Дисертації з теми "Monsoonal tropics"

Squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern Arizona and northwestern Mexico. Initial thunderstorm formation is over the mountains along the Continental Divide in the late afternoon. Satellite imagery, cloud-to-ground lightning strike data, and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than all the winds below 20 kPa so that most of the anvil clouds lag behind. The synoptic-scale circulation is anomalous with a strong ridge located over the western United States and a deep trough located over the eastern United States. West to northwest winds are found in the boundary layer over southern Arizona and northwest Mexico while a deep layer of east winds are observed above. As a result, most of the environmental wind shear is confined to the lowest 2.5 km above the ground. The low-level wind shear seems to be required for the westward propagation of thunderstorms and the formation of the squall lines. Extremely dry midtropospheric air develops in the easterly flow through some combination of advection and subsidence and also appears to be an important factor in the development of the squall lines. A two-dimensional, nonhydrostatic, numerical model was able to simulate many of the features observed in these squall lines. Solar heating of the elevated terrain in the model caused the initial thunderstorm to develop over the Continental Divide. Continued development of new thunderstorms to the west of the Divide produced a squall line that travelled westward by translation of cells and discrete propagation, wherein new cells would develop 10-25 km ahead of the old ones, at a speed greater than all the winds below 30 kPa. Upward motion produced by westward propagating gravity waves and by the strong low-level convergence found just ahead of the gust front appeared to cause several episodes of discrete propagation. The creation of horizontal potential temperature gradients and the vertical and horizontal advection of preexisting vorticity gradients combined to produce the vorticity field associated with the rear inflow jet that developed beneath the simulated squall line.

Myanmar remained largely closed to the world through political instability for several years, when it continued to suffer terribly at the hands of nature that remained largely unknown. Of note is the period between 2008 and 2013, during which the country suffered at least eight major natural calamities that killed more than 141,000 people and affected 3.2 million. The worst of these was Cyclone Nargis in May 2008 that killed more than 130,000. With an estimated $4 billion in damages, Nargis remains the deadliest and most destructive named cyclone ever to have occurred in the North Indian Ocean. Recent studies have shown that, due to increased greenhouse gases and aerosol loading in the atmosphere, more and stronger tropical cyclones (TCs) in the last three decades are tracking eastwards toward the Indochina peninsula. Unfortunately, the Burmese lack the capacity to deal with the impacts of such storms. Myanmar was left behind as the world made significant technological and industrial advancement; but agriculture, which employs at least 65% of the active labor force, has remained the backbone of the Myanmar economy – an industry that is heavily reliant on monsoon rainfall. The pre-monsoon TC season in the Bay of Bengal (BoB) precedes the onset of the Myanmar monsoon but sometimes the two (i.e.TC formation and the monsoon onset) occur in unison. This work studied the mechanism by which the Madden Julian Oscillation (MJO) modulates the Myanmar monsoon onset and TC activity collectively (i.e. ISO-Onset-TC connection). Avoiding TC destruction at the beginning of the planting season is crucial, so is the monsoon onset date critical for planning. Additional understanding of the aforementioned ISO-Onset-TC connection could provide further insight into predicting the Myanmar monsoon onset and aid in disaster planning for TC impact. This research is part of a two-year NASA funded project to study extreme climate and weather events.

Approved for public release; distribution is unlimited.
This study examines diurnal variation of convection over western India, the Bay of Bengal, Indochina and the northern South China Sea during the 1991 northern summer monsoon using combined Japanese (GMS) and Indian (INSAT) geostationary satellite data, ECMWF 850 hPa wind data, and NCEP sea surface temperature analyses. The diurnal cycle is examined in terms of spatial and temporal structure prior to onset and during the monsoon. The northern South China Sea is examined to determine how different periods of synoptic influences resulted in an anomalously strong diurnal signal during June. The wind and SST data are used to examine the relationship between the diurnal variation of convection and both low level convergence and vertical latent heat fluxes. Convection over west India is most common during May and June and starts as a diurnal system over land that becomes organized and propagates westward over the east Arabian Sea. The Bay of Bengal follows the classic land-sea breeze model and convection is modulated by convergence between the land breeze and large- scale monsoon flow. The diurnal cycle is generally enhanced over the ocean during active phases of convective activity. The maximum latent heat fluxes generally occurs prior to maximum convection due to strong monsoon flow enhancing evaporation

Rainfall variability in NW Madagascar and the kinematics and thermodynamics of flood-producing weather systems affecting the region are examined. Daily rainfall in the austral summer are used to select 16 cases for the composite analysis of flood events in the period 1987-1992. In addition two cases of flood events are studied individually. The first flood event occurred in 1991 and brought420 mm of rainfall in 24 hours at Maintirano during the passage of tropical cyclone (TC) Cynthia, an eastward moving TC from the Mozambique Channel. The second flood event occurred in 1992and brought 347 mm of rainfall in 24 hours at Mahajanga due to the passage of TC Bryna, a westward moving TC from the Indian Ocean. Inter-annual rainfall variability is correlated with the indices: TC day frequency, QBO and SOI, and a useful value is found with the first index but not with the two latter global indices. An intraseasonal analysis indicates that wet spells have a duration of 15 to 20 days. Daily spectra analysisof rainfall reveals the complexity of interplay between the convective forcing features. Cycles of 10-20 days are contributed by easterly waves, while 40 day cycles are produced by monsoon surges. The composite analysis identifies the following features: an eastward shift of active convection from Mozambique across Madagascar in conjunction with a surge of the NW monsoon and weakening of the easterlies in the lower troposphere. These factors promote TC genesis. Other supporting influences include the development of an anticyclonic circulation in the upper level over the island with north westward outflow. This often coincides with an east phase QBO, an upper level eastward moving mid-latitude trough and a persistent moisture flux convergence over the island throughout TC passage. The topography of Madagascar plays an important role in the intensification of the flood producing weather system by enhancing the cloud vortex in the NW region of the island.

Large-scale climate modes such as the El Niño/Southern Oscillation (ENSO), the Asian monsoon, and the Arctic Oscillation are responsible for much of the Earth’s climate variability. Despite the importance of these modes, we have limited understanding of how they vary on long (multidecadal to millennial) timescales due to the short length of instrumental climate records. Fortunately, climate information stored in natural archives can provide us with information on how these modes varied in the more distant past. Lake sediments are an ideal climate archive since they are continuous, have high temporal resolution, and contain many potential climate proxies. In the present study, I use lake sediment records to assess past climate and environmental changes associated with the El Niño/Southern Oscillation, the Asian monsoon, and the Arctic Oscillation. Exploring modern precipitation variability across the Asian monsoon region, I found that precipitation within this broad area is not coherent, which holds implications for paleorecords that are hypothesized to represent monsoon variability, including many lake sediment records on the Tibetan Plateau. Monsoon precipitation in the Arabian Sea is distinct from precipitation in India and China, and increased precipitation in the Arabian Sea coincides with decreased precipitation in the western North Pacific. Furthermore, only precipitation in southwestern Tibet responds to the Southwest monsoon, whereas precipitation in southeastern Tibet responds to the western North Pacific monsoon. In southwestern Tibet, I have reconstructed dust variability over the last millennium using the lake sediment record from Kiang Co. The sediment record shows a trend toward increasing dust over the 20th century, and our hypothesized dust proxy is positively correlated with the June-November Arctic Oscillation Index. A trend toward more positive Arctic Oscillation Index values as well as higher temperatures over the 20th century likely drove increased dustiness in southwestern Tibet, due the influence of temperature on glaciofluvial sediment availability in the Himalayas. Sediment trap, sediment core data, and modern measurements of local climate and lake water variables at Genovesa Crater Lake, Galápagos, indicate the lake and its sediments respond to local climate variability, with carbonate-rich sediments forming during prolonged dry periods (La Niña events), and organic-rich sediment forming during the warm season and El Niño events. The ratios of silica to calcium and strontium to calcium also reflect cool season SST. Thus, this lake sediment record has potential to provide a record of both seasonal and ENSO variability spanning the Holocene.

Thesis (M.S. in Meteorology) Naval Postgraduate School, September 1998.
"September 1998." Thesis advisor(s): Chih-Pei Chang. Includes bibliographical references (p. 105-106). Also Available online.

The interaction between atmosphere–land–ocean–biosphere systems plays a prominent role on the atmospheric dynamics and on the convective rainfall distribution over the West Africa monsoon area during the boreal summer. In particular, the initialization of convective systems in the Sub – Sahelian region has been directly linked to soil moisture heterogeneities identified as the major triggering, development and propagation of convective systems. The present study aims at investigating African monsoon large scale convective dynamics and rainfall diurnal cycle through an exploration of the hypothesis behind the mechanisms of a monsoon phenomenon as an emergence of a collective dynamics of many propagating convective systems. Such hypothesis is based on the existence of an internal self – regulation mechanism among the various components. To achieve these results a multiple analysis was performed based on remote sensed rainfall dataset, and global and regional modelling data for a period of 5 seasons: 2004 - 2008. Satellite rainfall data and convective occurrence variability were studied for assessing typical spatio – temporal signatures and characteristics with an emphasis to the diurnal cycle footprint. A global model and regional model simulation datasets, specifically developed for this analysis and based on Regional Atmospheric Modelling System – RAMS, have been analysed. Results from numerical model datasets highlight the evidence of a synchronization between the destabilization of the convective boundary layer and rainfall occurrence due to the solar radiation forcing through the latent heat release. This supports the conclusion that the studied interacting systems are associated with a process of mutual adjustment of rhythms. Furthermore, this rainfall internal coherence was studied in relation to the West African Heat Low pressure system, which has a prominent role in the large scale summer variability over the Mediterranean area since it is acting as one of dynamic link between sub tropical and midlatitudes variability.

