Ce travail de thèse est une étude multi-échelles de temps de la morphodynamique d'une île barrière soumise à des conditions de forçages paroxysmaux de mousson, de tempêtes tropicales et de typhons. Cette étude se concentre dans un premier temps, sur un large travail de terrain effectué dans le cadre du projet KUN-SHEN. Sept mois de mesures (de novembre 2011 à janvier 2012 et de mai à septembre 2012) ont permis l'acquisition d'un jeu de données inédit dans la littérature, exhaustif et de haute résolution temporelle et spatiale. Des mesures hydrodynamiques du large jusqu’à la zone de jet de rive ont été acquises en parallèle d'un suivi topographique de la plage émergée. Les mesures de vagues ont été acquises à partir d'une bouée au large, de deux houlographes-courantomètres en avant-côte (mesures en continu à 2 Hz) ainsi que de pressiomètres enfouis dans la plage sub-aérienne (mesures en continu à 5 Hz). Les levés topo-bathymétriques ont été réalisés au D-GPS (résolution centimétrique) une fois par semaine pendant la saison de mousson et juste après et avant chaque événements de tempêtes extrêmes pendant la saison des typhons. Le travail d'analyse cible ensuite la morphodynamique de la zone de jet de rive et de la plage émergée sur un panel d’échelles spatio-temporelles allant de l'instantanée à annuelle. A l’échelle instantanée, ce sont les variations d’élévation de la surface libre de l'eau et du lit sableux (quelques secondes) qui sont observées au cours de chacune des phases (montant, apex, tombant) de la tempête tropicale Talim (juin 2012, Hs = 10.34 m et Ts = 14.6 s). Pour le même événement de tempête, la réponse morphologique de toute la plage émergée est ensuite décrite et quantifiée dans le détail à l’échelle événementielle (quelques jours). Les bilans sédimentaires de chacune des saisons sont ensuite quantifiés dans le but de caractériser la dynamique saisonnière (quelques mois) à annuelle d'une barrière sableuse soumise à deux types de forçages extrêmes différents (mousson/typhons). L'impact sur le front de plage, des groupes de tempêtes faibles à modérées pendant l'hiver est ainsi comparé à l'impact des tempêtes extrêmes pendant l'été. On souligne finalement 1) l'importance d'un jeu de mesures in-situ de bonne qualité dans un travail d'analyse morphodynamique, 2) l'essentialité de l’emboîtement des échelles spatio-temporelle ainsi que 3) le rôle du profil morphologique héritée dans la réponse morphologique d'une plage émergée lorsqu'elle est soumise à des conditions de forçages de vent et de vague extrêmes. / This work focuses on a multi-scale analysis of a sand barrier morphodynamics exposed to paroxysmal forcings of monsoon, tropical storms, and typhoons. The study provided a large dataset of in-situ measures acquired in the framework of the KUNSHEN project. Seven months of field work (from November 2011 to January 2012 and from May to September 2012) provided a new, exhaustive and hight resolution measures. Measures of hydrodynamic conditions from offshore to the swash zone are acquired as well as high frequency topographic surveys of the emerged beach. Waves monitoring are from an offshore buoy, two current profilers in the shoreface (continuous measures at 2 Hz) and pressure sensors buried in the sub-aerial beach (continuous measures at 5 Hz). The topo-bathymetric surveys were acquired using D-GPS (centimetric resolution) one time a week during winter season of monsoon and just before and after each extreme storm during summer season of typhoons.Then, analysis focus in the morphodynamics of the swash zone and the emerged beach on a range of scales from instantaneous to one year. At the instantaneous scale, we analyze variations of the water surface fluctuation and sand bed elevation (seconds) during each phases (rising, apex and falling) of the Talim tropical storm (June 2012, Hs = 10.34 m and Ts = 14.6 s). Concerning the same storm, we characterize the morphologic changes of the whole emerged beach at the evenemential scale (days). Sand balances of each seasons are quantified in order to characterize the dynamics from seasonal scale (months) to annual scale of a sand barrier driven by two types of extreme forcings (monsoon/typhoons). On the beach-front, impact of groups of winter weak storms is compared to the impact of extremes tropical storms and typhoons. To conclude, we highlight 1) importance of in-situ measures in morphodynamic studies, 2) the major role of scales stacking and 3) the role of herited beach profile in the morphologic responses of the emerged beach driven by extremes forcings of wind and wave.

