Information from the TRMM-CSH and TRMM-2A12 datasets is used to examine the four-dimensional latent heating (LH) structures over the Asian monsoon region between 1998 and 2006. High sea surface temperatures, ocean-land contrasts and complex terrain produce large precipitation and atmospheric heating rates whose spatial and temporal characteristics are relatively undocumented. Analyses identify interannual and intraseasonal LH variations, with a large fraction of the interannual variability induced by internal intraseasonal variability. Also, the analyses identify a spatial dipole of LH anomalies between the equatorial Indian Ocean and the Bay of Bengal regions occurring during the summer active and suppressed phases of the monsoon intraseasonal oscillation. Comparisons made between the TRMM-CSH and TRMM-2A12 datasets indicate significant differences in the shape of the vertical profile of LH. Comparison of TRMM-LH retrievals with sounding budget observations made during the South China Sea Monsoon experiment shows a high correspondence in the timing of positive LH episodes during rainy periods. Negative values of LH, associated with radiative cooling and with higher troposphere cooling from non-precipitating clouds, are not captured by any of the TRMM datasets. In summary, LH algorithms based on satellite information are capable of representing the spatial and temporal characteristics of the vertically integrated heating in the Asian monsoon region. The TRMM-CSH presents better performance than TRMM-2A12. However, the vertical distribution of atmospheric heating is not captured accurately throughout all different convective phases. It is suggested that satellite derived radiative heating/cooling products are needed to supplement the LH products in order to give an overall better depiction of atmospheric heating.

It has been proposed that the Asian-Australasian monsoon system is influenced by large-scale sea-surface temperature (SST) variability in the three tropical oceans although how this influence is manifested has remained a largely open question. Closure of this issue is important because it is needed to explain trends in monsoon precipitation and circulation that have occurred in the last 30 years. Using an atmospheric general circulation model, we run a series of experiments with different configurations of global SST relating to various epochs occurring during the last century to evaluate their influence on the monsoon. Comparisons of circulation fields show that a colder SST configuration generates a weaker large-scale monsoonal circulation. On the other hand, warmer SST states generate stronger large scale circulations with more vigorous centers of divergence and convergence. Warmer SST configurations are associated with positive anomalies of precipitation in the eastern Bay of Bengal, Eastern Indian Ocean and South East Asia. Cooler SST configurations are associated with negative anomalies of precipitation in the Arabian Sea and Indian peninsula, especially at the beginning of the summer. Since SST gradients determine, to a large degree, the low level flow, they are also going to influence the transport of atmospheric moisture. Comparison of vertically integrated moisture transport fields between the different experiments show that cold SST configuration favors an increased inter-hemispheric flow of moisture but decreases in the westerly moisture flow in to the Bay of Bengal and India. Warm SST configurations, on the other hand, strengthens westerly flow into the eastern Indian Ocean. An increasing availability of moisture in a region of stronger convergence constitutes a favorable environment for the production of monsoonal precipitation. African easterly waves (AEW) constitute an important component of the African and tropical Atlantic Ocean climate during the boreal summer. An understanding of this component is essential since AEW are closely related with tropical Atlantic storm activity. We adopt an idealized modeling approach using the WRF model initialized with ERA-40 reanalysis data to study the mechanisms that trigger the formation and maintenance of AEW. The model domain includes the African continent, central and eastern Atlantic Ocean and the western Indian Ocean. Experiments are designed to test the relative importance of the thermal effect of the eastern African topography and the influence of the cross-equatorial pressure gradient, induced by the sea surface temperature (SST) on the origins and maintenance of AEW. Topography and SST variation are selectively added and removed. The control experiment shows that the model reproduces many of the mean features observed during the boreal summer. Westward propagating disturbances of 3-8 day period that originate between 30 and 40E at the surface levels and in the mid troposphere are well depicted. In addition, the model provides a reasonable representation of the AEJ. When all topographic features are removed, there is a weakening of the AEJ over land and ocean, however, longitude-time sections of meridional velocity still exhibit westward propagating disturbances that reach the western African coast at the surface and at the jet level with the same 3-8 day period. Spectral