This thesis examines the behaviour of Indian monsoon depressions (MDs), synopticscale systems that frequent the monsoon trough throughout the boreal summer. The original contribution to knowledge is a substantial advancement of our understanding on the structure, variability, and dynamics of MDs. A feature-tracking algorithm is developed to emulate the subjective methods used by the India Meteorological Department, based on surface wind speed and surface pressure. This is applied to reanalysis data spanning from 1979 to the present day (2016), extracting 106 events. These are used to build a statistical composite, with which we interrogate the structure and variability subject to external forcings. It is found, for example, that MDs significantly intensify during La Ni˜na and active monsoon spells, and have different spatial structure over ocean than land. These events are also examined in satellite data, where we discover and characterise a bimodal, diurnal cycle in surface precipitation; the structure of cloud type with dominating deep convection south of the centre; and present the first composite picture of vertical hydrometeor structure in MDs. A propagation mechanism whereby the MD behaves as a vortex in the presence of a wall (i.e. the Himalayan massif) is presented and shown to perform better than competing theories at predicting the velocity and heading of MDs. It is also shown, using a case study in the Met Office Unified Model (MetUM), that varying antecedent soil moisture conditions can significantly change the length of a MD track but not its direction. The sensitivity of MDs to changes in horizontal resolution in the MetUM are also explored across seven case studies at eight resolutions. It is found that the intensity is typically slightly overpredicted, correlating with too great a latent heat release in the mid-troposphere. Spatial structure is shown to improve with resolution but improvement saturates beyond N512 (40 km) resolution.

Thesis (M.S. in Meteorology and M.S. in Physical Oceanography) Naval Postgraduate School, December 1996.
Thesis advisor(s): C.P. Chang, Pete Chu. "December 1996." Includes bibliographical references (p. 127-129). Also available online.

Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第14669号
農博第1751号
新制||農||968(附属図書館)
学位論文||H21||N4442(農学部図書室)
UT51-2009-D381
京都大学大学院農学研究科森林科学専攻
(主査)教授 太田 誠一, 教授 谷 誠, 教授 舟川 晋也
学位規則第4条第1項該当

The eastern North Pacific Ocean has the highest density of tropical cyclone genesis events of any tropical basin in the world, and many of these systems form near land before moving westward. However, despite the level of tropical cyclone activity in this basin, and the proximity of the main genesis region to land, tropical cyclone behavior in the eastern North Pacific has been relatively unexplored. When synoptic conditions are favorable, moisture from northward-moving tropical cyclones can be advected into northern Mexico and the southwestern United States, often leading to the development of summertime thunderstorms during the North American monsoon season. An interaction with a mid-latitude trough produces the most rainfall, and the spatial variability of precipitation is greatly affected by the complex topography of the region. Moisture can be advected from a tropical cyclone around the subtropical ridge in place for much of the eastern North Pacific hurricane season and contribute to precipitation. This ridge, when it extends westward over the Pacific Ocean, can also prevent tropical cyclone moisture from impacting the southwestern United States. Northward-moving tropical cyclones often enter an environment with decreasing sea surface temperatures, increasing vertical wind shear, and meridional air temperature and moisture gradients. These key ingredients for extratropical transition are generally present in the eastern North Pacific, but the subtropical ridge prevents many named systems from moving northward, and only 9% of eastern North Pacific tropical cyclones from 1970 to 2011 complete ET according to cyclone phase space. However, over half of the systems that do not complete ET dissipate as cold core cyclones, a structural change that has yet to be explored in other tropical basins. It is difficult to estimate tropical cyclone intensity in a vast ocean area with few direct measurements available. The deviation angle variance technique, an objective method independent of the current techniques widely used today, was successfully applied to seven years of eastern North Pacific tropical cyclones. The RMS error of 13.5 kt for all seven years is comparable to the RMS errors found for other basins.

Då de flesta människor i Västafrika, framför allt de som bor innanför kustzonen, livnär sig på jordbruk så är pålitliga väderprognoser och säsongsförutsägelser ett viktigt hjälpmedel i det dagliga arbetet och planeringen. I den här rapporten har en litteraturstudie gjorts för att öka kunskapen om de komplexa, både lokala och storskaliga, väderfenomen som ger upphov till nederbörd i området.   Klimatet i Västafrika, ett område som mestadels täcks av regnskog eller savann, präglas av den västafrikanska monsunen som ger regnperiod under norra halvklotets sommar och torrperiod på vintern. Denna monsun visar på stor årlig variation när det gäller dess inledande faser, och prognoser som kan förutsäga dess början är nödvändigt när en jordbrukare ska planera säsongens verksamhet. Av många bidragande faktorer framgår det tydligt att ytvattentemperaturen i Guineabukten är en av de viktigaste parametrarna för monsunens startskede. Den intertropiska konvergenszonen, ITCZ, betraktas som monsunens nordligaste del och denna konvergenszon gör en plötsligt och relativt snabb förflyttning norrut, i fortsättningen benämnd som språnget, över ca 5 breddgrader, vilket av många ses som starten på regnperioden. En tillfällig tryckgradient som uppstår på grund av en tillfällig men skarp temperaturgradient är den bakomliggande orsaken till detta språng. Den i särklass viktigaste processen som ger upphov till regn i Västafrika är konvektion och även om mycket konvektiv nederbörd faller i samband med monsunen så uppstår det även många lokala och mesoskaliga konvektiva system inom monsunen, framförallt kopplat till ostliga vågor.
Since most people in West Africa, particularly those who live away from the shore, work within agriculture, reliable weather forecasts are important in the daily work. In this report, a literature study has been conducted to increase the knowledge about the complex and local weather phenomenon that causes rain.   West Africa is dominated by rainforest and savannah, and the climate is characterized by the West African monsoon which gives rise to a wet season in the northern hemisphere summer and a dry season during the winter. The monsoon shows wide annual variability in its initial stages and forecasts that can predict its beginning is necessary to the farmers who have to decide when and what to plant. Out of many contributing factors, the sea surface temperature seems to be one of the main parameters that have an impact on the starting stage of the monsoon. The inter tropical convergence zone, ITCZ, is regarded as the northernmost part of the monsoon and it makes a sudden and relatively quick jump over about 5 degrees latitude, which is widely regarded as the start of the rainy season. A temporary pressure gradient caused by differences in temperature adjacent to the ITCZ is the reason for this jump. The dominant process that causes rainfall in West Africa is convection. Although a lot of convective precipitation falls in connection with the northern part of the monsoon, convective systems can appear inside the monsoon flow, often linked to African easterly waves.

The Kukkal basin, Tamil Nadu, India, receives most of its rain from the southwest monsoon (SWM). A sediment core from Kukkal Lake preserves a continuous sediment record from the early-Holocene to present (9000 yr BP to present). The present lake is situated at an elevation of similar to 1887m a.s.l., in a small basin that appears to have alternated between a and wetland depositional environment. Climate proxies, including sediment texture, total organic carbon (TOC), total nitrogen (TN), C/N, pollen and geochemical composition indicate a steady progression to wetter conditions, with two stepwise changes at about 8000, and between 3200 and 1800 yr BP. The change at 8000 yr BP appears to correspond to a brief (100-150years) dry spell recorded elsewhere in India. The change at 3200-1800 yr BP consisted in a rapid intensification of the SWM, and may correlate with the initiation of the Roman Warm Period'. There is no clear evidence of changes at the times of the Medieval Warm Period' (MWP') and the Little Ice Age' (LIA'). The C/N ratio of the sediments ranges from 14.02 to 8.31, indicating that the organic matter originated from a mixture of lacustrine algae, vascular and terrestrial plants. Chemical weathering indices (Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA)) are consistent with extreme silicate weathering. Pollen data show a development from savanna vegetation prior to about 8000 yr BP, followed by grassland with palms, the appearance of ferns just prior to 3200 yr BP and the establishment of the tropical humid forest between 3200 and about 1800 yr BP.