The climate of West Africa is characterised by a monsoonal system that brings rainfall onto the subcontinent during an annual rainy season. From the late 60's to the mid-90's, rainfall levels significantly below average were observed, which brought severe socio-economic implications. The causes of the uncharacteristically long drought period, and indeed the mechanisms underpinning West African climate were poorly understood at the time, but have since attracted growing attention from the scientific community. Amongst the factors identified as critical is the interaction between the Earth surface and the atmosphere. To investigate these interactions over West Africa we have adopted an approach based upon regional climate modelling, an internationally recognised discipline enabling the representation of past and future climates, and the study of specific meteorological mechanisms. Using the regional climate model MAR, we have carried out simulations of the West African climate for the years 1986, 1987, and 1988. To improve the accuracy with which the model represents the biosphere, a new dataset describing the local vegetation was incorporated and a new scheme for the representation of roots implemented. A measure of the internal variability inherent to all results produced with this, and other such models, was determined. Subsequently, the influence of soil moisture anomalies on the model behaviour was investigated. The latest version of the model was validated by comparing it to observational data for selected years. Our results have prooven the ability of the improved MAR to simulate the West African climate, its monsoon and its spatial and temporal behaviour and provide strong evidence of its suitability for further investigation of the surface-atmosphere interactions over West Africa.

The climate of West Africa is characterised by a monsoonal system that brings rainfall onto the subcontinent during an annual rainy season. From the late 60's to the mid-90's, rainfall levels significantly below average were observed, which brought severe socio-economic implications. The causes of the uncharacteristically long drought period, and indeed the mechanisms underpinning West African climate were poorly understood at the time, but have since attracted growing attention from the scientific community. Amongst the factors identified as critical is the interaction between the Earth surface and the atmosphere. To investigate these interactions over West Africa we have adopted an approach based upon regional climate modelling, an internationally recognised discipline enabling the representation of past and future climates, and the study of specific meteorological mechanisms. Using the regional climate model MAR, we have carried out simulations of the West African climate for the years 1986, 1987, and 1988. To improve the accuracy with which the model represents the biosphere, a new dataset describing the local vegetation was incorporated and a new scheme for the representation of roots implemented. A measure of the internal variability inherent to all results produced with this, and other such models, was determined. Subsequently, the influence of soil moisture anomalies on the model behaviour was investigated. The latest version of the model was validated by comparing it to observational data for selected years. Our results have prooven the ability of the improved MAR to simulate the West African climate, its monsoon and its spatial and temporal behaviour and provide strong evidence of its suitability for further investigation of the surface-atmosphere interactions over West Africa.

The Indian summer monsoon (ISM) is one of the largest climate systems on earth and impacts the livelihood of nearly 40% of the world’s population. Despite dedicated efforts, a comprehensive picture of monsoon variability has proved elusive largely due to the absence of long term high resolution records, spatial inhomogeneity of the monsoon precipitation, and the complex forcing mechanisms (solar insolation, internal teleconnections for e.g., El Niño-Southern Oscillation, tropical-midlatitude interactions). My work aims to improve the understanding of monsoon variability through generation of long term high resolution palaeoclimate data from climatically sensitive regions in the ISM and westerlies domain. To achieve this aim I have (i) identified proxies (sedimentological, geochemical, isotopic, and mineralogical) that are sensitive to environmental changes; (ii) used the identified proxies to generate long term palaeoclimate data from two climatically sensitive regions, one in NW Himalayas (transitional westerlies and ISM domain in the Spiti valley and one in the core monsoon zone (Lonar lake) in central India); (iii) undertaken a regional overview to generate “snapshots” of selected time slices; and (iv) interpreted the spatial precipitation anomalies in terms of those caused by modern teleconnections. This approach must be considered only as the first step towards identifying the past teleconnections as the boundary conditions in the past were significantly different from today and would have impacted the precipitation anomalies. As the Spiti valley is located in the in the active tectonic orogen of Himalayas, it was