analysis of meridional velocity show that the variability associated with AEWs is reduced over East Africa and West Africa at 850-hPa and is reduced west of 20E along the southern flank of the jet and over northern Africa at the jet level. Maximum amplitude of the disturbances occurs right at the coast. The spatial distribution of barotropic and baroclinic energy conversions explains the reduction in AEWs over land and the intensification of these features at the coast. When the zonal SST gradient is removed, a weaker AEJ displaces southward and a weaker monsoon flow ensues. Spectral analysis of meridional velocity displays a variance reduction in the 3-8 day band at the 850-hP a level in western and eastern Africa and at the coast. At the 650-hPa level significant changes are not observed at the latitude of the AEJ (15N), however, a decrease in the variance associated with AEW occurs at the southern flank of the jet. A southward displacement of the jet favors a weakening of the baroclinic energy conversions. Barotropic conversions also appear to be weaker when the SST gradient is removed. The present study suggests that orography plays an important role in determining the variability of meridional wind associated with AEW over Eastern Africa at the lower levels. Further, zonal SST gradients over the Atlantic favor intensification of waves when they reach the coast and the maintenance of disturbances across the Ocean. Also, results could suggest that SST gradients support genesis of AEW just off the coast of Africa.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references. / The Indian monsoon affects the livelihood of over one billion people. Despite the importance of climate to society, knowledge of long-term monsoon variability is limited. This thesis provides Holocene records of monsoon variability, using sediment cores from river-dominated margins of the Bay of Bengal (off the Godavari River) and the Arabian Sea (off the Indus River). Carbon isotopes of terrestrial plant leaf waxes ([delta]¹³Cwax) preserved in sediment provide integrated and regionally extensive records of flora for both sites. For the Godavari River basin the ([delta]¹³Cwax record shows a gradual increase in aridity-adapted vegetation from ~4,000 until 1,700 years ago followed by the persistence of aridity-adapted plants to the present. The oxygen isotopic composition of planktonic foraminifera from this site indicates drought-prone conditions began as early as -3,000 years BP. The aridity record also allowed examination of relationships between hydroclimate and terrestrial carbon discharge to the ocean. Comparison of radiocarbon measurements of sedimentary plant waxes with planktonic foraminifera reveal increasing age offsets starting -4,000 yrs BP, suggesting that increased aridity slows carbon cycling and/or transport rates. At the second site, a seismic survey of the Indus River subaqueous delta describes the morphology and Holocene sedimentation of the Pakistani shelf and identified suitable coring locations for paleoclimate reconstructions. The ([delta]¹³Cwax record shows a stable arid climate over the dry regions of the Indus plain and a terrestrial biome dominated by C₄ vegetation for the last 6,000 years. As the climate became more arid ~4,000 years, sedentary agriculture took hold in central and south India while the urban Harappan civilization collapsed in the already arid Indus basin. This thesis integrates marine and continental records to create regionally extensive paleoenvironmental reconstructions that have implications for landscape evolution, sedimentation, the terrestrial organic carbon cycle, and prehistoric human civilizations in the Indian subcontinent. / by Camilo Ponton. / Ph.D.

In the current study, we have presented a systematic analysis of the diurnal cycle of rainfall over the Indian region using satellite observations, and evaluated the ability of the Weather Research and Forecasting Model (WRF) to simulate some of the salient features of the observed diurnal characteristics of rainfall. Using high resolution simulations, we also investigate the underlying mechanisms of some of the observed diurnal signatures of rainfall. Using the Tropical Rain-fall Measuring Mission (TRMM) 3-hourly, 0.25 ×0.25 degree 3B42 rainfall product for nine years (1999-2007), we extract the finer spatial structure of the diurnal scale signature of Indian summer monsoon rainfall. Using harmonic analysis, we construct a signal corresponding to diurnal and sub-diurnal variability. Subsequently, the 3-hourly time-period or the octet of rain-fall peak for this filtered signal, referred to as the “peak octet,” is estimated with care taken to eliminate spurious peaks arising out of Gibbs oscillations. Our analysis suggests that over the Bay of Bengal, there are three distinct modes of the peak octet of diurnal rainfall corresponding to 1130, 1430 and 1730 IST, from north central to south Bay. This finding could be seen to be consistent with southward propagation of the diurnal rainfall pattern reported by earlier studies. Over the Arabian sea, there is a spatially coherent pattern in the mode of the peak octet (1430 IST), in a region where it rains for more than 30% of