La majeure partie des côtes du Vietnam s'érode actuellement, un déséquilibre dans le bilan sédimentaire qui peut être causé par la surexploitation des sédiments fluviaux pour la construction, l'affaissement associé à la surexploitation des eaux souterraines, la surexploitation de la zone côtière, et peut-être aussi par une augmentation observée des typhons et de la mousson d'hiver. L'évaluation du bilan sédimentaire est nécessaire et nécessite une évaluation précise du transport des sédiments dans l'environnement côtier. Dans mon étude de doctorat, j'utilise deux systèmes de caméras à courte portée pour la surveillance des eaux littorales : une caméra terrestre pour l'observation de l'hydro-morphodynamique côtière à court, moyen et long terme et un drone pour la surveillance des événements à court terme. J'ai utilisé un système de caméra installé sur la plage de Nha Trang, au Viet Nam, du 05/2013 au 08/2016. Certains résultats sur les changements saisonniers et à court terme du littoral ont été analysés et publiés. Les profils transversaux, les positions du rivage et les caractéristiques des vagues (hauteur et période) extraits des données vidéo ont été étalonnés à l'aide de mesures in situ provenant de deux expériences sur le terrain et de mesures bathymétriques effectuées pendant le typhon de Haiyan. L'étude montre une évolution saisonnière marquée du littoral de Nha Trang. L'impact des typhons de catégorie 5 Nari et Haiyan sur le littoral est également dramatique avec des changements de 4 à 8 m dans chaque cas. Cependant, la reprise aux événements individuels est rapide. Nous avons également comparé l'effet des moussons d'hiver avec celui des tempêtes. Nos observations vidéo continues montrent pour la première fois que les épisodes de mousson de longue durée ont un impact plus persistant (phase de récupération de la plage plus longue) que les typhons. À l'aide d'un modèle d'équilibre riverain, nous estimons que c'est l'enveloppe des événements intrasaisonniers plutôt que la moyenne mensuelle des vagues qui détermine le comportement saisonnier du rivage. Enfin, l'étude suggère que l'interaction entre l'intensité et la durée des événements intrasaisonniers peut être d'une importance capitale. [...]
Most of the coast of Vietnam is currently eroding, an imbalance in the sediment budget that may be caused by overuse of river sediments for construction, subsidence associated with overuse of groundwater, over-exploitation of the littoral zone (coastal squeeze), and possibly by an increase in typhoon frequency and winter monsoon events. Assessment of sediment budget is needed and requires accurate evaluation of sediment transport in the coastal environment. In my PhD study, I use two close-range camera systems for nearshore monitoring: a land-based nearshore camera system for observing short-, medium- and long-term coastal hydro-morphodynamics and a drone for monitoring short-term events. Specifically, I used a camera system installed in Nha Trang beach, Viet Nam, from 05/2013 to 08/2016. The cross-shore profiles, shoreline positions and wave characteristics (height and period) extracted from the video data are calibrated with in-situ measurement from two field experiments during the Haiyan typhoon event. Then, the results on short-term and seasonal shoreline changes are analyzed -- and published. The study shows a marked seasonal evolution of Nha Trang shoreline and dramatic impact of cat-5 typhoons Nari and Haiyan with changes of 4 to 8 m in each case. However, the recovery to individual events is fast, as opposed to the effect of winter monsoon events. Our continuous video observations show for the first time that long-lasting monsoon events have more persistent impact (longer beach recovery phase) than typhoons. Using a shoreline equilibrium model, we estimate that the envelope of intra-seasonal events rather than monthly-averaged waves drives the seasonal shoreline behavior. Finally, the shoreline study suggests that the interplay between intensity and duration of intra-seasonal events may be of key significance. In the second part of this PhD study, a video-based bathymetry inversion technique is applied to long-term data with varying wave environment from swell to wind wave conditions.[...]

This thesis presents investigations on sediments from two African lakes which have been recording changes in their surrounding environmental and climate conditions since more than 200,000 years. Focus of this work is the time of the last Glacial and the Holocene (the last ~100,000 years before present [in the following 100 kyr BP]). One important precondition for this kind of research is a good understanding of the present ecosystems in and around the lakes and of the sediment formation under modern climate conditions. Both studies therefore include investigations on the modern environment (including organisms, soils, rocks, lake water and sediments). A 90 m long sediment sequence was investigated from Lake Tswaing (north-eastern South Africa) using geochemical analyses. These investigations document alternating periods of high detrital input and low (especially autochthonous) organic matter content and periods of low detrital input, carbonatic or evaporitic sedimentation and high autochthonous organic matter content. These alternations are interpreted as changes between relatively humid and arid conditions, respectively. Before c. 75 kyr BP, they seem to follow changes in local insolation whereas afterwards they appear to be acyclic and are probably caused by changes in ocean circulation and/or in the mean position of the Inter-Tropical Convergence Zone (ITCZ). Today, these factors have main influence on precipitation in this area where rainfall occurs almost exclusively during austral summer. All modern organisms were analysed for their biomarker and bulk organic and compound-specific stable carbon isotope composition. The same investigations on sediments from the modern lake floor document the mixed input of the investigated individual organisms and reveal additional influences by methanotrophic bacteria. A comparison of modern sediment characteristics with those of sediments covering the time 14 to 2 kyr BP shows changes in the productivity of the lake and the surrounding vegetation which are best explained by changes in hydrology. More humid conditions are indicated for times older than 10 kyr BP and younger than 7.5 kyr BP, whereas arid conditions prevailed in between. These observations agree with the results from sediment composition and indications from other climate archives nearby. The second lake study deals with Lake Challa, a small, deep crater lake on the foot of Mount Kilimanjaro. In this lake form mm-scale laminated sediments which were analyses with micro-XRF scanning for changes in the element composition. By comparing these results with investigations on thin sections, results from ongoing sediment trap studies, meteorological data, and investigations on the surrounding rocks and soils, I develop a model for seasonal variability in the limnology and sedimentation of Lake Challa. The lake appears to be stratified during the warm rain seasons (October – December and March – May) during which detrital material is delivered to the lake and carbonates precipitate. On the lake floor forms a dark lamina with high contents of Fe and Ti and high Ca/Al and low Mn/Fe ratios. Diatoms bloom during the cool and windy season (June – September) when mixing down to c. 60 m depth provides easily bio-available nutrients. Contemporaneously, Fe and Mn-oxides are precipitating which cause high Mn/Fe ratios in the light diatom-rich laminae of the sediments. Trends in the Mn/Fe ratio of the sediments are interpreted to reflect changes in the intensity or duration of seasonal mixing in Lake Challa. This interpretation is supported by parallel changes in the organic matter and biogenic silica content observed in the 22 m long profile recovered from Lake Challa. This covers the time of the last 25 kyr BP. It documents a transition around 16 kyr BP from relatively well-mixed conditions with high detrital input during glacial times to stronger stratified conditions which are probably related to increasing lake levels in Challa and generally more humid conditions in East Africa. Intensified mixing is recorded for the time of the Younger Dryas and the period between 11.4 and 10.7 kyr BP. For these periods, reduced intensity of the SW monsoon and intensified NE monsoon are reported from archives of the Indian-Asian Monsoon region, arguing for the latter as a probable source for wind mixing in Lake Challa. This connection is probably also responsible for contemporaneous events in the Mn/Fe ratios of the Lake Challa sediments and in other records of northern hemisphere monsoon intensity during the Holocene and underlines the close interaction of global low latitude atmospheric circulation.
In dieser Arbeit werden Ergebnisse von Untersuchungen an den Sedimenten zweier afrikanischer Seen vorgestellt, die ein Archiv für Klimaveränderungen über einen Zeitraum von mehr als 200.000 Jahren darstellen. Der Schwerpunkt liegt in dieser Arbeit auf dem letzten Glazial und dem Holozän (ca. 100.000 Jahre vor heute [nachfolgend als 100 kyr BP bezeichnet] bis heute). Grundlegende Voraussetzung für solche Studien ist ein gutes Verständnis der Ökosysteme in und um den See, sowie des gegenwärtigen Sedimentationsgeschehens. Deswegen beinhalten beide Seestudien Untersuchungen der heutigen Organismen, Böden, Gesteine, Wasserchemie und Sedimentablagerungen. Im Tswaing-See im nordöstlichen Südafrika wurden anhand eines 90 m langen Sedimentprofils Studien zur Sedimentzusammensetzung und Untersuchungen der Zusammensetzung und Qualität des organischen Materials durchgeführt. Sie zeigen einen Wechsel zwischen Phasen hohen detritischen Eintrags, während derer v.a. kaum autochthones organisches Material im See erhalten blieb, mit Phasen geringen Eintrags und dafür karbonatischer oder evaporitischer Sedimentation, die hohe Gehalte v.a. autochthonen organischen Materials aufweisen. Diese Phasen werden als relativ feuchte bzw. trockene Perioden interpretiert und folgen bis vor ca. 75 kyr BP Schwankungen der lokalen solaren Einstrahlung. Dieser Einfluss nimmt nach 75 kyr BP ab und azyklische feuchte Phasen werden beobachtet. Mögliche Ursachen sind Veränderungen in der ozeanischen Zirkulation und Verschiebungen in der Lage der Innertropischen Konvergenzzone (ITCZ); beides sind auch heute Haupteinflussfaktoren auf die Niederschläge in der Region. Die heute lebenden Organismen des Tswaing-Kraters wurden mittels Analysen der Biomarkerzusammensetzung und der Kohlenstoffisotopie charakterisiert und ihr Einfluss auf die heutigen Seeablagerungen untersucht. Dabei konnten zusätzlich Indikatoren für die Aktivität methanotropher Bakterien nachgewiesen werden. Der Vergleich heutiger Sedimente mit denen des Zeitraumes 14 bis 2 kyr BP zeigt deutliche Veränderungen sowohl in der Zusammensetzung, als auch in der Kohlenstoffisotopie der Biomarker, die mit Veränderungen in der Hydrologie erklärt werden können. Die gefundenen Hinweise auf feuchtere Bedingungen im Zeitraum älter als 10 kyr BP, für trockenere Verhältnissen zwischen 10 und 7.5 kyr BP und für die nachfolgende Wiederzunahme an Feuchtigkeit werden durch die sedimentologischen Ergebnisse unterstützt. Objekt der zweiten Seestudie ist der Challa-See am Fuß des Kilimanjaro. Hier werden heute im mm-Maßstab laminierte Sedimente gebildet, die mit Mikro-XRF-scanning auf Veränderungen in der Elementzusammensetzung untersucht wurden. Zusammen mit Untersuchungen der Mikrofazies und im Vergleich mit ersten Ergebnissen noch laufender Sedimentfallenstudien, mit meteorologischen Daten und Analysen des Umgebungsgesteins werden die saisonalen Veränderungen in der Temperaturverteilung, der Durchmischungstiefe, dem detritischen Eintrag und der Bioproduktivität des Sees in den Sedimenten nachvollziehbar. Der See ist in den feucht-warmen Perioden von Oktober bis Dezember und von März bis Mai stratifiziert. Während dieser Zeit erfolgt der Eintrag detritischen Materials und Kalziumkarbonat fällt aus; eine dunkle Lage mit hohen Gehalten an Fe und Ti und mit hohen Ca/Al- und niedrigen Mn/Fe-Verhältnissen bildet sich am Boden des Sees. Diatomeen blühen während der kühlen, windigen Periode von Juni bis September, wenn die Durchmischung bis auf etwa 60 m Tiefe Nährstoffe verfügbar macht. Die Ausfällung von Fe- und Mn-oxiden sorgt für hohe Mn/Fe-Verhältnisse; es bildet sich eine helle Lage auf dem Sediment. Trends im Mn/Fe-Verhältnis werden als Signal für Veränderungen in der Intensität oder Dauer der saisonalen Durchmischung interpretiert. Dies wird unterstützt durch parallele Trends im Gehalt an organischem Material und an biogenem Silizium, wie durch Analysen an einem 22 m langen Bohrkern gezeigt werden kann. Nach gut durchmischten und von erhöhtem Eintrag von außen geprägten Verhältnissen während des letzten Glazials erfolgt gegen 16 kyr BP ein Übergang zu stärker stratifizierten Bedingungen. Diese korrespondieren mit einem steigenden Seespiegel und verbreiteten Hinweisen auf feuchte Bedingungen im tropischen Ostafrika. Stärkere Durchmischung herrschte während der Jüngeren Dryas und von 11.4 bis 10.7 kyr BP. Diese Perioden entsprechen Zeiten verringerter Südwest- und vermutlich verstärkter Nordostmonsunintensität im Bereich des Indisch-Asiatischen Monsuns und spiegeln eine global beobachtete südliche Verschiebung der ITCZ wider. Nach einer kurzen stabilen, feuchten Phase im frühen Holozän nimmt die Durchmischung des Sees im Verlauf des Holozän wieder zu. Abrupte Ereignisse während des Holozän scheinen im Challa-See zeitgleich mit Veränderungen der Monsunintensität der Nordhemisphäre aufzutreten und bezeugen die starke klimatische Kopplung der niederen Breiten in globalem Maßstab.

