Dissertationen zum Thema „Wind mixing“

Thesis (M.S. in Meteorology and Physical Oceanography)--Naval Postgraduate School, September 2003.
Thesis advisor(s): Wendell A. Nuss, David S. Brown. Includes bibliographical references (p. 65-66). Also available online.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006.
Includes bibliographical references.
Two years of moored oceanographic and automatic weather station data which span the winter ice seasons of 2001-2003 within Marguerite Bay on the western Antarctic Peninsula (wAP) shelf were collected as part of the Southern Ocean Global Ocean Ecosystems Dynamics program. In order to characterize the ice environment in the region, a novel methodology is developed for determining ice coverage, draft and velocity from moored upward-looking acoustic Doppler current profiler data. A linear momentum balance shows the importance of internal ice stresses in the observed motion of the ice pack. Strong inertial, not tidal, motions were observed in both the sea ice and upper ocean. Estimates of upward diapycnal fluxes of heat and salt from the Upper Circumpolar Deep Water to the surface mixed layer indicate almost no contribution from double diffusive convection. A one-dimensional vertical mixed layer model adapted for investigation of mixing beneath an ice-covered ocean indicates that the initial wind event, rather than subsequent inertial shear, causes the majority of the mixing. This work points towards episodic wind-forced shear at the base of the mixed layer coupled with static instability from brine rejection due to ice production as a major factor in mixing on the wAP shelf.
by Jason Hyatt.
Ph.D.

Dans cette thèse, nous étudions la détection homodyne (ou rétroinjection optique, ou self-mixing) appliquée à la mesure de la vitesse du vent. Il n’existe actuellement pas sur le marché de solution de mesure du vent par des moyens optiques à faible coût. L’objectif de cette thèse est de développer un tel prototype, en se basant sur le principe de la rétroinjection optique. Les différentes solutions d’anémométrie existant actuellement sont étudiées (anémométrie à coupelles, à ultrasons et à fil chaud, sondes de Pitot, PIV/PTV, sodars et lidars), avec un bref descriptif des avantages et inconvénients de chacune d’entre elles. Les équations de fonctionnement d’un laser soumis à rétroinjection optique sont redémontrées dans cette thèse. Les expressions résultantes sont des équations différentielles non linéaires et à retard, qui peuvent être réduites en un modèle statique couramment utilisé dans la littérature. Ce modèle prévoit une variation périodique de la puissance du laser pour un déplacement à vitesse constante de la cible responsable de la rétroinjection. Pour une puissance réinjectée suffisamment grande, ce modèle prévoit des discontinuités dans la puissance du laser. Nous développons un nouveau modèle issu des équations complètes linéarisées, qui permet de décrire le comportement dynamique du laser. Ce modèle prévoit notamment la présence d’oscillations amorties lors des discontinuités du modèle statique. Les caractéristiques de ces oscillations sont liées à la distance et au coefficient de réflexion de la cible. Les prédictions de ce modèle ont été vérifiées expérimentalement, et les résultats en ont été publiés dans la revue Optics Letters. Le corps de cette thèse se concentre sur l’acquisition et le traitement du signal de self-mixing, issu du passage de particules portées par le vent dans le faisceau de la diode laser utilisée. Comme la fréquence du signal est proportionnelle à la vitesse de la particule projetée sur l’axe optique du laser, nous avons mis en place un traitement dans le domaine fréquentiel par transformée de Fourier discrète. La ongueur de la transformée à utiliser est un compromis dicté par la recherche d’un rapport signal-sur-bruit optimal, le temps d’interaction de la particule avec le faisceau, et les ressources disponibles pour les calculs. Après avoir fixé ce compromis, nous calculons la fréquence de fausses détections qui en résulte. Nous étudions le biais qu’introduisent ces fausses détections sur la mesure de vitesse, ainsi qu’un algorithme permettant de compenser ce biais. Nous étudions ensuite les configurations optiques permettant de mesurer la vitesse du vent dans le plan horizontal (donnée qui intéresse les acheteurs potentiels du produit). Nous démontrons qu’il est nécessaire d’utiliser au moins quatre têtes optiques pour obtenir une acquisition fiable du signal. Des essais réalisés en soufflerie indiquent que le capteur mesure effectivement la vitesse du vent qui lui est présenté. Un démonstrateur autonome avec une unique voie de mesure a été monté sur un mât de prospection éolienne. Les mesures en résultant indiquent que le capteur est sensible à la température ambiante. Après correction par rapport à la température, la mesure effectuée est bien corrélée à une mesure de référence par un anémomètre et une girouette. Cette thèse a donc permis de développer un démonstrateur autonome permettant de mesurer la vitesse du vent en utilisant la rétroinjection optique, en conditions extérieures
In this thesis, we study the homodyne detection (or self-mixing) applied to wind speed measurements. At the moment, there is no commercially available optical anemometer with a low price point. The objective of this thesis is to develop such a prototype, which will be using the self-mixing phenomenon. Existing anemometers are studied, with a short comparison of advantages and drawbacks of each solution (cup, ultra-sonic and hot-wire anemometers, Pitot probes, PIV/PTV, sodars and lidars). The equations describing the behavior of a self-mixing laser are demonstrated in this thesis. The resulting expressions are nonlinear delayed differential equations. These equations can be reduced to a static model that is commonly used in the relevant literature. This model predicts a periodic variation of the laser power for a linear displacement of the target responsible for self-mixing. If the reflection coefficient of the target is big enough, this model predicts discontinuities in the laser power. We develop a new model from the complete equations. This new model allows for the study of the dynamical behavior of the laser. It notably predicts damped oscillations where the static model presents discontinuities. The characteristics of these oscillations are related to the distance of the target and its reflectivity. The predictions of this new model were confirmed experimentally, and the corresponding results were published in the Optics Letters journal. The main part of this thesis is focused on the acquisition and processing of the self-mixing signal, which is produced by particles carried by the wind in the laser beam. The frequency of the resulting signal is proportional to the speed of the particle projected onto the optical axis. Therefore, we use a discrete Fourier transform to study the signal in the frequency domain. The length of the Fourier transform is a compromise between the necessity of an optimal signal-to-noise ratio that can trigger the detection, the interaction time between the particle and the beam, and the resources available for computing. After choosing the right compromise, we compute the resulting false detection frequency. We study the bias arising from these false detections, and we create an algorithm that can be used to compensate this bias. Finally, we study the optical configurations that allows for the measurement of wind speed in the horizontal plane (it is this data that is interesting for the potential clients). We demonstrate that at least four optical heads are necessary to obtain a reliable acquisition. The tests conducted in a wind tunnel show that the sensor actually measures the wind speed. An autonomous demonstrator with one measuring channel has been put on a measuring mast. The resulting measurements show that the sensor is temperature sensitive. When the measurements are corrected against the temperature, they are well correlated to a reference measurement made by a cup anemometer and a wind vane. This thesis has led to the development of an autonomous demonstrator that measures the wind speed by self-mixing in a laser diode, in outdoor conditions