the time. In the equatorial Indian Ocean, while most of the western part shows a late night/early morning peak, the eastern part does not show a spatially coherent pattern in the mode of the peak octet, owing to the occurrence of a dual maxima (early morning and early/late afternoon). The Himalayan foothills were found to have a mode of peak octet corresponding to 0230 IST, whereas over the Burmese mountains and the Western Ghats (west coast of India) the rainfall peaks during late afternoon/early evening (1430-1730 IST). This implies that the phase of the diurnal cycle over inland orography (e.g., Himalayas) is significantly different from coastal orography (e.g., Western Ghats). We also find that over the Gangetic plains, the peak octet is around 1430 IST, a few hours earlier compared to the typical early evening maxima over land. The second part of our study involves evaluating the ability of the Weather Research and Fore-casting Model (WRF) to simulate the observed diurnal rainfall characteristics. It also includes conducting high resolution simulations to explore the underlying physical mechanisms of the observed diurnal signatures of rainfall. The model (at 54km resolution) is integrated for the month of July 2006 since this period was particularly favourable for the study of diurnal cycle. We first evaluate the sensitivity of the model to the prescribed sea surface temperature (SST) by using two different SST datasets, namely Final Analyses (FNL) and Real-time Global (RTG). The overall performance of RTG SST was found to be better than FNL, and hence it was used for further model simulations. Next, we investigated the impact of different parameterisations (convective, microphysical, boundary layer, radiation and land surface) on the simulation of diurnal cycle of rainfall. Following this sensitivity study, we identified the suite of physical parameterisations in the model that “best” reproduces the observed diurnal characteristics of Indian monsoon rainfall. The “best” model configuration was used to conduct two nested simulations with one-way, three-level nesting (54-18-6km) over central India and Bay of Bengal. While the 54km and 18km simulations were conducted for July 2006, the 6km simulation was carried out for the period 18-24 July 2006. This period was chosen for our study since it is composed of an active period (19-21 July 2006), followed by a break period (22-24 July 2006). At 6km grid-spacing the model is able to realistically simulate the active and break phases in rainfall. During the chosen active phase, we find that the observed rainfall over central India tends to reach a maximum in the late night/early morning hours. This is in contrast to the observed climatological diurnal maxima of late evening hours. Interestingly, the 6km simulation for the active phase is able to reproduce this late night/early morning maxima. Upon further analysis, we find that this is because of the strong moisture convergence at the mid-troposphere during 2030-2330 IST, leading to the rainfall peak seen during 2330-0230 IST. Based on our analysis, we conclude that during both active and break phases of summer monsoon, mid-level moisture convergence seems to be one of the primary factors governing the phase of the diurnal cycle of rainfall. Over the Bay of Bengal, the 6km model simulation is in very good agreement with observations, particularly during the active phase. The southward propagation observed during 19-20 July 2006, which was not captured by the coarse resolution simulation (54km), is exceedingly well captured by the 6km simulation. The positive anomalies in specific humidity attain a maxima during 2030-0230 IST in the north and during 0830-1430 IST in the south. This confirms the role of moisture convergence in the southward propagation of rainfall. Equally importantly we find that while low level moisture convergence is dominant in the north Bay, it is the mid-level moisture convergence that is predominant in the south Bay.

No description available.

Le climat asiatique est aujourd'hui caractérisé par une forte dualité entre un climat de moussons au Sud-Est et un climat aride en Asie centrale. Ces climats sont tous les deux définis par une saisonnalité marquée, que ce soit en terme de précipitations pour le premier ou de températures pour le second. Si l'intensification des moussons asiatiques au Néogène, liée à l'influence du soulèvement final du plateau tibétain sur les climats asiatiques, semble faire consensus dans la communauté scientifique, la caractérisation des climats paléogènes est encore peu établie. Ainsi la question de savoir quand cette dualité climatique s'est installée en Asie reste encore ouverte. Au Paléogène, les reliefs liés à la collision entre les plaques indienne et eurasiatique étaient encore naissant et la distribution entre les terres et les mers très différente de l'actuelle. Notablement, une vaste mer épicontinentale et peu profonde (la Proto-Paratethys) s'étendait à travers l'Europe et l'Asie Centrale. À la fin du Paléogène, la Proto-Parathetys se retire de l'Asie Centrale, et les hautes topographies asiatiques se mettent en place. Dans ce contexte géodynamique, cette thèse cherche à caractériser les