Evapotranspiration is a nexus for planetary energy and carbon cycles, as yet poorly constrained. Here I use stable isotopes of oxygen and hydrogen to partition flux of water due to plant transpiration from the direct evaporative flux from soils, water bodies and plant. The study areas, Langat and Kelantan watersheds represent examples of domains dominated by the respective Southwest and Northeast monsoons on the two sides of the main orographic barrier (Titiwangsa mountain range). Mean annual rainfall for the Langat watershed, obtained from 30 years of hydrological data, is 2145 ± 237 mm. Tentatively, 48% of this precipitation returns to the atmosphere via transpiration (T), with 33% partitioned into discharge (Q), 8% into interception (In), and 11% into evaporation (Ed). In the Kelantan watershed, the mean annual rainfall, also based on the 30 year hydrological data, is 2383 ± 120 mm. Similar to Langat, the T accounts for 43% of precipitation (P), 45% is discharged into South China Sea (Q), 12% partitioned into interception (In) and tentatively 0% for evaporation (Ed). Ed for the Langat watershed represents only a small proportion in terms of volumetric significance, up to almost ~11% with strong effect on the isotopic fingerprints of waters associated with the summer Southwest Monsoon (SWM). Note, however, that insignificant Ed for the Kelantan watershed may be an artefact of rain and river water sampling at only coastal downstream portion of the watershed. High humidity (80%) also was recorded for the Malaysian Peninsula watershed. T appropriates about half of all solar energy absorbed by the continents, here ~1000*103 g H2O m-2 yr-1 similar to other tropical regions at 900-1200*103 g H2O m-2 yr-1. The associated carbon fluxes are ~ 1300 g C m-2yr-1, independent of P. Vegetation responses to solar irradiance, via T and photosynthesis reflects the importance of stomatal regulation of the water and carbon fluxes. In order to maintain high transpiration in the tropical region, “constant” water supply is required for continuous pumping of water that delivers nutrients to the plant, suggesting that water and carbon cycle are co-driven by the energy of the sun. The existence of the water conveyor belt may be precondition for nutrient delivery, hence operation of the carbon cycle. Potentially, this may change our perspective on the role that biology plays in the water cycle. In such perspective, the global water cycle is the medium that redistributes the incoming solar energy across the planet, and the anatomical structures of plants then help to optimize the loop of energy transfer via evaporation and precipitation in the hydrologic cycle. The main features of aquatic geochemistry of the Langat and Kelantan rivers inferred from the Principal Component Analysis are controlled by three components that explain 80% and 82% of total variances. These components are reflecting of the geogenic factor with superimposed pollution, the latter particularly pronounced in urbanized sections of the Langat river and dominant in downstream of the Kelantan river. There is no correlation between seasonal variations in major ion chemistry and environmental variables such as precipitation, discharge, temperature or solar activity.

La région océanique située au large des côte sénégalaises et mauritaniennes est l’une des zones le plus productives dans le monde en raison du système d’upwelling qui a lieu en hiver. Cette saisonnalité est très spécifique à cette région. Ce système d’upwelling se distingue donc des autres upwellings classiquement appelés upwelling de bord est (EBUSs: Eastern Boundary Upwelling Systems) tels que le système des Canaries, de la Californie, du Pérou-Chili et du Benguela et qui sont maximum en été. Le devenir de ces upwellings sous l’effet du changement climatique a récemment reçu beaucoup d’attention. Les premières études ont suggéré que leur intensité pourrait augmenter à l’avenir, mais les observations récentes ne permettent pas de dégager des preuves solides. Le système d’upwelling sénégalo-mauritanien est en grande partie exclu de ces études, malgré son rôle crucial pour le développement socio-économique des populations riveraines, dont la nourriture et le revenu dépendent fortement des ressources halieutiques. Dans ce contexte, cette thèse propose une caractérisation de la représentation de ce système dans les modèles de climat de la basse CMIP5 et de sa réponse sous l’effet de l’augmentation des gaz à effet de serre. Notre analyse est basée sur les caractéristiques de l’upwelling en termes de forçage du vent et de signature de la température de surface de la mer. Malgré une certaine diversité dans la capacité des modèles à représenter ce système d’upwelling, les résultats suggèrent que son intensité pourrait au contraire diminuer à l’avenir, principalement en raison de la réduction de la force du vent. D’autre part, cette thèse propose une première analyse de la variabilité inter-annuelle à décennale de l’intensité de ce système à partir de réanalyses. Les indices de caractérisation de l’intensité de l’upwelling ne sont pas toujours cohérents entre eux sur la période historique mais ils suggèrent une variabilité à l’échelle de temps décennale marquée. Le lien avec l’AMV n’est pas robuste, à la différence du lien avec l’intensité de la mousson au cours de la saison opposée. Le mécanisme liant ces deux systèmes est en partie expliqué par les modulations saisonnières de la position hivernale de la zone de convergence intertropicale
The oceanic region located off the Senegalese and Mauritanian coasts is one of the most productive one in the world ocean. This is due to the upwelling system, which occurs during the winter season in this region. This seasonality is very specific. In particular, it differs from the well-known upwelling systems located along the eastern coast of the tropical oceans but further poleward such as along Morocco and the Iberian peninsula, the Californian coast, the Peru-Chili and the Benguela. These upwelling systems are maximum in summer. Several studies have investigated their sensitivity to global warming. Early studies have suggested that their intensity may increase in the future, but recent observations do not clear give robust evidence of this behavior. The winter senegalo-mauritanian upwelling system has been largely excluded from these studies, in spite of its crucial role for the socio-economical development of the populations of the surrounding region, whose food and income strongly depend on the halieutic resources. In this context, this study proposes an evaluation of the representation of this system in the CMIP5 climate models, and its response to climate change. Our analysis is based on characteristics of the upwelling in terms of wind forcing and sea surface temperature signature. In spite of some diversity in the model’s ability to represent the senegalo-mauritanian upwelling system, the results suggest that its intensity may rather decrease in the future, primarily because of a reduction of the wind forcing. In a second hand we propose an analysis of the inter-annual to decadal variability of the intensity of the upwelling based on recent reanalyses. This study focuses on the link with the monsoon and with large scale climate modes such as the North Atlantic Oscillation (NAO), Atlantic Multi- decadal Oscillation (AMO). The link with the decadal variations of the West African Monsoon (WAM) during the opposite season is relatively clear. It is associated with anomalous trajectories of the ITCZ. The indices characterizing the upwelling are not always correlated with each other, showing that they are not yet very robust. They nevertheless indicate a marked variability at the decadal timescale partly associated with the AMO