The mixing layer (ML) heights inferred from radiosondes, wind profilers, airborne lidar, airborne microwave temperature profiler (MTP), and in-situ aircraft data were compared during the Texas 2000 Air Quality Study in the Houston area. The comparisons and resulting good agreement between the separate instruments allowed for the spatial and temporal evolution of the ML height distribution to be determined across the Houston area on September 1, 2000. A benchmark method was created for determining ML heights from radiosonde data. The ML heights determined using this method were compared to ML heights determined using wind profiler data. The airborne lidar and MTP heights were also compared to the wind profiler heights. This was the first time the MTP was used for estimating ML heights. Because of this, the MTP heights were also compared to the ML heights determined by in-situ aircraft data. There was good agreement between the ML estimates when the instruments were co-located. The comparisons between the benchmark method and the wind profilers were independent of the quality of the profiler heights. The statistics for lidar and the wind profilers were better for the inland profiler comparisons. Even so, the results for coastal profilers were similar to the other comparisons. The results between the MTP and the wind profilers were comparable with the results found between the other instruments, and better, in that the statistics were similar for the both the inland and coastal profilers. The results between the MTP and in-situ aircraft data provided additional support for the use of MTP for determining ML heights. The combination of the inland and coastal wind profilers with the airborne instruments provided adequate information for the spatial and temporal evolution of the ML height to be determined across the Houston area on September 1, 2000. By analyzing the ML height distribution, major features were evident. These features included the shallow ML heights associated with the marine air from Galveston Bay and the Gulf of Mexico, and the sharp gradient of increasing ML heights north of Houston associated with the variation in the inversion depth found on this day.