fluctuations à haute fréquence du climat en Asie Centrale afin de comprendre l'évolution de la saisonnalité au cours du Paléogène, et plus précisément pendant la période de l'Éocène (-55 à -34 Ma). Pour cela une approche originale utilisant une méthode géochimique multi- proxy sur des coquilles d'huîtres a été établie. Grâce à l'apport de l'analyse incrémentielle de marqueurs élémentaires et isotopiques sur les coquilles nous accédons aux variations saisonnières de la température et de la salinité de l'eau de mer. Ceci nous permet de mieux cerner les bilans hydriques et thermiques à l'échelle de l'année et ainsi de caractériser le climat d'Asie Centrale à très haute résolution. Combinant cette approche géochimique avec une étude sédimentologique et une étude numérique à plus grande échelle, cette thèse cherche à mieux établir les causes de l'évolution du climat régional au cours du Paléogène. / The modern Asian climate is mainly characterized by a monsoonal duality between humid summers in southern and eastern Asia and arid winters in Central Asia resulting in a strong seasonality in terms of precipitation and temperature in these respective regions. Although Neogene monsoonal intensification - mainly attributed to Tibetan plateau uplift - is well established, Paleogene Asian climate is still poorly understood such that the question of how and when this climate duality was established remains open. During Paleogene times, paleoreliefs due to the ongoing Indo-Asia collision and the land-sea distribution were very different compared to modern. Notably, a shallow epicontinental sea (the Proto-Paratethys) covered part of Europe and Central Asia. During the Eocene (-55 to -34 Ma), the Proto-Paratethys retreated westward while high Asian topographies formed. In this peculiar context, this PhD thesis aims to characterize the evolution of high-frequency climatic fluctuations in Central Asia in order to better constrain the seasonality changes associated with sea retreat, topographic uplift or nascent monsoons. We develop a novel approach using a geochemical multi-proxy methodology on oyster shells. Thanks to incremental analyses of elements and isotopes on bivalve shells, we estimate seasonal variations of temperature and salinity in seawater at high resolution. This enables to constrain precisely the annual-scale water and thermal balances and, by applying this technique to successive oyster bearing deposits widely distributed over Central Asia, aims to characterize Central Asian climate evolution. Combining this geochemical approach with a sedimentological and a numerical studies at larger time- and geographic- scale, this PhD thesis is aiming at better understanding the causes of the Eocene regional climate evolution.

L'objectif principal de cette thèse est de restituer l'évolution passée des moussons asiatiques au cours du Quaternaire et d’en évaluer leurs impacts sur l’érosion continentale et les transferts sédimentaires terre-mer, à partir de l’étude de carottes marines collectées dans la Baie du Bengale, l'ouest de la mer des Philippines et la mer d'Arabie. La stratégie scientifique mise en œuvre implique des analyses minéralogiques (argiles), sédimentologiques (granulométrie laser) et géochimiques (⁸⁷Sr/⁸⁶Sr et εNd) afin de restituer les zones sources sédimentaires, les conditions d’érosion et de transfert sédimentaires à l’océan. Les analyses de la concentration en élément des terres rares et des valeurs de l’εNd ont également été faites sur des échantillons d’eau de mer et de foraminifères collectés dans la Baie du Bengale afin de contraindre l’utilisation de ce traceur dans un contexte de très forts changements saisonniers de débit des fleuves Himalayens. Cette stratégie nous a permis, entre autre, de restituer les précipitations de mousson du domaine ouest tropical Pacific au cours du Quaternaire et d’établir un lien avec l’évolution à long terme de la dynamique de circulation méridienne de type ENSO. Nous avons également apporté de nouvelles contraintes sur l’utilisation du traceur εNd dans les foraminifères de la Baie du Bengale en vue d’en restituer la dynamique passée de l’érosion himalayenne. / The main objective of this PhD study is to reconstruct the evolution of the Asian monsoons during the Quaternary and their impacts on the continental erosion and sedimentary transfers from land to sea by the investigation of sediments cores collected in the Northern Bay of Bengal, the western Philippines Sea and the Arabian Sea. The implemented scientific strategy involves mineralogical (clay size fraction), sedimentological (grain-size laser) and geochemical (⁸⁷Sr/⁸⁶Sr and εNd) analyses in order to establish sedimentary sources, conditions of erosion and transfer of sediments to the Ocean. The analyses of the concentration of Rare Earth Elements (REE) and εNd were also made on seawater and foraminifera samples to better constrain the εNd as a proxy of weathering in a context of strong seasonal variations of sediment discharges by Himalayan rivers. Clay mineralogy and laser grain-size analyses have been conducted on sediments from core MD06-3050 collected on the Benham Rise (Philippines Sea). Siliciclastic grain-size results indicate variations of the