La population et les écosystèmes habitant des zones influencées par des systèmes de mousson sont extrêmement vulnérables aux phénomènes atmosphériques générateurs de sècheresses ou d’inondations ainsi qu’aux possibles changements de régime des précipitations associés au réchauffement climatique. C'est notamment le cas des Andes tropicales de l’hémisphère sud, où le principal pic de précipitation se produit en été austral, en lien avec la phase la plus développée de la mousson sud-américaine (MSA). Cette unique saison de pluie est la principale source d’eau pour les écosystèmes dans cette région semi-aride et pour les différentes activités socio-économiques et le réseau d’approvisionnement d’eau potable. Par ailleurs, , il y a des évènements météorologiques extrêmes propres à la saison sèche hivernale (juin-août), qui causent des dégâts importants au bétail et à la production agricole, et peuvent provoquer des pertes en vies humaines.Ainsi, la meilleure compréhension de la climatologie de la pluie dans cette région, pour prévoir les changements à long terme et l’amélioration de la prévision météorologique dans les cas d'évènements extrêmes, prend une dimension cruciale. Pour en revenir au point précèdent, il est très important de commencer à comprendre les mécanismes atmosphériques liés à la variabilité de pluie dans cette région andine, et ce, en prenant en compte les échelles de temps qui vont de la synoptique à des oscillations multi-décennales. C'est dans ce contexte précis que ces travaux de thèse ont été encadrés.Les études climatiques dans cette région sont rendues difficiles par l’absence d’un réseau de stations météorologiques et pluviométriques de bonne qualité. C’est pourquoi cette thèse s’est appuyée sur une utilisation combinée de différentes bases de données : mesures in situ, produits satellites, données de réanalyse et sorties de modélisation climatique.Dans un premier temps, l’analyse du régime pluviométrique des Andes tropicales entre 20°S et 1°N a conduit à identifier une sous-région comprise entre 20°S et 8°S (et qualifiée par la suite d’Andes Tropicales Sud), où le cycle saisonnier est unimodal avec des pluies concentrées sur les mois de décembre à mars. La pluie y est en effet fortement liée à la phase mature de la MSA. Plus au nord (8°S - 1°N), la MSA a moins d’influence et c’est la migration vers le sud de la zone de convergence intertropicale (ZCIT) qui détermine un pic saisonnier des pluies entre Février et Avril. La phase initiale de croissance de la MSA (octobre-novembre), provoque néanmoins une seconde saison des pluies entre 8°S et 5°S. En saison sèche (Juin-Août), les analyses synoptiques révèlent que la convection atmosphérique dans la zone occidentale de l’Amazonie peut se conjuguer avec des perturbations atmosphériques des latitudes extratropicales pour générer des évènements de pluie extrêmes dans les zones à haute altitude (> 3000 m) des Andes Tropicales Sud.En se basant sur les réanalyses et des simulations climatiques réalisées avec le modèle WRF on met également pour la première fois en évidence que la variabilité interannuelle des pluies d’été sur les Andes Tropicales Sud est contrôlée par la convection dans l’ouest de l’Amazonie, outre l'anticyclone de Bolivie. Nos résultats révèlent que l’anticyclone de Bolivie a été le facteur dominant de variabilité entre 1982 et 2002, mais que la convection atmosphérique dans l’ouest de l’Amazonie est devenue le facteur dominant depuis le début du 21ième siècle. Cette bascule est lié à l’intensification de l'activité convective dans l’ouest de l’Amazonie, qui explique aussi l’amoindrissement durable de la saison des pluies et le déficit de précipitation annuelle. En conclusion, nous avons identifié une liaison atmosphérique entre l’Amazonie occidentale et la zone de haute altitude des Andes Tropicales Sud sur plusieurs échelles de temps
Population and ecosystems in monsoon regions are highly vulnerable in face of weather- and climate-related hazards as droughts, floods, and precipitation changes due to global warming, among others. This is the case of the southern tropical Andes, which includes the southern Peruvian Andes and the Bolivian and Chilean Andes north of 20S. In this Andean region, the unique seasonal precipitation maximum occurs during the December-March season, which is the period of the mature phase of the South America Monsoon System (SAMS). Indeed, December-March precipitation is the principal water source for ecosystems and socio-economic activities as livestock farming, agriculture, and human consumption. On the other hand, extreme precipitation events occurring in the dry season (June-August), when SAMS does not exist, is a source of hazard for the population by killing livestock, devastating crop fields, and causing losses of human lives. Improving our knowledge of the rainfall climatology of this region, anticipating possible long term changes, and improving our forecasting skills is thus of crucial importance. This requires to identify the atmospheric mechanisms controlling the austral summer and winter precipitation variability in this Andean region over a wide range of time-scales, from synoptic to multidecadal, and this is the object of this thesis.The lack of a good quality network of meteorological stations and the complex topography of this region have hindered the proper identification of these mechanisms. For this reason, different data sets as in situ and satellite-based precipitation products, as well as reanalysis and climate modeling data sets are used in this thesis.Analyzing the regional precipitation over the tropical Andes from 20S and 1N, we defined the southern tropical Andes as the region from 20S to 8S, where the unimodal annual cycle of precipitation with a seasonal maximum in the December-March season dominates. Indeed, the region south of 12S and above 3000 m.a.s.l is the most influenced, in terms of precipitation, by the mature phase of the SAMS. We also found that the demise of the SAMS together with the southward migration of the Intertropical Convergence Zone (ITCZ) in the February-April season involves a seasonal precipitation increment north of 8S. The onset of the SAMS, occurring in the September-November season, also creates a secondary seasonal precipitation maximum in the tropical Andes between 8S and 5S. In the season when the SAMS does not exist (June-August), western Amazon convection in association with extratropical perturbations trigger the most severe extreme precipitation events in the upper-elevation southern tropical Andes (>3000 m.a.s.l.).The analysis of the interannual variability indicates that, in addition to the Bolivian High, western Amazon convection is also a controlling mechanism of December-February precipitation over the upper-elevation southern tropical Andes. The existence of the relationship between precipitation and the two mechanisms, which are components of the SAMS, was confirmed by the use of the WRF model. While the Bolivian High explains the precipitation variability in the 1982-2002 period, western Amazon convection plays a more important role from 2002 onwards. This change in the controlling mechanism is, indeed, related to the long-term intensification of western Amazon convection, which is also associated with the less frequency of dry years in this Andean region. Thus, our results document for the first time and highlight the atmospheric connection existing between the western Amazon and the upper-elevation southern tropical Andes on different time-scales

Tese de doutoramento, Biologia (Biologia Evolutiva), Universidade de Lisboa, Faculdade de Ciências, 2018
How speciation, range shifting and reticulation through climatic oscillations combine to shape current patterns of local and regional diversity remains a key question in evolutionary biology. This can be investigated by using multi-scale analyses of closely related species assemblages in diverse communities, such as the lizards of Australia’s tropical savannas. An example of these is the understudied group of rainbow skinks (Carlia) that are broadly co-distributed across the Australian Monsoonal Tropics (AMT) region, and that stands out from most well-studied Carlia species occurring along the Australian east coast. The aims of this thesis were to: i) identify lineage diversity between two sympatric and closely related skinks and to use an integrative taxonomic approach to statistically test major lineages as species; ii) compare past responses of species with different climatic niche breadth that experienced the same climatic fluctuations; iii) infer the phylogenetic relationships of the rainbow skinks in a collaborative study; and iv) explore the occurrence of introgression in a group of six species that broadly cooccur in the AMT, using a dataset by target exon capture (>1000 loci). The investigation of lineage diversity discovered cryptic lineages mostly in the Kimberley region (north west of Australia). In an integrative taxonomic approach, these lineages were then statistically validated as new species using multispecies coalescent methods and morphological analyses, and subsequently described as C. insularis sp. nov and C. isostriacantha sp. nov. with genetic and morphological characters. The exploration of how concordant were the species responses with late Pleistocene climatic changes, identified contrasting responses by species with different climatic niches, suggesting that the narrow climatic specialist species was more sensitive to these changes. In addition, using multiple phylogenomic approaches, it contributed to a much improved and well-supported phylogeny for the rainbow skinks in comparison with a previous, poorly resolved tree. The new tree showed that the six Carlia species that co-occur in the AMT are actually closely related. Lastly, the analysis for the presence of introgression between the AMT sympatric species did not detect evidence of recent admixture, but identified patterns of ancestral introgression before the divergence of sister species, and some instances of introgression in the more climatic unstable Kimberley region. It also showed that by accounting for reticulated evolution with phylogenetic networks methods, a distinct topology from the overall well supported species tree can be observed, in this case with the clade inferred as introgressed appearing as more ancestral in the phylogeny. The results of this thesis have implications for the understanding of the processes driving cryptic species diversity and responses to past climatic change across this richm and understudied Australian Monsoonal Tropics biome.

Understanding what has driven the distribution of biodiversity across the world is a central question in biogeography. One possible explanation is the persistence of organisms in refugia, areas of stability that allow taxa to persist during times of unfavourable climate, with the potential to expand from there in later times. In this dissertation, I explore measures of refugia and stability, and the contribution of refugia to the diversity patterns of lizards in the Australian Monsoonal Tropics. Stability and refugia are widely used concepts in macroecology and biogeography, but their use is not always clearly defined. In Chapter 1 I present a conceptual framework for thinking through concepts of stability, focusing on five key questions. Thinking through these questions provides a clear understanding of any definition of stability, assisting with comparing studies and interpreting the biological implications of their results. In Chapter 2 I empirically test the importance of using this framework, comparing four different measures of stability to see which best predicts reptile diversity in the Australian Monsoonal Tropics. I find large differences in the performances of the stability measures, demonstrating the importance of carefully considering the biological meaning of stability measures before including them in studies. Chapter 3 investigates drivers of diversity patterns across the Australian Monsoonal Tropics, including stability as well as current climate, geology and disturbance. Finally, in the fourth chapter I explore the impact of refugia on the biogeography of lizards in the Kimberley region. I test refugia across different spatial scales (fine and coarse grain) and temporal scales (Holocene and Pleistocene) to examine the drivers of diversity patterns in the region.