A new theory for predicting the windprofile over a canopy has been evaluated. The theory was first presented by Harman and Finnigan (2007). The theory relies on the forming of a mixing-layer above the canopy, due to different mean wind in and above the canopy. Characteristics from both mixing-layer and Monin Obukhov similarity theory have been used to develop the governingequations that give the wind profile. The theory has been used to calculate wind profiles for sixdifferent atmospheric stabilities. In order to evaluate the theory, profiles from the theory have beencompared to measurements from Jädraås forest, Sweden. Profiles from Monin Obukhov similarity theory were also used for comparison.In general the mixing-layer theory gives better results than Monin Obukhov similarity theory. Agreement with measurements is good in neutral conditions, but fails when the atmospheric stability is altered, especially in convective conditions. This is believed to be due to the canopy lacking in thickness. The mean wind speed is systematically underestimated and this is also believed to be caused by insufficient thickness of the canopy. A correction for this behaviour is proposed. The theory gives higher values of the mean wind speed in convective conditions with the correction and the calculated values of mean wind speed are closer to the measurements.

Atmospheric models are strongly dependent on the turbulent exchange of momentum, sensible heat and moisture (latent heat) at the surface. Oceans cover about 70% of the Earth’s surface and understanding the processes that control air-sea exchange is of great importance in order to predict weather and climate. In the atmosphere, for instance, hurricane development, cyclone intensity and track depend on these processes. Ocean waves constitute an obvious example of air-sea interaction and can cause the air-flow over sea to depend on surface conditions in uniquely different ways compared to boundary layers over land. When waves are generated by wind they are called wind sea or growing sea, and when they leave their generation area or propagate faster than the generating wind they are called swell. The air-sea exchange is mediated by turbulent eddies occurring on many different scales. Field measurements and high-resolution turbulence resolving numerical simulations have here been used to study these processes. The standard method to measure turbulent fluxes is the eddy covariance method. A spatial separation is often used between instruments when measuring scalar flux; this causes an error which was investigated for the first time over sea. The error is typically smaller over ocean than over land, possibly indicating changes in turbulence structure over sea. Established and extended analysis methods to determine the dominant scales of momentum transfer was used to interpret how reduced drag and sometimes net upward momentum flux can persist in the boundary layer indirectly affected by swell. A changed turbulence structure with increased turbulence length scales and more effective mixing was found for swell. A study, using a coupled wave-atmosphere regional climate model, gave a first indication on what impact wave mixing have on atmosphere and wave parameters. Near surface wind speed and wind gradients was affected especially for shallow boundary layers, which typically increased in height from the introduced wave-mixing. A large impact may be expected in regions of the world with predominant swell. The impact of swell waves on air-sea exchange and mixing should be taken into account to develop more reliable coupled Earth system models.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988.
Includes bibliographical references (leaves 163-172).
A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross shore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion.
by Derek Allen Fong.
Ph.D.

Field measurements and numerical modelling of the shallow coastal waters offshore in south-western Australia were used to describe changes in the water column's vertical structure and the biological response on temporal scales of the order of hours and days. A cycle of chlorophyll a concentration, primary production, and photosystem II function on a diel timescale, which was related to changes in the solar irradiance and thermal structure, was identified. The diel cycle included (1) vertically well-mixed (or weakly linear) conditions in density and chlorophyll a early in the morning, resulting from vertical mixing through penetrative overnight convection; (2) depleted chlorophyll a concentration in the surface layer during the middle of the day due to photoinhibition; (3) an increased chlorophyll a concentration in the bottom layer by late afternoon due to optimum light conditions; and (4) the formation of a chlorophyll a break point (CBP) at the thermocline, which migrated downwards with the deepening surface mixed layer. On a longer timescale (days), moored acoustic instruments were used to derive echo level (EL), which approximated suspended particulate matter (SPM). Wind events ultimately controlled SPM, a conclusion based on (1) elevated EL during high windgenerated turbulence and bed shear stress, (2) positive time-lagged correlations between wind speed and EL at three field sites with different exposures to wave action, and (3) significant negative correlations between wind speed and depth-differentiated echo level (d(EL)/dz) at all sites. Sea breezes produced a similar response in EL through the water column to a small storm event, and wind-driven SPM resuspension resulted in a reduction in the sub-surface light climate (kd). Near-bed dissolved oxygen concentrations varied in accord with elevated wind speeds, EL and kd, highlighting a possible suppression of photosynthesis. One-dimensional modelling revealed that wind stirring was most often the dominant process in these waters. It was found that for a brief period during thermal stratification there was shear production of turbulent instabilities that migrated from the thermocline to the surface and the seabed. Convective cooling was not able to mix the water column entirely overnight without the addition of wind, and minimum wind speeds were determined for this complete vertical mixing. Bottom-generated turbulence was limited to a small region above the bed, and was deemed insignificant compared with mixing generated at the surface. Minimum wind speeds required for de-stratification and prevention of stratification were determined for summer, autumn and winter. A hypothetical desalination outfall was simulated for all seasons and it was concluded that positioning of the discharge at middepth was preferable compared to at the seabed. The results of this thesis advance the current knowledge of coastal biophysical oceanography and provide new insights into the temporal dynamics of the coastal water column of south-western Australia.