essential to understand the role of regional tectonics to make valid interpretations of catchment erosion and detrital influx into the lake. My approach of using integrated structural/morphometric and geomorphic signatures provided clear evidence for active tectonics in this area and demonstrated the suitability of these lacustrine sediments as palaleoseismic archives. The investigations on the lacustrine outcrops in Spiti valley also provided information on changes in seasonality of precipitation and occurrence of frequent and intense periods (ca. 6.8-6.1 cal ka BP) of detrital influx indicating extreme hydrological events in the past. Regional comparison for this time slice indicates a possible extended “break-monsoon like” mode for the monsoon that favors enhanced precipitation over the Tibetan plateau, Himalayas and their foothills. My studies on surface sediments from Lonar lake helped to identify environmentally sensitive proxies which could also be used to interpret palaeodata obtained from a ca. 10m long core raised from the lake in 2008. The core encompasses the entire Holocene and is the first well dated (by 14C) archive from the core monsoon zone of central India. My identification of authigenic evaporite gaylussite crystals within the core sediments provided evidence of exceptionally drier conditions during 4.7-3.9 and 2.0-0.5 cal ka BP. Additionally, isotopic investigations on these crystals provided information on eutrophication, stratification, and carbon cycling processes in the lake. / Der Indische Sommer Monsun (ISM) ist eines der bedeutendsten Klimaphänomene auf der Erde und hat großen Einfluss auf die Lebensbedingungen und -grundlagen von nahezu 40% der Weltbevölkerung. Trotz großer Bemühungen ist es bisher nicht gelungen ein genaues und umfassendes Verständnis der Monsun-Variabilität zu gewinnen. Hauptgründe dafür sind das Fehlen von langjährigen und hochaufgelösten Klimazeitreihen, räumlichen Inhomogenitäten in den Niederschlagsverteilungen und die Komplexität der treibenden klimatischen Mechanismen (Sonneneinstrahlung, interne Wechselwirkungen des Klimasystems, wie z.B. zwischen Tropen und mittleren Breiten oder die Auswirkungen der El Niño Oszillation). Die Zielsetzung der hier vorgestellten Arbeit ist ein verbessertes Verständnis der Monsun-Variabilität zu entwickeln, auf Basis von hochaufgelösten und weit reichenden Paläoklimazeitreihen aus klimasensitiven Regionen des ISM und der Westwindzone. Um die Zielsetzung umzusetzen habe ich: (i) Proxys identifiziert (sedimentologische, geochemische, isotopische, und mineralogische), die empfindlich auf Umweltveränderungen reagieren; (ii) die identifizierten Proxys zur Erzeugung von langjährigen Paläoklima-Daten für zwei klimasensible Regionen verwendet, eine im NW des Himalaja (Übergangs-Westwindzone und ISM Gebiet von Spity Valley) und eine in der Kernzone des Monsun (Lonar-See) in Zentralindien; (iii) Übersichts-"Momentaufnahmen" der regionalen klimatischen Bedingungen für ausgewählte Zeitpunkte der Vergangenheit erzeugt; und (iv) räumliche Niederschlagsanomalien in Hinblick auf heutige Wechselbeziehungen im Klimasystem interpretiert. Dieser Ansatz stellt allerdings nur einen ersten Schritt zur Identifizierung von paläoklimatischen Wechselbeziehungen im Monsunsystem dar, da sich die Randbedingungen in der Vergangenheit deutlich von den heutigen unterscheiden und diese einen signifikanten Einfluss auf die Niederschlagsanomalien haben. Da das Spity Valley im tektonisch aktiven Himalaja-Orogen lokalisiert ist, ist es von entscheidender Bedeutung die regionalen tektonischen Prozesse zu verstehen, um Erosionsvorgänge des Einzugsgebiets und die Einfuhr von Detritus in den See korrekt interpretieren zu können. Mein Ansatz der Nutzung kombinierter strukturell/morphometrischer und geomorphologischer Charakteristiken lieferte klare Beweise für aktive Tektonik im untersuchten Gebiet und demonstrierte damit die Eignung dieser lakustrinen Sedimente als paläoseismisches Archiv. Die Untersuchung lakustriner Aufschlüsse in Spity Valley lieferte auch Informationen saisonale Änderung der Niederschlagsverteilung sowie das Auftreten von häufigen und intensiven Perioden (ca. 6,8-6,1 cal ka BP) detritischer Einfuhr, welche auf extreme hydrologische Ereignisse in der Vergangenheit schließen lässt. Ein regionaler Vergleich dieser Periode deutet auf einen möglicherweise erweiterten „break-monsoon-like“ Modus für den Monsun hin, welcher hohe Niederschläge über dem Tibetischen Plateau, dem Himalaja und seinen Gebirgsausläufern begünstigt. Meine Studien an den Oberflächensedimenten des Lonar-Sees haben dazu beigetragen umweltsensitive Proxys zu identifizieren, die auch zur Interpretation von Paläodaten von einem ca. 10 m langen Sedimentkern genutzt wurden, der 2008 erbohrt wurde. Der Kern umfasst das gesamte Holozän und stellt das erste gut 14C-datierte Archiv aus der Kernmonsunzone Zentralindiens dar. Die Identifizierung von authigenen Evaporit-Kristallen (Gaylussite) innerhalb der Sedimente liefert einen Beweis für ungewöhnlich trockene Bedingungen in den Perioden zwischen 4,7-3,9 und 2,0-0,5 cal ka BP. Darüber hinaus lieferten Isotopen-Untersuchungen dieser Kristalle Informationen zur Eutrophierung, Stratifikation und zum Kohlenstoff-Kreislauf des Sees.