Rehabilitation is seen as a key strategy for minimising biodiversity losses. Although most rehabilitation strategies aim to provide habitat for fauna, they usually focus entirely on establishing vegetation. Successful vegetation establishment, however, does not necessarily provide habitat to the same species that are threatened by habitat loss. Improved understanding of faunal response to rehabilitation is required if rehabilitation techniques are to be refined and deliver the hoped for biodiversity outcomes. This study aimed to assess to what extent post-mining rehabilitation on the Weipa bauxite plateau has restored the bird habitat values of the pre-mining native forest. Bird assemblages, vegetation, and landscape functionality were compared between: (1) Eucalyptus tetrodonta open forest reference sites representative of the pre-mining native forest; (2) two reference land units of Eucalyptus tetrodonta tall woodland that have previously been nominated as ecologically appropriate analogues for the post-mining landscapes; and (3) a chronosequence of post-mining rehabilitation sites up to 23 years old. Bird species richness and mean bird abundance increased with rehabilitation age. Bird species composition also changed and became more similar to native forest bird assemblages with increasing age. Significant differences remained, however, in mean bird abundance and composition of the bird assemblages between the oldest age class of mine rehabilitation and reference native forest land units. The mean bird species shortfall index in the oldest age class of mine rehabilitation was 63%, compared to a mean species shortfall index of 27% for pre-mining native forest sites. There were also significant differences in vegetation composition and structure between reference native forest land units and post-mining rehabilitation sites. Most importantly, the framework plant species that dominate the native vegetation community occurred at much lower densities in mine rehabilitation than in reference land units. Site detection rates of birds were strongly related to vegetation composition and structure. It is concluded that mining and post-mining rehabilitation on the Weipa bauxite plateau has so far resulted in habitat conversion rather than habitat restoration. It therefore contributes to the causes of biodiversity decline. Post-mining rehabilitation created new habitat for 18 bird species not sourced from the pre-mining native forest. It also provided partial habitat for many of the generalist native forest bird species recorded, although their presence in the landscape remained dependent on access to native forest. However, rehabilitation did not provide the habitat resources that are required by habitat specialists and foraging specialists. This study found that the native forest bird species most sensitive to habitat loss, and most in need of habitat restoration, may be the last to return to rehabilitation if they return at all. The findings of this study have implications for rehabilitation practices, biodiversity conservation on the Weipa bauxite plateau, as well as broader implications for policies that rely on the assumption that rehabilitation can offset biodiversity losses.
ANU Graduate School Scholarship. Rio Tinto Aluminium Weipa

Research Doctorate - Doctor of Philosphy (PhD)
The climate evolution of the southern sub-equatorial tropics during marine isotope stage (MIS)5a/b and the Holocene is explored using geochemical tracers from speleothems on Flores island, Indonesia. Oxygen isotope measurements from two precisely-dated stalagmites reveal that the Australian-Indonesian monsoon increased during the Younger Dryas (YD) cooling event, when Atlantic meridional overturning circulation was relatively weak. Monsoon precipitation intensified even more rapidly from 11 to 7 ka ago, when the Indonesian continental shelf was flooded by global sea-level rise. Analysis of oxygen (δ18O) and hydrogen (δD) isotope ratios from speleothem fluid inclusions shows that inclusion-δ18O values vary in phase with speleothem calcite δ18O during the Holocene, confirming that calcite δ18O primarily reflects variations in the δ18O of meteoric rainfall. Cave drip-water temperatures, reconstructed from coupled measurements of δ18O in speleothem calcite and fluid inclusions, remained relatively constant through the Holocene but were significantly cooler during the YD, consistent with the high northern latitudes. To help confirm the stable isotope records, trace elements were used to reconstruct the position of the austral summer inter-tropical convergence zone and east Indonesian rainfall variability during the Holocene. Mg/Ca and Sr/Ca ratios correlate significantly with one another, and with δ18O and δ13C, throughout the record suggesting that the trace element ratios were dominated by prior calcite precipitation, a process whereby degassing in the vadose zone during periods of low recharge causes deposition of calcite and disproportionate loss of Ca2+ ions (relative to Mg2+ and Sr2+) ‘upstream’ of the stalagmite. Comparison of speleothem δ18O time-series from Flores and Borneo shows that they vary in unison for much of the Holocene. However, there is an exception during the mid-Holocene when a distinct anomaly in δ18O in the Borneo record, possibly caused by a change in the circulation of the Australian-Indonesian summer monsoon (AISM) in response to a period of positive IOD-like conditions in the eastern Indian Ocean, occurred between the two regions. A stalagmite reconstruction of Indo-Pacific climate through the interval 84 - 91 ka shows that the lower-frequency oxygen isotope trend indicates that the AISM was largely controlled by local summer insolation during this time, while the carbon isotopes show a pattern that is closer linked with northern polar-latitude ice-core records. Most notably, an abrupt decrease in the temperature-controlled δ13C values at the MIS 5a/b transition occurs in parallel with GIS 21 in the GISP2 δ18O and CH4 records highlighting the strong connection between the IPWP and North Atlantic during the last glacial period.

Thesis (Ph.D.)--University of Hawaii at Manoa, 2008.
On the intraseasonal timescale, extreme active and break phases of the ISM often bring about devastating floods and severe droughts. The concurrent buildup of the anomalous high over Central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period over the NISM region. Two predictors may be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over Central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit 1.0, an extreme phase is anticipated to occur over northern India after six to seven days.
This study also reveals a significant, coupled intraseasonal variation between a Rossby wavetrain across the Eurasian continent and the summer monsoon convection in northwestern India and Pakistan (referred to as NISM hereafter). The time-lagged SVD analysis shows that the mid-latitude wavetrain originates from the northeastern Atlantic and traverses Europe to central Asia. The wavetrain enhances the upper-level high pressure and reinforces the convection over the NISM region; meanwhile, it propagates further toward East Asia along the wave guide provided by the westerly jet. After an outbreak of NISM convection, the anomalous central Asian high retreats westward.
This study investigates the most recurrent coupled pattern of interannual variability between the Northern Hemisphere mid-latitude circulation and the tropical convection (between 15°S and 30°N) during the northern summer (June to September). The leading singular value decomposition (SVD) pattern reveals a significant, coupled interannual variation between a circumglobal teleconnection (CGT) pattern and strong tropical convection pattern associated with the La Nina phase of Equatorial Eastern Pacific sea surface temperature. The CGT, having a zonal wavenumber five structure, is primarily positioned within a waveguide associated with the westerly jetstream. The spatial phases of CGT tend to lock to preferred longitudes. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of West Europe, European Russia, India, East Asia and North America. This implies that the CGT may be a source of climate variability and predictability in the midlatitude regions.
Includes bibliographical references (leaves 141-152).
Also available by subscription via World Wide Web
152 leaves, bound 29 cm

Monsoons and Intertropical Convergence Zones (ITCZ) exhibit variability at various temporal and spatial scales. The temporal scale of variability encompasses scales from the intraseasonal through interannual to interdecadal time scales. Anthropogenic climate change can also have an impact on ITCZ and monsoons. Thus it is necessary to assess the ability of coupled ocean atmospheric models (commonly known as AOGCM) to simulate these aspects of variability of tropical climate. This has been studied with simulations from 20 AOGCMs and their AGCM from IPCCAR4 archive. In addition, we have used our own 100 year simulation with CCSM2 and also simulations with its AGCM viz. CAM2. Our analysis shows that most model have significant bias in tropical rainfall and SST. Most models underestimate SST except over a few regions such as the Eastern boundaries of Atlantic and Pacific Oceans. The AGCMs which are forced with observed SSTs have much higher annual mean rainfall as compared to AOGCMs. There is a strong correlation between error in shortwave reflectance at the top of the atmosphere and error in SST. The ability of coupled ocean-atmosphere models and their atmosphere-alone counterparts to simulate the seasonal cycle of rainfall over major monsoon regions and also over oceanic ITCZ. It is found that over the Indian monsoon region, most AGCMs overestimate the seasonal cycle while AOGCMs have a more realistic seasonal cycle. This inspite of the fact that most AOGCMs underestimate the SST over the Indian region. It is shown that this is related to errors in precipitable water-rainfall relationship in most models i.e. for a given amount of precipitable water, most models overestimate the rainfall. Thus lower SST reduces the precipitable water and hence the amount of rainfall is reduced. Therefore, the mutual cancellation of errors leads to a more realistic seasonal cycle in AOGCMs. The seasonal cycle over Africa was analysed with the help of a diagnostic model. Over Southern Africa, most models show simulate a less stable atmosphere and hence the rainfall is overestimated. A technique based on Continous Wavelet Transform in Space and Time (CWTST) has been modified to seperate northward and southward propagating modes of BSISO over the Indian and West Pacific regions. It was seen that over the Indian region, northward propagating modes were more prominent in comparison to southward modes. It was also found that the predominant spatial scale (of about 30o) did not show much interannual variability but the associated temporal scale showed significant variation. Both AOGCMs and AGCMs simulations were analysed to investigate the impact of coupling on intraseasonal activity. Most AOGCMs were able to simulate the predominant spatial scale but were unable to simulate the associated temporal scale correctly. These problems persisted with AGCMs also. It was also found that for AGCMs, there were some variations between ensemble members of the AGCMs. Comparing BSISO in increased GHG scenarios with present day simulations we found that in general, power in the spectrum increases. This could be related to higher mean precipitation that has been simulated by most AOGCMs when GHG are increased. The interannual variability in the tropics with special reference to Tropical Biennial Oscillation (TBO) and ENSO has been studied. The changes in these modes of variability due to anthropogenic climate change has also been assessed. We found that in most models over the Nino3.4 region, the mode of variation shifts from a near-four period (in pre-industrial simulations) to that of TBO mode in increased GHG (green house gas) scenario. This suggests that with increasing GHGs, ENSO quasi-periodicity might shift to about two years. It is also interesting to note that for observed rainfall, OLR and 850 hPa winds, the TBO mode has higher variance over the Eastern Indian Ocean, indicating that the TBO mode might be related to Indian Ocean Dipole Mode and EQUINOO (Equatorial Indian Ocean Oscillation).

