massa induzida pelo calor sensível, na ausência de FCSL. No experimento sem FCSL, ocorre também desacoplamento entre o ciclone em superfície e a onda em níveis médios e altos, com consequente enfraquecimento do sistema. O aumento da estabilidade estática e o mecanismo de convergência de Ekman são responsáveis por menor convergência nas regiões frontais na ausência de FCSL. A relação de fase entre os campos de altura geopotencial e temperatura em baixos níveis e o perfil vertical de aquecimento diabático também mostram condições mais favoráveis ao desenvolvimento do ciclone na presença dos FCSL. O ciclone 2 não teve a trajetória alterada entre as duas simulações. A advecção de temperatura e a convergência em baixos níveis devido ao calor sensível foram semelhantes, explicando a similaridade na trajetória nos experimentos com e sem FCSL. A convergência de Ekman diferenciou-se entre as duas simulações, especialmente no final do ciclo de vida do ciclone, mostrando que este processo também altera a intensidade de ciclones fracos. A influência dos FCSL mostrou-se dependente do mecanismo dominante de formação dos ciclones. O ciclone 1, com forçante dinâmica menos intensa, sofreu grandes variações em trajetória e tempo de vida na ausência de FCSL. Já o ciclone 2, sob forçante dinâmica mais definida e intensa, mostrou-se menos dependente dos processos de superfície para o seu deslocamento. Os mecanismos de aprofundamento foram mais intensos no ciclone 1.
The role of latent and sensible heat fluxes (LSHF) between ocean and atmosphere during the development of two extratropical cyclones over the southwestern Atlantic Ocean is analyzed using the WRF (Weather Range and Forecast) Mesoscale Model, version 2.2. In cyclone 1, the circulation has originated in low levels and propagated to the middle troposphere; the cyclone 2s circulation has originated in middle levels propagating towards the surface during its life cycle. The trajectory of cyclone 1 was strongly influenced by the surface heat fluxes, showing an incorrect displacement and a shorter lifetime in the absence of these fluxes. This behavior is associated with changes in low level temperature advection and the reduction of low level mass convergence is induced by sensible heat fluxes from surface. In the absence of LSHF there is also a decoupling of the surface low and the upper level wave, causing the weakening of the system. Without surface fluxes, the higher static stability and the weaker Ekman convergence mechanism are responsible for less convergence in the frontal regions of the cyclone. The lagging of the geopotential wave and the temperature wave in low levels, and the diabatic heating profile in the troposphere also show more favorable conditions to the cyclone deepening in the presence of surface fluxes. The trajectory of cyclone 2 showed no significant modification in the absence of LSHF. The temperature advection field is similar and the low level convergence related to sensible heat fluxes didn´t has an impact on the displacement of this system. The Ekman convergence had smaller magnitude in the no-LSHF simulation, especially in the final stages of the cyclone life cycle, indicating that this mechanism can be important also for the deepening of weak systems and not only for explosive systems, as considered in previous studies. This work shows that the role of the LSHF seems to be dependent on the cyclone development main mechanisms. In cyclone 1, where the dynamic forcings are less intense, the absence of surface fluxes had a great impact on the trajectory, intensity and duration of the system. In cyclone 2, with more intense dynamic forcings, the displacement was less influenced by surface processes. The deepening mechanisms had greater impact on the cyclone 1.