This study investigated the influences of sea-land breezes (SLBs) and northeastern monsoon (NEM) on the transportation and dispersion of air pollutants over coastal region in southern Taiwan. The physicochemical properties of particulate matter (PM) was simultaneously sampled and analyzed at both inland and offshore sites during eight intensive sampling periods. This study further used a SURFER 2-D plotting software, a backward trajectory model, a 3-D meteorological model (MM5), and a comprehensive air quality model (CAMx) to simulate surface wind fields and spatial distribution of air pollutants over the coastal region during the intensive sampling periods of SLBs and NEM. According to the meteorological condition and the synoptic weather patterns of the observation data showed that the SLBs sampling periods commonly occurred the weather patterns were zone of low pressure, pacific high pressure, and west stretch of the pacific high pressure when the main prevailing wind direction were west wind and southwest wind. During the NEM sampling periods, the weather patterns were strong northeastern monsoon, standard northeastern monsoon, and outflow rebound with high pressure in southern Taiwan, and then the main prevailing wind direction were northwest wind and northeast wind. However, during the MIX sampling periods, the weather patterns were outflow circulation of typhoon, weak northeastern monsoon, and outflow rebound with high pressure, while the wind directions didn¡¦t change regular. Thus, at coastal sites, the sea-land breezes induce an inland transport of air pollutants during the daytime and a seaward return of air pollutants at nighttime, causing a recirculation of air pollutants back to inland regions each day during the SLBs sampling periods. During the NEM sampling period was mainly brought from the northeastern wind which transported air pollutants from the northern region to Kaohsiung metropolitan area. The results of PM concentration and size distribution indicated that the inland sites had a higher fraction of fine particles (PM2.5), whereas the offshore sites had a higher fraction of coarse particles (PM2.5-10). These phenomena were attributed to the fact that marine aerosols are generally abundant in the coarse particles. PM concentration is relatively higher during the NEM sampling periods than during the SLBs and MIX sampling periods. For PM concentration, the order of secondary inorganic aerosols (SIA) was NEM > MIX > SLBs, while the SIA/Ions ratio of PM2.5 were approximately 50% during sampling periods. The [NO3-]/[SO42-] ratios of PM2.5 and PM2.5-10 during the SLBs sampling periods were always lowest than those during the NEM and MIX sampling periods. It is suggested that the PM concentrations during the SLBs sampling periods were highly influenced by stationary sources emissions. The crustal elements indicated that the Al, Ca, Fe, and K contributed major composition of particles. Artificial metals, such as Mg, Pb, V, and Zn were also enriched in the atmospheric PM during the NEM sampling periods. In addition, the higher concentration of Fe was attributed to local anthropogenic emission and weak northeastern monsoon during the MIX sampling periods. Regardless of inland or offshore sites, a high concentration of secondary organic carbon (SOC) during the NEM and MIX sampling periods was consistent with OC/EC ratio higher than 2.2 indicates the potential formation of secondary aerosols. Chloride deficit of PM at inland sites were lost easily for approximately 40.28% during the NEM sampling periods. Moreover, the lowest [Cl-]/[Na+] ratio occurred during the sampling periods when the chloride deficit was relative high at inland sites. According to the results of neutralization ratio (NR), regardless of the periods (SLBs, NEM, and MIX), the particulates of inland and offshore sites were both acid. During the NEM and MIX sampling periods, sulphur oxidation ratio (SOR) of PM2.5 over coastal region in southern Taiwan were above 0.25. It is suggested that the results of SOR during those sampling periods were highly influenced by long transportation. In addition, the nitrogen oxidation ratio was lower influenced than SOR over coastal region in southern Taiwan. It is suggested that air quality of Kaohsiung metropolitan area were influenced by the industrial source emissions. During the SLBs sampling period, sea breezes blown in the morning transported the offshore PM10 back to the inland sites in the Kaohsiung metropolitan area. In contrast, the air mass observed during the NEM sampling period was brought to the Kaohsiung metropolitan area mainly by a northerly wind which transported air mass originating in the northern region (i.e. Chiayi counties). The backward trajectory modeling of the MIX sampling periods suggested that the winds were dominated by the Northeastern Monsoon, and as such, likely inhibited the influences of sea-land breezes. Local surface air mass circulation over southern Taiwan obtained from MM5 model influenced by the southwestern monsoon during the SLBs sampling period. The weather in southern Taiwan during the NEM intensive sampling period was occasionally influenced by the Northeastern Monsoon as well as by complex terrain. During the MIX sampling periods, air mass recirculation was frequently observed in the coastal region of southern Taiwan. The results of CAMx model showed that air pollutions were occurred apparent sea breezes in the afternoon and land breezes at night during the SLBs sampling period. During the NEM sampling period, a strong Northeastern Monsoon blew air pollutions from the north to the southern region. We have compared the model simulation with field measured O3 and PM10 concentrations for inland and offshore regions. The order of correlation coefficients of ¬the model simulation and the measurement for O3¬ and PM10 in sampling periods is SLBs>MIX>NEM and NEM>MIX>SLBs, respectively. Overall, the correlation coefficients of the model simulation and the measurement were middle and high correlation.

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.