This thesis describes the development of an open-loop control allocation function also known as a mixing function for aircraft with an unconventional control surface setup (i.e. not consisting of a conventional elevator, rudder and ailerons) by using mathematical optimisation. The techniques used to design the control allocation and mixing used on the unconventional configuration when flying it without artificial stability or control augmentation is provided. A typical application of this control mixing would be to enable a pilot to operate an unconventional unmanned aerial vehicle (UAV) as if it was a conventional model aircraft during flight testing or as a backup mode should any sensor failures occur during a typical flight test program. The allocation can also be used to simplify the inner control structure of a UAV autopilot or stability augmentation system. Although this type of mixing would be straightforward on a conventional airframe, an unconventional configuration has several unique characteristics that complicate the modelling and design process. A custom six degree of freedom (6DOF) formulation for flight simulation was made available to model the aircraft and run various scripts to evaluate the aircraft response when the control allocation function is implemented. The simulation model was used to develop the mixing function that maps conventional input commands to the unconventionally situated control surfaces in the most optimal way. The design process was formulated as a multi-objective optimisation problem, which was solved using a custom sequential quadratic programming and custom leapfrog programming method. A methodology was proposed to define the constraints, which can be customised for a particular aircraft or application. The control allocation function was implemented in two different simulation environments to investigate the suitability of candidate designs. A robustness study was performed to evaluate the impact of actuator failures on the aircraft control response using the designed control allocation system. The proposed control allocation design methodology can also be used to design the inner control loops of more sophisticated control systems such as stability augmentation and automatic flight control, which is also briefly discussed in this thesis.
Dissertation (MEng)--University of Pretoria, 2015.
tm2016
Mechanical and Aeronautical Engineering
MEng
Unrestricted

Turbulent mixing plays an important role in the distribution of sunlight, carbon dioxide, and nutrients for algae in the raceway ponds. For large-scale raceway ponds the choice of mixing technology still needs to be evaluated in order to prevent algae sedimentation and to enhance light utilization efficiency. In open ponds, mixing the algae culture is of great significance in terms of input energy costs and particularly productivity. A very small amount of research has been performed previously using different vortex generators in the algal raceway ponds, but the quantification of mixing depth relationships is not defined well. By accepting the premise from the literature review that mixing increases algal production, delta wings were selected to study mixing characteristics in the raceway. The main objective of this research was to study algae-raceway hydrodynamics with an emphasis on increasing vertical mixing. A clear acrylic raceway was designed and constructed for flow visualization studies. Experimental investigations were performed to quantify the vertical mixing with and without delta wings in a lab-scale raceway at approximately the same power input to the paddle wheel. Velocity vector profiles and turbulence parameters were measured using an Acoustic Doppler Velocimeter (ADV) at various locations along the entire length of the raceway. The results indicated that the addition of delta wings increases the vertical mixing intensity or circulation of algae cells over the raceway depth. Vortices were observed in the raceway up to a distance of around 3 m downstream of the delta wing. This sort of systematic vertical mixing plays an important role to produce the flashing light effect (light-dark cycles) on algae mass culture. In addition, turbulence dissipation rates were evaluated to compare them with the published literature and to estimate the microscales using the Kolmogorov hypothesis. Also, an energy model was developed to operate the paddlewheel-driven raceway with the delta wing.