relative proportion of three grain-size sub-populations corresponding to eolian dusts (EM2 about 9-11 μm) and Luzon rivers inputs (EM1 about 2-5 μm and EM3 about 19-25 μm). The long-term evolutions of the EM1/EM2 and smectite/(illite+chlorite) ratios permit to reconstruct variations of the contribution of detrital material deriving from the volcanic arc of Luzon and rainfall intensity of this tropical region. At long time scale, periods of intensification of monsoon rainfall on Luzon are associated to a reduction of precipitation on central China. These periods are also associated to an increase of the zonal gradient of sea surface temperatures on the equatorial Pacific Ocean suggesting a strengthening of El Niña conditions. These results highlight for the first time a strong role of the dynamics of the meridian circulation of ENSO on the long-term changes of rainfall of the tropical western Pacific during the Quaternary. In the Arabian Sea, clay mineralogy, siliciclastic grain-size, ⁸⁷Sr/⁸⁶Sr ratio and εNd were analysed on Quaternary sediments of the IODP site U1457. Our results suggest a change in the relative proportions of sediments from the Deccan Trapps (smectite) and the Indus river (mainly illite and chlorite). Variability of sedimentary sources and sediment transport (turbidites activity) to the Indus Fan have been reconstructed and attributed to monsoon rainfall and the sea level variations. The concentrations of REE combined with εNd were analysed on seawater samples collected in June 2012 along a North-South cross section in the Bay of Bengal. We highlighted from normalized REE patterns that the contributions of dissolved REE from the Ganges-Brahmaputra river system was the main source of the dissolved REE of surface waters of the Bay of Bengal, whereas the desorption of lithogenic particles dominate the dissolved REE of the intermediate and deep waters masses. We then revalued the residence time of the dissolved REE in the Bay of Bengal. A comparison of εNd, obtained just before the increase of the Ganges-Brahmaputra river discharge inferred by Indian monsoon rainfall, with the results obtained by Singh and al. (2012) for seawater samples collected after the peak of river discharge, allowed us to highlight for the first time a seasonal variability of seawater εNd of the Bay of Bengal. εNd have been analysed on planktonic foraminiferas of core MD77-176 located at 1375 m water depth to reconstruct for the first time the seawater εNd record of the intermediate waters masses of northern Bay of Bengal for the last 27 kyr. This new seawater εNd record of the Northern Bay of Bengal give us new constrain for this proxy already used to reconstruct past changes of the Himalayan weathering.

The Miocene of the Lidam area, Eastern Province, Saudi Arabia, was studied to examine the interaction of glacio-esustasy during moderate Antarctic glaciation, within a small back bulge basin on the slowly subsiding distal Arabian foreland, distal from the active Zagros fold-thrust belt. Low subsidence rates of 1 to 4 cm/k.y generated the long-term accommodation, which were considerable slower than those in the proximal foredeep in Iran. Deposition of the siliciclastics was driven by lowered sea levels, and moderately humid to arid climate. Rising sea levels pushed the siliciclastics updip allowing mixed siliciclastics and carbonates to form downdip, under semi-arid climate and locally hypersaline conditions. Maximum transgression slightly predated the Middle Miocene climatic optimum when prograding siliciclastics migrated across the platform. Falling triggered siliciclastic deposition under semi-arid climate. Sequences appear to relate to long-term obliquity (~1.2 m.y. cycles) and long-term eccentricity (400 k.y.) cycles. The succession contains numerous missing beats reflecting the updip position of the study area, and sea level changes of tens of meters that frequently exposed the platform. Siliciclastic units commonly are incised into muddy sediments beneath sequence boundaries. Multiple exposure surfaces occur within Hadrukh brecciated palustrine carbonates. Within Dam carbonates, parasequence boundaries commonly are capped by tidal flat laminites (some of which are incipiently brecciated). High frequency negative excursions of ∂¹³C within the succession appear to relate to near-surface diagenesis by soil gas depleted in ∂¹³C beneath sequence boundaries. Positive C isotope excursions in the Lidam Miocene section can be tied to similar excursions in Qatar and UAE, where Sr isotope dates constrain the ages of the units. The overall C isotope profile at Lidam shows depleted values early in the Miocene to heavy values in the Middle Miocene, becoming lighter again in the late Miocene. The profile appears to follow the long-term global ∂¹³C curve. Incursion of meteoric groundwaters into the study area was driven by the long-term global sea level changes. Oxygen isotopes are surprisingly light, extending down to -12.5 ‰VPDB. The very light δ¹⁸O values of the meteoric waters may be explained by rainfall associated with enhanced Miocene Indian monsoons, and with far travelled air mass trajectories migrating across north Africa and from the polar region. / Ph. D.