The poor representation of cloud processes in general circulation models (GMCs) has been recognized for decades as one of the major sources of uncertainties in weather and climate predictions. Because of the coarse spatial resolution of GCMs, subgrid- scale cloud and convection processes are modelled by parameterization schemes that provide a statistical representation of the subgrid-scale processes in terms of the large- scale, gridbox fields. This thesis focuses on the stochastic multicloud parameterization of Khouider et al. (2010), which is based on the three cloud types (congestus, deep, and stratiform) that are most observed in tropical convective systems. A rigorous parameter estimation model based on the Bayesian paradigm is developed to infer from data a set of seven convective timescales that determine the transition rates from one cloud type to another in the multicloud framework. The Bayesian posterior is given in terms of a costly model likelihood function that must be approximated numerically using high-performance linear algebra routines for parallel distributed computing. The Bayesian procedure is applied to the Giga-LES dataset of Khairout- dinov et al. (2009), a large-eddy simulation of tropical deep convection that covers a physical domain comparable to that of a typical horizontal grid cell in a GCM. The stochastic multicloud model and its deterministic version are then coupled to a zonally iv symmetric atmospheric model to study the meridional Hadley circulation and mon- soon dynamics. The main model is based on the hydrostatic Boussinesq equations on a rotating sphere, and is composed of a deep convective troposphere and a dynamical planetary boundary layer to sustain shallow convection. The resulting equations form a system of nonconservative partial different equations, which is solved numerically using high order non-oscillatory finite volume methods. Results from deterministic and stochastic simulations reveal a mean local Hadley cell structure with some fea- tures of organized convection. In the stochastic case, the Giga-LES parameter regime best captures the Hadley-type circulation and monsoon trough features, compared to a parameter regime used in a different study.
Graduate
mdelachev@gmail.com

Arctic geoengineering wherein sunlight absorption is reduced only in the Arctic has been suggested as a remedial measure to counteract the on-going rapid climate change in the Arctic. Several modelling studies have shown that Arctic geoengineering can minimize Arctic warming but will shift the Inter-tropical Convergence Zone (ITCZ) southward, unless offset by comparable geoengineering in the Southern Hemisphere. In this study, we investigate and quantify the implications of this ITCZ shift due to Arctic geoengineering for the global monsoon regions using the Community Atmosphere Model version 4 coupled to a slab ocean model. A doubling of CO2 from pre-industrial levels leads to a warming of ~ 6 K in the Arctic region and precipitation in the monsoon regions increases by up to ~15 %. In our Arctic geoengineering simulation which illustrates a plausible latitudinal distribution of the reduction in sunlight, an addition of sulfate aerosols (11 Mt) in the Arctic stratosphere nearly offsets the Arctic warming due to CO2 doubling but this shifts the ITCZ southward by ~1.5⁰ relative to the pre-industrial climate. The combined effect from this shift and the residual CO2-induced climate change in the tropics is a decrease/increase in annual mean precipitation in the Northern Hemisphere /Southern Hemisphere monsoon regions by up to -12/+17%. Polar geoengineering where sulfate aerosols are prescribed in both the Arctic (10 Mt) and Antarctic (8 Mt) nearly offsets the ITCZ shift due to Arctic geoengineering, but there is still a residual precipitation increase (up to 7 %) in most monsoon regions associated with the residual CO2 induced warming in the tropics. The ITCZ shift due to our Global geoengineering simulation, where aerosols (20 Mt) are prescribed uniformly around the globe, is much smaller and the precipitation changes in most monsoon regions are within ±2 % as the residual CO2-induced warming in the tropics is also much less than in Arctic and Polar geoengineering. Further, global geoengineering nearly offsets the Arctic warming. Based on our results we infer that Arctic geoengineering leads to ITCZ shift and leaves residual CO2 induced warming in the tropics resulting in substantial precipitation changes in the monsoon regions.

碩士
國立臺灣師範大學
地球科學系
99
Monsoon trough is one of the major factors that influence the climate and typhoon activities in the west North Pacific Ocean. However, the structure of monsoon trough and tropical storms are often poorly simulated by the Global Climate Models (GCMs) due to their coarse resolutions. In this study, Weather Research and Forecasting (WRF) Regional Climate Model (RCM) at 30-km resolution is adopted to simulate the climatological feature of monsoon trough and tropical storms from 2000 to 2009. Numerical experiments with three different domains including NWP (105°E-180°, 0°-40°N), NP(105°E-120°W, 5°S-45°N), and IONWP (45°E-180°, 20°S-40°N) are conducted to investigate the relative contributions from tropical East Pacific Ocean, Indian Ocean and land-sea contrast to the monsoon trough. The results show that the gross features of large-scale circulations are well captured by all three experiments. Nevertheless, monsoon troughs are too strong, while subtropical highs are too weak in NWP and NP experiments. This causes the low-level convergence region simulated in NWP and NP experiments to the east of the observation. With the inclusion of Indian Ocean and land-sea contrast, the strength of monsoon trough, subtropical high, and low-level convergence are well represented in IONWP experiment. These large-scale circulations are closely related to the typhoon activities in all three experiments.

In this thesis, we have presented a systematic analysis of the change of cloud properties due to variation in aerosol concentration over Indian region using satellite observations, and Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) simulations. The Tropical Rainfall Measurement Mission (TRMM) based Microwave Imager (TMI) estimates (2A12) have been used to compare and contrast the characteristics of cloud liquid water and ice over the Indian land region and the surrounding oceans, during the pre-monsoon (May) and monsoon (June–September) seasons. Based on the spatial homogeneity of rainfall, we have selected five regions for our study (three over ocean, two over land). In general, we find that the mean cloud liquid water and cloud ice content of land and oceanic regions are different, with the ocean regions showing higher amount of CLW. A comparison across the ocean regions suggests that the cloud liquid water over the or graphically influenced Arabian Sea (close to the Indian west coast) behaves differently from the cloud liquid water over a trapped ocean (Bay of Bengal) or an open ocean (Equatorial Indian Ocean). Specifically, the Arabian Sea region shows higher liquid water for a lower range of rainfall, whereas the Bay of Bengal and the Equatorial Indian Ocean show higher liquid water for a higher range of rainfall. Apart from geographic differences, we also documented seasonal differences by comparing cloud liquid water profiles between monsoon and pre-monsoon periods, as well as between early and peak phases of the monsoon. We find that the cloud liquid water during the lean periods of rainfall (May or June) is higher than during the peak and late monsoon season (July-September) for raining clouds over central India. However, this is not true over the ocean. As active and break phases are important signatures of the monsoon progression, we also analyzed the differences in cloud liquid water during various phases of the monsoon, namely, active, break, active-to-break (a2b) and break-to-active (b2a) transition phases. We find that the cloud liquid water content during the b2a transition phase is significantly higher than that during the a2b transition phase over central India. We speculate that this could be attributed to higher amount of aerosol loading over this region during the break phase. We lend credence to this aerosol-liquid water/rain association by comparing the central Indian cloud liquid water with Southeast Asia (where the aerosol loading is significantly smaller) and find that in the latter region, there are no significant differences in cloud liquid water during the different phases of their monsoon. The second part of our study involves evaluating the ability of the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) to simulate the observed variation of cloud liquid water and rain efficiency. We have used no chemistry option, and the model was run with constant aerosol concentration. The model simulations (at 4.5 km resolution) are done for the month of June–July 2004 since this period was particularly favorable for the study of an active–break cycle of the monsoon. We first evaluate the sensitivity of the model to different parameterizations (microphysical, boundary layer, land surface) on the simulation of rain over central India and Bay of Bengal. This is done to identify an “optimal” combination of parameterizations which reproduces the best correlation with observed rain over these regions. In this default configuration (control run), where the aerosol concentration is kept constant throughout the simulation period, the model is not able to reproduce the observed variations of cloud liquid water during the different phases of an active-break cycle. To this end, we proceeded to modify the model by developing an aerosol-rain relation, using Aerosol Robotic Network (AERONET) and TRMM 3B42 data that realistically captures the variation of aerosol with rain. It is worth highlighting here that our goal was to primarily isolate the indirect effect of aerosols in determining the observed changes in cloud liquid water (CLW) during the active-break phases of the Indian monsoon, without getting into the complexity of a full chemistry model such as that incorporated in WRF-Chem. Moreover, the proposed modification (modified run) is necessitated by the lack of realistic emission estimates over the Indian region as well as the presence of inherent biases in monsoon simulation in WRF. The main differences we find between the modified and control simulations is in the mean as well as spatial variability of CLW. We find that the proposed modification (i.e., rate of change of aerosol concentration as a function of rain rate) leads to a realistic variation in the CLW during the active-break cycle of Indian monsoon. Specifically, the peak value of CLW in the b2a (a2b) phase is larger (smaller) in the modified as compared to the control run. These results indicate a stronger change in CLW amount in the upper levels between the two transition phases in the modified scheme as compared to the control simulation. More significantly, we also observe a change in sign at the lower levels of the atmosphere, i.e., from a strong positive difference in the control run to a negative difference in the modified simulation, similar to that observed. Additionally, we investigated the impact of the proposed modification, via CLW changes, on cloud coverage, size of clouds and their spatial variability. We find that the transformation of optically thin clouds to thick clouds during the break phase was associated with larger cloud size in modified compared to the control simulation. Moreover, the higher rate of decay of the spatial variability of CLW with grid resolution, using the modified scheme, suggests that clusters of larger clouds are more in the modified compared to control simulation. Taken together, the interactive aerosol loading proposed in this thesis yields model simulations that better mimic the observed CLW variability between the transition phases.