Open channel raceway bioreactors are a low-cost system used to grow algae for biofuel production. Microalgae have many promises when it comes to renewable energy applications, but many economic hurdles must be overcome to achieve an economic fuel source that is competitive with petroleum-based fuels. One way to make algae more competitive is to improve vertical mixing in algae raceway bioreactors. Previous studies show that mixing may be increased by the addition of mechanisms such as airfoils. The circulation created helps move the algae from the bottom to top surface for necessary photosynthetic exchange. This improvement in light utilization allowed a certain study to achieve 2.2-2.4 times the amount of biomass relative to bioreactors without airfoils. This idea of increasing mixing in open channel raceways has been the focus of the Utah State University (USU) raceway hydraulics group. Computational Fluid Dynamics (CFD), Acoustic Doppler Velocimetry (ADV), and Particle Image Velocimetry (PIV) are all methods used at USU to computationally and experimentally quantify mixing in an open channel raceway. They have also been used to observe the effects of using delta wings (DW) in increasing vertical mixing in the raceway. These efforts showed great potential in the DW in increasing vertical mixing in the open channel bioreactor. However, this research begged the question, does the DW help increase algae growth? Three algae growth experiments comparing growth in a raceway with and without DW were completed. These experiments were successful, yielding an average 27.1% increase in the biomass. The DW appears to be a promising method of increasing algae biomass production. The next important step was to quantify vertical mixing and understand flow patterns due to two DWs side-by-side. Raceway channels are wider as they increase in size; and arrays of DWs will need to be installed to achieve quality mixing throughout the bioreactor. Quality mixing was attained for several paddle wheel (PW) speeds. Also, an optimal spacing between the DWs in an array was found to be the width of the DW. This optimal spacing allows for the best increase in vertical mixing along the width of the channel. Dimensional analysis was performed using experimental data to estimate vertical mixing index (VMI) results for data obtained by larger scale DW experiments. This rough analysis showed that the VMI may be estimated from small to large scale within 26.6% and 26.5% when equating Reynolds and Froude numbers, respectively. These results suggest that quality mixing would still be present at a larger DW scale.

The South China Sea is a marginal basin with a complex circulation influenced by the East Asian Monsoon, river discharge and intricate bathymetry. As a result, both the mesoscale eddy field and the near-inertial energy distribution display large spatial variability and they strongly influence the oceanic transport and mixing. With an ensemble of numerical integrations using a regional ocean model, this work investigates how the temporal resolution of the atmospheric forcing fields modifies the horizontal and vertical velocity patterns and impacts the transport properties in the basin. The response of the mesoscale circulation in the South China Sea is investigated under three different forcing conditions: monthly, daily and six-hourly momentum and heat fluxes. While the horizontal circulation does not display significant differences, the representation of the vertical velocity field displays high sensitivity to the frequency of the wind forcing. If the wind field contains energy at the inertial frequency or higher (daily and six-hourly cases), then Vortex Rossby waves and near inertial waves are excited as ageostrophic expression of the vigorous eddy field. Those waves dominate the vertical velocity field in the mixed layer (vortex Rossby waves) and below the first hundred meters (near inertial waves) and they are responsible for the differences in the vertical transport properties under the various forcing fields as quantified by frequency spectra, vertical velocity profiles and vertical dispersion of Lagrangian tracers.

Algae-derived biodiesel is currently under investigation as a suitable alternative to traditional fossil-fuels. Though it possesses many favorable characteristics, algae remains prohibitively expensive to mass produce and distribute. The most economical means of growing algae are large-scale open pond raceways. These, however, suffer from low culture densities; this fact impacts the cost directly through diminished productivity, as well as indirectly by raising costs due to the necessity of dewatering low culture density raceway effluent. Algae, as a photosynthetic organism, achieves higher culture densities when sufficient light is provided. In open ponds this can be accomplished by frequently cycling algae to the raceway surface. The current work examined delta wing vortex generators as a means of instigating this cycling motion. In particular the impact of spacing and angle of attack was analyzed. These vortex generators were found to significantly increase vertical mixing when placed in a series, developing precisely the motion desired. Their impact on power requirements was also examined. Specifically it was shown that increases in spacing and decreases in angle of attack result in lower power consumption. It was demonstrated that the most efficient mixing generation is achieved by larger spacings and smaller angles of attack. The impact that these devices had on raceway turbulence as measured by dissipation rate was also investigated and compared to published values for algae growth. Raceways were found to be significantly more turbulent than standard algae environments, and adding delta wings increased these levels further.