L’objectif de cette thèse est d’étudier les caractéristiques physiques et biogéochimiques de l’habitat épipélagique (0-200 m), ses variabilités spatio-temporelles et les processus qui les gouvernent dans le Golfe de Tadjourah (Djibouti). L’analyse spectrale singulière (SSA) et la fonction empirique orthogonale (EOF) sont appliquées à deux jeux des données satellitales dérivées du radiomètre AVHRR_MetopA et des capteurs de la couleur de l’océan (Modis et Meris). Cette analyse statistique montre que les variabilités de la température de la surface de la mer (SST) et de la concentration de la chlorophylle a (CHLa) sont essentiellement expliquées par les cycles annuels et semi-annuels. Le cycle annuel de la SST montre l’alternance des eaux chaudes d’avril à octobre et des eaux froides de novembre à mars. Le cycle semi-annuel indique une légère baisse de la SST entre juillet et aout, particulièrement à l’ouest du golfe. Pour la CHLa, la variabilité est entièrement représentée par le cycle annuel. Celui-ci indique l’enrichissement des eaux du large avec un fort gradient côte-large de juillet à novembre et une tendance inverse de décembre à juin. En outre, l’analyse spectrale singulière multi-canal (M-SSA) et la fonction de corrélation croisée avec fenêtre de 120 jours, appliquées à l’ensemble des paramètres océanique (SST et CHLa) et atmosphériques (Vent, Température et humidité spécifique de l’air) révèlent que le cycle annuel de la SST est relié aux flux de chaleur à l’interface air-mer. En revanche, le refroidissement des eaux durant juillet-aout associé au pic de la CHLa, a été attribué au phénomène d’upwelling. Dans la seconde partie, afin d’élucider l’influence océanique sur la SST et la CHLa, les structures thermohalines and biogéochemiques de la couche supérieure (0-200 m) sont étudiées à l’aide des données collectées durant juillet-aout 2013, septembre 2013 et Février 2014. Les résultats montrent qu’en juillet-aout, la couche superficielle se composait d’une couche de mélange (CM) s’étendant sur environ 20-30 m de profondeur, suivie d’une thermocline localisée entre 30 and 50 m. La CM était réduite à l’ouest et au sud-est du golfe où le gradient thermique et la CHLa étaient plus élevées proche de la surface. En septembre, cette stratification persistait mais la CM était plus chaude et salée. En Février, la CM s’étendait sur environ 120 m de profondeur et la thermocline était moins prononcée. La comparaison des courants mesurés avec les courants de dérive d’ekman et les courants géostrophiques ont révélé que les structures thermohalines et biogéochimiques sont influencées par les vents de moussons du Sud-Ouest (MSO) et du Nord-Est (MNE). Les MSO qui soufflent de juin à aout, déplacent les eaux de surface du Golfe de Tadjourah vers le Golfe d’Aden et induisent la remontée des eaux profondes à l’ouest et l’intrusion par le nord-est des eaux salée de la thermocline. Celle-ci se rapproche de la surface particulièrement à l’ouest où elle se mélange avec les eaux de surface. En revanche, les vents de moussons du nord-est (MNE) prédominants de novembre à mai, emmènent les eaux froides vers le Golfe de Tadjourah. Le mélange convectif profond épaissit la CM. Cette thèse montre que les vents de moussons et leur renverse saisonnière jouent un rôle crucial dans la stratification de la colonne d’eau et que la topographie du bassin influence et module leurs effets. Durant l’été, la forme en cuvette du bassin et la pente continentale plus allongée à l’ouest favorise l’upwelling à l’ouest du golfe où les anomalies de la SST et de fortes concentrations de la CHLa ont été observées. / The objective of this thesis is to study the physical and biogeochemical characteristics of the pelagic habitat (0-200m), its spatio-temporal variability and the processes that govern them in the Gulf of Tadjourah (Djibouti).Singular spectrum analysis (SSA) and empirical orthogonal function (EOF) were applied to two data sets derived from AVHRR_MetopA radiometry and sea colour sensors (Modis and Meris). These statistical analysis shows that the variability of sea surface temperature (SST) and chlorophyll a concentration (CHLa) are mainly explained by annual and semi-annual cycles. The annual cycle of SST consists of an alternation between warmer (April to October) and cooler (November to March) surface water. The semi-annual cycle shows a slight drop of SST between July and August, particularly in the west of the gulf. For the CHLa, the variability is fully dominated by the annual cycle indicating summer enrichment of seashore water (July-November) with a strong seaward gradient which is