The past climate in central Asia, and especially on the Tibetan Plateau (TP), is of great importance for an understanding of global climate processes and for predicting the future climate. As a major influence on the climate in this region, the Asian Summer Monsoon (ASM) and its evolutionary history are of vital importance for accurate predictions. However, neither the evolutionary pattern of the summer monsoon nor the driving mechanisms behind it are yet clearly understood. For this research, I first synthesized previously published Late Glacial to Holocene climatic records from monsoonal central Asia in order to extract the general climate signals and the associated summer monsoon intensities. New climate and vegetation sequences were then established using improved quantitative methods, focusing on fossil pollen records recovered from Tibetan lakes and also incorporating new modern datasets. The pollen-vegetation and vegetation-climate relationships on the TP were also evaluated in order to achieve a better understanding of fossil pollen records. The synthesis of previously published moisture-related palaeoclimate records in monsoonal central Asia revealed generally different temporal patterns for the two monsoonal subsystems, i.e. the Indian Summer Monsoon (ISM) and East Asian Summer Monsoon (EASM). The ISM appears to have experienced maximum wet conditions during the early Holocene, while many records from the area affected by the EASM indicate relatively dry conditions at that time, particularly in north-central China where the maximum moisture levels occurred during the middle Holocene. A detailed consideration of possible driving factors affecting the summer monsoon, including summer solar insolation and sea surface temperatures, revealed that the ISM was primarily driven by variations in northern hemisphere solar insolation, and that the EASM may have been constrained by the ISM resulting in asynchronous patterns of evolution for these two subsystems. This hypothesis is further supported by modern monsoon indices estimated using the NCEP/NCAR Reanalysis data from the last 50 years, which indicate a significant negative correlation between the two summer monsoon subsystems. By analogy with the early Holocene, intensification of the ISM during coming decades could lead to increased aridification elsewhere as a result of the asynchronous nature of the monsoon subsystems, as can already be observed in the meteorological data from the last 15 years. A quantitative climate reconstruction using fossil pollen records was achieved through analysis of sediment core recovered from Lake Donggi Cona (in the north-eastern part of the TP) which has been dated back to the Last Glacial Maximum (LGM). A new data-set of modern pollen collected from large lakes in arid to semi-arid regions of central Asia is also presented herein. The concept of "pollen source area" was introduced to modern climate calibration based on pollen from large lakes, and was applied to the fossil pollen sequence from Lake Donggi Cona. Extremely dry conditions were found to have dominated the LGM, and a subsequent gradually increasing trend in moisture during the Late Glacial period was terminated by an abrupt reversion to a dry phase that lasted for about 1000 years and coincided with the first Heinrich Event of the northern Atlantic region. Subsequent periods corresponding to the warm Bølling-Allerød period and the Younger Dryas cold event were followed by moist conditions during the early Holocene, with annual precipitation of up to about 400 mm. A slightly drier trend after 9 cal ka BP was then followed by a second wet phase during the middle Holocene that lasted until 4.5 cal ka BP. Relatively steady conditions with only slight fluctuations then dominated the late Holocene, resulting in the present climatic conditions. In order to investigate the relationship between vegetation and climate, temporal variations in the possible driving factors for vegetation change on the northern TP were examined using a high resolution late Holocene pollen record from Lake Kusai. Moving-window Redundancy Analyses (RDAs) were used to evaluate the correlations between pollen assemblages and individual sedimentary proxies. These analyses have revealed frequent fluctuations in the relative abundances of alpine steppe and alpine desert components, and in particular a decrease in the total vegetation cover at around 1500 cal a BP. The climate was found to have had an important influence on vegetation changes when conditions were relatively wet and stable. However, after the 1500 cal a BP threshold in vegetation cover was crossed the vegetation appears to have been affected more by extreme events such as dust storms or fluvial erosion than by the general climatic trends. In addition, pollen spectra over the last 600 years have been revealed by Procrustes analysis to be significantly different from those recovered from older samples, which is attributed to an increased human impact that resulted in unprecedented changes to the composition of the vegetation. Theoretical models that have been developed and widely applied to the European area (i.e. the Extended R-Value (ERV) model and the Regional Estimates of Vegetation Abundance from Large Sites (REVEALS) model) have been applied to the high alpine TP ecosystems in order to investigate the pollen-vegetation relationships, as well as for quantitative reconstructions of vegetation abundance. The modern pollen–vegetation relationships for four common pollen species on the TP have been investigated using Poaceae as the reference taxa. The ERV