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2019
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 113-121).
Submesoscale ocean dynamics and instabilities, with characteristic scales 0.1-10 kin, can play a critical role in setting the ocean's surface boundary layer thickness and associated density stratification. Submesoscale instabilities contribute to lateral stirring and tracer dispersal. These dynamics are investigated in the Bay of Bengal, motivated by the upper ocean's potentially coupled interactions with Monsoon winds and convection. The region's excess precipitation and runoff generates strong salinity gradients that typically set density fronts and stratification in the upper 50 m. Since we cannot synoptically measure currents containing fast-evolving and oscillating components across the submesoscale range, we instead analyze passive tracer distributions (spice = density-compensated temperature (T) and salinity (S) anomalies), identifying signatures of flows and testing dynamical theories.
The analysis is based on over 9000 vertical profiles of T and S measured along ~4800 km of ship tracks in the Bay of Bengal during ASIRI and MISO-BOB expeditions in 2013, 2015, and 2018. Observations in the surface mixed layer reveal ~1 km scale-selective correlation of surface T and S, with compensation reducing cross-front density gradients by ~50%. Using a process study ocean model, we show this is caused by submesoscale instabilities slumping fronts, plus surface cooling over the resultant enhanced salinity stratification, potentially thwarting the forward cascade of energy. In the stratified interior, we present a spectral analysis of horizontal spice variance statistics from wavenumber k ~0.01 cpkm to ~1 cpkm. At scales <10 km, stratified layers that are closer to the surface exhibit redder passive tracer spectra (power spectra k⁻³, gradient spectra k⁻¹) than predicted by quasi-geostrophic or frontogenetic theories.
Complimentary observations reveal spice patterns with multiple, parallel, ~10 m thin layers, crossing isopycnals with O(10⁻⁴) slopes, coherent over at least 30-80 kin, with coincident layers of stratification anomalies. Comparison with shear measurements, and a numerical process study, suggest that both submesoscale sheared eddies, and thin near-inertial waves, form such layers. Fast formation timescales and large aspect ratios suggest they enhance horizontal mixing by shear dispersion, reducing variance at ~1-10 km scales.
by Gualtiero Victor Rudi Spiro Jaeger.
Ph. D.
Ph.D. Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)

The spatial structure and temporal evolution of sea breeze and the latitudinal distribution of propagation and mixing of sea breeze driven near-inertial ocean response in the Gulf of Mexico are investigated using comprehensive data sets and a non-linear numerical model. Near 30�N, inertial oceanic response is significantly enhanced by a near-resonant condition between inertial and diurnal forcing frequencies. Observational results indicate that sea breeze variability peaks in summer and extends at least 300 km offshore with continuous seaward phase propagation. The maximum near-inertial oceanic response occurs in June when there is a shallow mixed layer, strong stratification, and an approximately 10-day period of continuous sea breeze forcing. Near-inertial current variance decreases in July and August due to the deepening of the mixed layer and a more variable phase relationship between the wind and current. River discharge varies interannually and can significantly alter the oceanic response during summer. During 1993, the ?great flood? of the Mississippi River deepens the summer mixed layer and reduces the sea breeze response. The near-inertial currents can provide considerable vertical mixing on the shelf in summer, as seen by the suppression of bulk Richardson number during strong near-inertial events. Three-dimensional idealized simulations show that the coastal oceanic response to sea breeze is trapped poleward of 30� latitude, however, it can propagate offshore as Poincare waves equatorward of 30� latitude. Near 30� latitude, the maximum oceanic response to sea breeze moves offshore slowly because of the near-zero group speed of Poincare waves at this latitude. The lateral energy flux convergence plus the energy input from the wind is maximum near the critical latitude, leading to increased vertical mixing. This local dissipation is greatly reduced at other latitudes. Simulations with realistic bathymetry of the Gulf of Mexico confirm that a basin-wide ocean response to coastal sea breeze forcing is established in the form of Poincare waves. This enhanced vertical mixing is consistent with observations on the Texas-Louisiana Shelf. Comparison of the three-dimensional and one-dimensional models shows some significant limitations of one-dimensional simplified models for sea breeze simulations near the critical latitude.

Previous numerical and theoretical results based on constant diapycnal diffusivity suggested the abyssal meridional overturning circulation (MOC) should weaken as winds over the Southern Ocean intensify. We corroborate this result in a simple ocean model, but find it does not hold in more complex models. First, models with a variable eddy transfer coefficient and simple yet dynamic atmosphere and sea-ice models show an increase, albeit slightly, of the abyssal MOC under increasing winds. Second, the abyssal MOC significantly strengthens with winds when diapycnal diffusivity is parameterized to be energetically supported by the winds. This tests the emerging idea that a significant fraction of the wind energy input to the large-scale ocean circulation is removed by mesoscale eddies and may then be transferred to internal lee waves, and thence to bottom-enhanced diapycnal mixing. A scaling theory of the abyssal MOC is extended to incorporate this energy pathway, corroborating our numerical results.
Graduate
0415
gstanley@uvic.ca