reversed from October to May. Multichannel spectrum analysis (M-SSA) and cross correlation function analysis applied to the oceanic (SST, CHL-a) and atmospheric parameters (wind speed, air temperature and humidity) showed that the annual cycle of SST is connected to heat flux at the air-sea interface, while the summer drop of SST and its associated CHL-a increase were attributed to upwelling. In the second part, in order to elucidate this oceanic influence on SST and CHL-a, the vertical thermohaline and biogeochemical structures of the upper layer (0-200 m) were studied using high-resolution hydrographic data collected in July-August 2013, September 2013 and February 2014. During summer, the superficial layer consisted of the mixed layer (ML) extending to a depth of about 20-30 m, followed by the thermocline located between 30 and 50 m depth. The ML was thicker in the west and the southeast where the thermal gradient and chlorophyll a concentrations were particularly high. During September, this stratification persisted but the ML became warmer and saltier and the thermocline moved slightly deeper. In February, the ML extended to about 120 m, and the thermocline was less pronounced. A comparison of the directly measured currents to the wind induced Ekman currents and to geostrophic current profiles revealed that the thermohaline and the biogeochemical features were related to the southeast and northeast monsoon winds (SWM & NEM). Between June and August, the SWM drives surface water from the Gulf of Tadjourah to the Gulf of Aden and thus induces the westward intrusion of high salinity thermocline water from the Gulf of Aden. This near surface flow mixes surface waters in the extreme west of the Gulf of Tadjourah. In contrast, the NEM which blow from September to May, bring cold water toward the Gulf of Tadjourah and thickens the ML through convective mixing. Our thesis shows that the monsoon winds and their seasonal reversal play a crucial role in the stratification of the water column, but that their effects are additionally influenced by basin topography. During summer the bowl-shape of the basin and its elongated slope in the west enhance the upwelling in this area where negative sea surface temperature anomalies and high chlorophyll a concentrations were observed.

Thesis (Ph.D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2000. / Includes bibliographical references (p. 217-222). / Estimation of the upper ocean heat budget from one year of observations at a moored array in the north central Arabian Sea shows a rough balance between the horizontal advection and time change in heat when the one-dimensional balance between the surface heat flux and oceanic heat content breaks down. The two major episodes of horizontal advection, during the early northeast (NE) and late southwest (SW) monsoon seasons, are both associated with the propagation of mesoscale eddies. During the NE monsoon, the heat fluxes within the mixed layer are not significantly different from zero, and the large heat flux comes from advected changes in the thermocline depth. During the SW monsoon a coastal filament exports recently upwelled water from the Omani coast to the site of the array, 600 km offshore. Altimetry shows mildly elevated levels of surface eddy kinetic energy along the Arabian coast during the SW monsoon, suggesting that such offshore transport may be an important component of the Arabian Sea heat budget. The sea surface temperature (SST) and mixed layer depth are observed to respond to high frequency (HF, diurnal to atmospheric synoptic time scales) variability in the surface heat flux and wind stress. The rectified effect of this HF forcing is investigated in a three-dimensional reduced gravity thermodynamic model of the Arabian Sea and Indian Ocean. Both the HF heat and wind forcing act locally to increase vertical mixing in the model, reducing the SST. Interactions between the local response to the surface forcing, Ekman divergences, and remotely propagated signals in the model can reverse this, generating greater SSTs under HF forcing, particularly at low latitudes. The annual mean SST, however, is lowered under HF forcing, changing the balance between the net surface heat flux (which is dependent on the SST) and the meridional heat flux in the model. A suite of experiments with one-dimensional upper ocean models with different representations of vertical mixing processes suggests that the rectified effect of the diurnal heating cycle is dependent on the model, and overstated in the formulation used in the three-dimensional model. / by Albert Sok Fischer. / Ph.D.