Submodel 2 yielded relatively high PPEs for the steppe and desert taxa (Artemisia Chenopodiaceae), and low PPEs for the Cyperaceae that are characteristic of the alpine Kobresia meadows. The plant abundances on the central and north-eastern TP were quantified by applying these PPEs to four post-Late Glacial fossil pollen sequences. The reconstructed vegetation assemblages for the four pollen sequences always yielded smaller compositional species turnovers than suggested by the pollen spectra, indicating that the strength of the previously-reported vegetation changes may therefore have been overestimated. In summary, the key findings of this thesis are that (a) the two ASM subsystems show asynchronous patterns during both the Holocene and modern time periods, (b) fossil pollen records from large lakes reflect regional signals for which the pollen source areas need to be taken into account, (c) climate is not always the main driver for vegetation change, and (d) previously reported vegetation changes on the TP may have been overestimated because they ignored inter-species variations in pollen productivity. / Das Paläoklima in Zentralasien, besonders in der Hochebene von Tibet (HT), ist von großer Bedeutung um globale Klimaprozesse zu verstehen und mögliche Voraussagung für die zukunft zu treffen. Als wichtigstes Klimaphänomen nehmen der asiatische Sommermonsun (ASM) und seine Entwicklungsgeschichte eine Schlüsselposition ein. Dennoch sind derzeit weder das Entwicklungsschema noch der antreibende Vorgang ausreichend verstanden. Dies gilt insbesondere für das Holozän, für welches große Kimaschwankungen und regionale Diskrepanzen weithin belegt sind. Deshalb habe ich zuerst holozäne Klimadaten zusammengefasst. Bereits veröffentlichte Publikationen aus den Monsungebieten Zentralasiens dienten als Grundlage, um die wichtigsten Klimasignale und die zugehörigen Intensitäten des Sommermonsuns heraus zu arbeiten. Anhand von Pollensequenzen aus tibetischen Seen erzeugte ich neue Klima- und Vegetationssequenzen, welche auf verbesserten quantitativen Methoden und rezenten Datensätzen beruhen. Außerdem wurden die Verhältnisse Pollen-Vegetation und Vegetation-Klima bewertet, um Schlussfolgerungen fossiler Pollensequenzen zu verbessern. Die Zusammenfassung der zuvor veröffentlichten, niederschlagsbezogenen Paläoklimadaten im Monsungebiet Zentralasiens ergab generell unterschiedliche Muster für die zwei Teilsysteme des ASMs, den Indischen Sommermonsun (ISM) und den Ostasiatischen Sommermonsun (OASM). Der ISM weist maximale feuchte Bedingungen während des frühen Holozöns auf, während viele Datensätze aus dem Gebiet des OASMs einen relativ trockenen Zustand anzeigen, besonders im nördlichen Zentralchina, wo maximale Niederschläge während des mittleren Holozäns registriert wurden. Genaue Betrachtungen der Antriebsfaktoren des Sommermonsuns ergaben, dass der ISM hauptsächlich durch Veränderungen der Sonneneinstrahlung auf der Nordhemisphäre angetrieben wird, während der OASM potentiell durch den ISM beherrscht wird - dies führt zu asynchronen Entwicklungen. Diese Hypothese wird durch rezente Monsunindizes gestützt. Sie weisen eine signifikant negative Korrelation zwischen den beiden Sommermonsun-Teilsystemen auf. Für die quantitative Klimarekonstruktion von Pollensequenzen wurde ein Sedimentkern aus dem See Donggi Cona im Nordosten der HT analysiert, der bis zum letzten glazialen Maximum (LGM) zurückdatiert wurde. Aufgrund der Tatsache, dass Donggi Cona ein relativ großer See ist, wird hiermit ein neuer Pollen-Klima-Kalibrierungsdatensatz auf Grundlage großer Seen in ariden und semiariden Regionen Zentralasiens vorgelegt. Das Konzept des Pollenherkunftsgebietes wurde in diese rezente, pollenbasierte Klimakalibrierung eingebracht und auf die Pollensequenz von Donggi Cona angewendet. Die Auswertung ergab, dass extrem trockene Bedingungen während des LGM (ca. 100 mm/yr) vorherrschten. Ein ansteigender Trend von Niederschlägen während des späten Glazials wurde durch einen abrupten Rückgang zu einer etwa 1000-jährigen Trockenphase beendet, welche mit Heinrich-Ereignis 1 in der Nordatlantik-Region übereinstimmt. Danach entsprechen die Klimaperioden dem warmen Bølling/Allerød und dem Kälteereignis der Jüngeren Dryas. Anschließend herrschten feuchte Bedingungen im frühen Holozän (bis zu 400 mm/yr). Ein etwas trockenerer Trend nach dem Holozänen Klimaoptimum wurde dann von einer zweiten Feuchtphase abgelöst, welche bis 4,5 cal. ka vor heute andauerte. Relativ gleichmäßige Bedingungen dominierten das späte Holozän bis heute. Die Klimadynamik seit dem LGM wurde vor allem durch Entgletscherung und Intensitätsschwankungen des ASM bestimmt. Bei der Betrachtung des Vegetation-Klima-Verhältnisses habe ich die zeitlichen Variationen der bestimmenden Faktoren hinsichtlich der Vegetationsdynamik auf der nördlichen HT untersucht. Dabei wurden hochauflösende holozäne Pollendaten des Kusai-Sees verwendet. Eine Redundanzanalyse (RDA) wurde angewendet um die Korrelation zwischen Pollenvergesellschaftungen und individuellen sedimentären Klimaanzeigern als auch die damit verbundene Signifikanz zu bewerten. Es stellte sich heraus, dass das Klima einen wichtigen Einfluss auf den Veränderungen in der Vegetation besaß, wenn die Bedingungen relativ warm und feucht waren. Trotzdem scheint es, dass, dass die Vegetation bei zu geringer Bedeckung stärker durch Extremereignisse wie Staubstürme oder fluviale Erosion beeinflusst wurde. Pollenspektren der vergangen 600 Jahre erwiesen sich als signifikant unterschiedlich verglichen mit den älterer Proben, was auf verstärkten anthropogenen Einfluss