This thesis examines the structure of the Tropical Easterly Jet (TEJ) in a General Circulation Model (GCM). The TEJ is observed only during the Indian summer monsoon period and is strongest during July and August. The jet structure simulated by an atmospheric GCM (CAM-3.1) in July has been compared with reanalysis data. The simulated TEJ was displaced westward by ~ 25◦ when compared to observations. The removal of orography had no impact on the jet structure. This demonstrated that the Tibetan Plateau did not play an important role in the location and structure of the jet. The changes in cumulus scheme in the GCM had a large influence on the location of the jet maxima. To examine the factors which control the location and structure of the jet, a series of experiments were conducted using an aqua-planet version of the model. The impact of different sea surface temperature (SST) profiles was studied. The rainfall in the GCM was primarily in the regions where the SST attained a maximum. By altering the location of SST maximum (and hence the rainfall maximum), the impact of location of rainfall maximum on the location and structure of the jet was studied. When the rainfall maximum was located close to the equator, it did not generate a strong jet but had an influence on the vertical structure of the jet. A large number of simulations were conducted with multiple rainfall maxima and the need for these was demonstrated since only then was the observed jet structure well simulated. Based on the simulations, it was concluded that the simulation of the TEJ by CAM-3.1 was unrealistic because of large unrealistic rainfall over Saudi Arabia in this GCM. Equatorial heating has been shown to be important to simulate proper jet structure. The zonal structure of the jet was also influenced by rainfall in the Pacific Ocean. Although the aqua-planet configuration of the CAM-3.1 GCM provided several useful insights, the simulation was not perfect on account of errors in the simulation of the temperature profile in the lower troposphere. An ideal-physics configuration of the GCM was used. This removed the cumulus physics and instead imposed the observed heating pro-files. Both upper tropospheric friction and radiative-convective atmospheric temperatures were required to simulate the TEJ. The problems with the simulation of structure in the jet exit region was corrected by using radiative-convective atmospheric temperatures that were qualitatively similar to those observed in northern hemisphere summer time. The ideal-physics configuration reconfirmed that the Saudi Arabian rainfall was responsible for the westward shift of the TEJ in the simulations. The ideal-physics simulations showed that the simple analytical model proposed by Gillin1980 was not suitable for the simulation of TEJ. The above the simulations indicate that a shift in the location of the jet is related to a shift in the rainfall pattern. Based on this insight one would expect that the jet location will be different in good and bad monsoon periods. This is indeed the case. In July 2002 the Indian monsoon failed after beginning well in June. In June the TEJ is consequently located west ward compared to July. The same situation prevails even in good and poor monsoon years. In a good monsoon year (July 1988) the jet maximum is located westward when compared to a bad monsoon year (July 2002). In this thesis we have clearly demonstrated the role of anomalous rainfall on the location of the TEJ. This thesis has shown that an accurate simulation of the TEJ depends upon the accurate simulation of various rainfall centers that act as multiple heat sources in the atmosphere. The rainfall in the equatorial region does not influence the strength of the TEJ but alters the vertical structure of the jet. The strength the jet is dependent on the intensity of rainfall and the latitudinal distance from the equator. The complex vertical structure of the jet is not simulated by simple analytical models of the jet.

In this thesis, we investigated modes of intraseasonal variability (ISV) observed in the Indian monsoon rainfall and how these modes modulate rainfall over India. We identified a decreasing trend in the intensity of low-frequency intraseasonal mode with increasing strength in synoptic variability over India. We also made an attempt to understand the reason for these observed trends using numerical simulations. In the first part of the thesis, satellite rainfall estimates are used to understand the spatiotem-poral structures of convection in the intraseasonal timescale and their intensity during boreal sum-mer over south Asia. Two dominant modes of variability with periodicities of 10–20-days (high-frequency) and 20–60-days (low-frequency) are found, with the latter strongly modulated by sea surface temperature. The 20–60-day mode shows northward propagation from the equatorial In-dian Ocean linked with eastward propagating modes of convective systems over the tropics. The 10–20-day mode shows a complex space-time structure with a northwestward propagating anoma-lous pattern emanating from the Indonesian coast. This pattern is found to be interacting with a structure emerging from higher latitudes propagating southeastwards. This could be related to ver-tical shear of zonal wind over northern India. The two modes exhibit variability in their intensity on the interannual time scale and contribute a significant amount to the daily rainfall variability in a season. The intensities of the 20–60-day and 10–20-day modes show significantly strong inverse and direct relationship, respectively, with the all-India June–September rainfall. This study also establishes that the probability of occurrence of substantial rainfall over central India increases significantly if the two intraseasonal modes simultaneously exhibit positive anomalies over the region. There also exists a phase-locking between the two modes. In the second part of the thesis, we investigated the changing nature of these intraseasonal modes over Indian region, and their association with extreme rainfall events using ground based observed rainfall. We found that the relative strength of the northward propagating 20–60-day mode has a significant decreasing trend during the past six decades, possibly attributed to the weakening of large-scale circulation in the region during monsoon. This reduction is compensated by a gain in synoptic-scale (3–9 days) variability. The decrease in the low-frequency ISV is associated with a significant decreasing trend in the percentage of extreme events during the active phase of the monsoon. However, this decrease is balanced by a significant increasing trend in the percentage of extreme events in break phase. We also find a significant rise in occurrence of extremes during early- and late-monsoon months, mainly over the eastern coastal regions of India. We do not observe any significant trend in the high-frequency ISV. In the last part of the thesis, we used numerical simulations to understand the observed changes in the ISV features. Using the atmospheric component of a global climate model (GCM), we have performed two experiments: control experiment (CE) and heating experiment (HE). The CE is the default simulation for 10 years. In HE, we prescribed heating in the atmosphere in such a way that it mimics the conditions for extreme rainfall events as observed over central India during June– September. Heating is prescribed primarily during the break phase of the 20–60-day mode. This basically increases the number of extremes, majority of which are in break phase. The design of the experiment reflects the observed current scenario of increased extreme events during breaks. We found that the increased extreme events in the HE decreased the intensity of the 20–60-day mode over the Indian region. This reduction is associated with a reduction of rainfall in active phase and increase in the length of break phase. A reduction in the seasonal mean over India is also observed. The reduction of active phase rainfall is linked with an increased stability of the atmosphere over central India. Lastly, we propose a possible mechanism for the reduction of rainfall in active phase. We found that there is a significant reduction in the strength of the vertical easterly shear over the northern Indian region during break–active transition phase. This basically weakens the conditions for the growth of Rossby wave instability, thereby elongating break phase and reducing the rainfall intensity in the following active phase. This study highlights the redistribution of rainfall intensity among periodic (low-frequency) and non-periodic (extreme) modes in a changing climate scenario, which is further tested in a modeling study. The results presented in this thesis will provide a pathway to understand, using observations and numerical model simulations, the ISV and its relative contribution to the Indian summer monsoon. It can also be used for model evaluation.

The atmospheric boundary layer (ABL) is the lowest layer of the atmosphere where surface effects are felt on time scales of about an hour. While its properties are determined by the surface characteristics, season and synoptic conditions, they in turn determine convective cloud properties and are required for the representation of cloud processes in atmospheric models. Further, interaction of the ABL with the surface layer of the ocean is a key component of ocean-atmosphere coupling. ABL characteristics over ocean surrounding the sub-continent become very important for understanding the monsoon processes during the monsoon season because the roots of many monsoon systems, that give rain to India, are over there. In this thesis data used are from three major field experiments namely the Bay of Bengal Monsoon Experiment (BOBMEX, 1999), Arabian Sea Monsoon Experiment (ARMEX, in two phases, ARMEX-I during 2002 and ARMEX-II in 2003), and Continental Tropical Convergence Zone (CTCZ) experiment (Pilot in 2009) which were carried out under the Indian Climate Research Programme (ICRP). While there have been few studies on ABL characteristics for individual cruises, a comprehensive study considering all available radiosonde data from the above cruises has been missing. This study fills this gap and focuses on the vertical structure of ABL using more than 400 high resolution Vaisala GPS radiosonde data collected over Bay of Bengal and Arabian Sea. The study attempts at first to look at the ABL characteristics of individual cruises and then compare and contrast them over the Bay of Bengal and Arabian Sea. ABL height Hm, estimated by using virtual potential temperature (θv) profile, shows diurnal variation during weak phase of convection while maximum in early morning during active phase of convection. Different variables i.e. moist static energy (h), specific humidity (q), convective available potential energy (CAPE), virtual potential temperature (θv) and equivalent potential temperature (θe) also differ during weak and active convection periods. Conserved variables mixing line approach gives the height up to which ground thermals penetrate in the vertical. This height, denoted by MH that represents the actual ABL height, is 2-3 times larger than Hm when shallow convective clouds are present. In general both Hm and MH are 20-30% larger over Arabian Sea compares to that over Bay of Bengal. Comparison of surface convective available potential energy (CAPE) and equivalent potential temperature (θe) between normal and deficit monsoon years shows that convective instability was as large in deficit years. This means that dynamic and not thermodynamics, controlled the occurrence of convection.