hindeutet. Dieser resultierte in einem beispiellosen Wandel in der Zusammensetzung der Vegetation. In Hinsicht auf das Pollen-Vegetation-Verhältnis und der quantitativen Rekonstruktion der Vegetationshäufigkeit habe ich theoretische Modelle, welche für europäische Regionen entwickelt und weithin angewendet wurden, respektive die Modelle "Extended R-Value" (ERV) sowie "Regional Estimates of Vegetation Abundance from Large Sites" (REVEALS), auf die hochalpinen Ökosysteme der HT überführt. Dafür wurden rezente Pollen-Vegetations-Verhältnisse von vier weit verbreiteten Pollen-Arten der HT überprüft. Poaceae wurden als Referenztaxa verwendet. Bei der Anwendung dieser Verhältnisse auf vier Pollensequenzen, welche die Paläoumweltbedingungen seit dem letzten Glazial widerspiegeln, wurden die Häufigkeiten von Pflanzen auf der zentralen und nordöstlichen HT quantifiziert. Anteile von Artimisia und Chenopodiaceae waren dabei im Vergleich zu ihren ursprünglichen Pollenprozenten deutlich verringert. Cyperaceae hingegen wies eine relative Zunahme in dieser Vegetationsrekonstruktion auf. Die rekonstruierten Vegetationsvergesellschaftungen an den Standorten der vier Pollensequenzen ergaben stets geringere Umwälzungen in der Artenzusammensetzung, als durch die Pollenspektren zu vermuten gewesen wäre. Dies kann ein Hinweis darauf sein, dass die Intensität der bislang angenommenen Vegetationsveränderungen überschätzt worden ist. Zusammengefasst sind die Hauptresultate dieser Dissertation, dass (a) die zwei ASM Teilsysteme asynchrone Muster während des Holozäns und heute aufweisen, dass (b) fossile Pollensequenzen großer Seen regionale Klimasignale widerspiegeln sofern die Herkunftsgebiete der Pollen berücksichtigt werden, dass (c) Klima nicht immer der Haupteinflussfaktor für Vegetationswandel ist und dass (d) das Ausmaß von Vegetationsveränderungen in zuvor veröffentlichten Studien auf der Hochebene von Tibet überschätzt worden sein kann, weil Diskrepanzen der Pollenproduktivität zwischen den Arten nicht einbezogen wurden.

abstract: Ponderosa pine forests are a dominant land cover type in semiarid montane areas. Water supplies in major rivers of the southwestern United States depend on ponderosa pine forests since these ecosystems: (1) receive a significant amount of rainfall and snowfall, (2) intercept precipitation and transpire water, and (3) indirectly influence runoff by impacting the infiltration rate. However, the hydrologic patterns in these ecosystems with strong seasonality are poorly understood. In this study, we used a distributed hydrologic model evaluated against field observations to improve our understandings on spatial controls of hydrologic patterns, appropriate model resolution to simulate ponderosa pine ecosystems and hydrologic responses in the context of contrasting winter to summer transitions. Our modeling effort is focused on the hydrologic responses during the North American Monsoon (NAM), winter and spring periods. In Chapter 2, we utilized a distributed model explore the spatial controls on simulated soil moisture and temporal evolution of these spatial controls as a function of seasonal wetness. Our findings indicate that vegetation and topographic curvature are spatial controls. Vegetation controlled patterns during dry summer period switch to fine-scale terrain curvature controlled patterns during persistently wet NAM period. Thus, a climatic threshold involving rainfall and weather conditions during the NAM is identified when high rainfall amount (such as 146 mm rain in August, 1997) activates lateral flux of soil moisture and frequent cloudy cover (such as 42% cloud cover during daytime of August, 1997) lowers evapotranspiration. In Chapter 3, we investigate the impacts of model coarsening on simulated soil moisture patterns during the NAM. Results indicate that model aggregation quickly eradicates curvature features and its spatial control on hydrologic patterns. A threshold resolution of ~10% of the original terrain is identified through analyses of homogeneity indices, correlation coefficients and spatial errors beyond which the fidelity of simulated soil moisture is no longer reliable. Based on spatial error analyses, we detected that the concave areas (~28% of hillslope) are very sensitive to model coarsening and root mean square error (RMSE) is higher than residual soil moisture content (~0.07 m3/m3 soil moisture) for concave areas. Thus, concave areas need to be sampled for capturing appropriate hillslope response for this hillslope. In Chapter 4, we investigate the impacts of contrasting winter to summer transitions on hillslope hydrologic responses. We use a distributed hydrologic model to generate a consistent set of high-resolution hydrologic estimates. Our model is evaluated against the snow depth, soil moisture and runoff observations over two water years yielding reliable spatial distributions during the winter to summer transitions. We find that a wet winter followed by a dry summer promotes evapotranspiration losses (spatial averaged ~193 mm spring ET and ~ 600 mm summer ET) that dry the soil and disconnect lateral fluxes in the forested hillslope, leading to soil moisture patterns resembling vegetation patches. Conversely, a dry winter prior to a wet summer results in soil moisture increases due to high rainfall and low ET during the spring (spatially averaged 78 mm ET and 232 mm rainfall) and summer period (spatially averaged 147 mm ET and 247 mm rainfall) which promote lateral connectivity and soil moisture patterns with the signature of terrain curvature. An opposing temporal switch between infiltration and saturation excess runoff is also identified. These contrasting responses indicate that the inverse relation has significant consequences on hillslope water availability and its spatial distribution with implications on other ecohydrological processes including vegetation phenology, groundwater recharge and geomorphic development. Results from this work have implications on the design of hillslope experiments, the resolution of hillslope scale models, and the prediction of hydrologic conditions in ponderosa pine ecosystems. In addition, our findings can be used to select future hillslope sites for detailed ecohydrological investigations. Further, the proposed methodology can be useful for predicting responses to climate and land cover changes that are anticipated for the southwestern United States. / Dissertation/Thesis / Ph.D. Geological Sciences 2012

This study contributes to developing terrestrial and marine palynological indicators of winter or summer monsoon signals as well as oceanographic environments of the South China Sea (SCS). The high-resolution reconstructions of Holocene East Asian Monsoon (EAM) climate and oceanographic condition of the northern SCS provide insights into regional climate events in the western low-latitude Pacific Ocean and their impacts on local oceanography and ecology. Sediment trap samples from the southwest Taiwan waters of the SCS in winter monsoon (March-April) and summer monsoon (July-August) seasons identify abundances of Pinus and Ulmus pollen as indicators of the winter monsoon whereas fern spores appeared to be indicators of the summer monsoon. The increased fluxes of dinoflagellate cyst (DC) taxa during summer are correlated with decreased sea-surface salinity (SSS) associated with nutrient-rich river inputs. DC distributions across the SCS show that some taxa are good indicators of changes in sea-surface temperature (SST), SSS, water depth and chlorophyll-a (chl-a) concentrations associated with EAM and oceanographic conditions. In particular, the concenrations of Brigantedinium spp. and cysts of Protoperidinium together with Echinidinium spp. are positively correlatd with SST in January and SST in July, and chl-a concentrations, respectively, which are linked to past monsoon strength and primary productivity. In total, four high cyst concentration regions have been observed off southern Vietnam, Borneo, Hainan, and South China. High-resolution palynological records from a sediment core in the northern SCS reflect several EAM climatic and oceanographic events over the last 12.5 kyr. A short-term Impagidinium decrease implied that the Taiwan Strait opened at ~11.7–11.0 cal kyr BP, with reduced Kuroshio Current influence when the East China Sea waters entered through the strait. Three Holocene relative sea-level stages were identified in the palynomorph records. The highest herb pollen abundances were observed before ~10.4 cal kyr BP, reflecting the shortest distance from the grassland sources on the exposed shelf at the low sea-level stand. High Brigantedinium and cysts of Protoperidinium abundances also indicate a near-shore environment. During ~10.4- ~6.8-6.0 cal kyr BP at the rising sea-level stage, fern spore abundances increased and DC abundances decreased. Consistently low total DC concentrations and high fern spore abundance were observed after ~6.8-6.0 cal kyr BP when the present oceanographic conditions were formed. Increased abundances of Pinus pollen reflected three strengthened winter monsoon intervals at ~5.5, 4.0 and 2.5 cal kyr BP under the present oceanographic conditions. The highest Dapsilidinium pastielsii abundances reflected the warmest interval at ~6.8-5.5 cal kyr BP of the northern SCS. / Graduate / 2019-12-13

abstract: The Dhofar Cloud Forest is one of the most diverse ecosystems on the Arabian Peninsula. As part of the South Arabian Cloud Forest that extends from southern Oman to Yemen, the cloud forest is an important center of endemism and provides valuable ecosystem services to those living in the region. There have been various claims made about the health of the cloud forest and its surrounding region, the most prominent of which are: 1) variability of the Indian Summer Monsoon threatens long-term vegetation health, and 2) human encroachment is causing deforestation and land degradation. This dissertation uses three independent studies to test these claims and bring new insight about the biodiversity of the cloud forest. Evidence is presented that shows that the vegetation dynamics of the cloud forest are resilient to most of the variability in the monsoon. Much of the biodiversity in the cloud forest is dominated by a few species with high abundance and a moderate number of species at low abundance. The characteristic tree species include Anogeissus dhofarica and Commiphora spp. These species tend to dominate the forested regions of the study area. Grasslands are dominated by species associated with overgrazing (Calotropis procera and Solanum incanum). Analysis from a land cover study conducted between 1988 and 2013 shows that deforestation has occurred to approximately 8% of the study area and decreased vegetation fractions are found throughout the region. Areas around the city of Salalah, located close to the cloud forest, show widespread degradation in the 21st century based on an NDVI time series analysis. It is concluded that humans are the primary driver of environmental change. Much of this change is tied to national policies and development priorities implemented after the Dhofar War in the 1970’s. / Dissertation/Thesis / Doctoral Dissertation Geography 2015