Dissertations / Theses on the topic 'Monsoons East Asia'

Observational evidence is presented to show the existence of a boreal summer teleconnection between the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM). Based on station rainfall data, the dominant patterns of variability in monthly and seasonal rainfall over India-East Asia region are investigated mainly through composite analysis. The association between the midlatitude circulation and Indian monsoon rainfall on interannual time scale has also been examined by using 54-year NCEP/NCAR reanalysis data. The major results are as follows: (1) Associated with the year-to-year fluctuation of Indian summer rainfall, a well-organized upper level teleconnection pattern is obvious over the Eurasia continent with two anomalous anticyclonic (cyclonic) circulations in the strong (weak) monsoon year. One center is located over West Tibet, and the other with an equivalent barotropic structure resides in northeast Asia. This teleconnection establishes a linkage between two monsoon systems. (2) Within the summer season from May to September, above mentioned midlatitude teleconnection pattern undergoes a different structure with the strongest intensity in June and August. Hence, depending on the condition of Indian monsoon, the 'window of linkage' between Indian subcontinent rainfall and East Asian monsoon seems to open only in June or August. (3) On the other hand, a global anomalous wavetrain with favored longitudinal phase has been found in the upper and middle troposphere in each summer month. During June and August, this geographically fixed wavetrain characteristic of a circumglobal feature coincides with the ISM-EASM teleconnection. Based on all these observational results, three possible scenarios are proposed to explain the establishment of the teleconnection. And it seems that the upper level climatological westerly jet, Indian monsoon heating and midlatitude stationary wave activity are important factors that control the establishment of the teleconnection. On intraseasonal time scale, simultaneous and lagged correlation statistics have been calculated between height in northern hemisphere and convection over north India. It is found that prior to the breakout of the convection in north India two anomalous ridges have existed over north of Pakistan and northeast Asia, respectively. A plausible mechanism of this interaction between the westerly flow in the midlatitude and Indian monsoon is also briefly discussed.
ix, 89 leaves

The East Asian Summer Monsoon (EASM) is a major component in Asian climate. It is largely driven by climatic factors such as humidity, solar insolation and temperature. For at least 50 years the EASM has been studied extensively by scientists regarding its current strength. Models have been recreating past monsoon intensity as well as attempted to predict future intensity. As the monsoon undergoes changes, the climatic shifts responsible for them leave various traces behind; geochemical as well as biological, and these have been preserved and recorded in various locales on the planet. The most significant climatic change is the variation between glacial and interglacial periods which have been alternating for the last 2.6 million years and the EASM has changed in tune with the climate during this time. The EASM follows the δ18O-record in speleothems found in Eastern Asia as well as in ice cores from Greenland. Various geochemical and biological tracers seem to reflect these fluctuations in climate locally as well as globally over a 200 kyr period. The current intensity of the EASM seems to be one of decreasing strength, a phase that has persisted since the Holocene climatic optimum 8.5 kyr ago. Recently however a decrease in the East Asian Winter Monsoon has been confirmed, indicating an increase in EASM intensity. During the Holocene the EASM reached peak intensity during the Holocene climatic optimum but has fluctuated largely in tune with solar insolation. This is also true for the Eemian period although some events such as the mid-Eemian cooling show that factors other than solar insolation regulate monsoon intensity over large time periods. The future of the EASM seems to be one of increased strength due to climate change and models predict both increased wind speeds and an increasing occurrence of extreme precipitation despite decreasing solar insolation.

Motivation | Societal and economic needs of East Africa rely entirely on the availability of water, which is governed by the regular onset and retreat of the rainy seasons. Fluctuations in the amounts of rainfall has tremendous impact causing widespread famine, disease outbreaks and human migrations. Efforts towards high resolution forecasting of seasonal precipitation and hydrological systems are therefore needed, which requires high frequency short to long-term analyses of available climate data that I am going to present in this doctoral thesis by three different studies. 15,000 years - Suguta Valley | The main study of this thesis concentrated on the understanding of humidity changes within the last African Humid Period (AHP, 14.8-5.5 ka BP). The nature and causes of intensity variations of the West-African (WAM) and Indian Summer monsoons (ISM) during the AHP, especially their exact influence on regional climate relative to each other, is currently intensely debated. Here, I present a high-resolution multiproxy lake-level record spanning the AHP from the remote Suguta Valley in the northern Kenya Rift, located between the WAM and ISM domains. The presently desiccated valley was during the AHP filled by a 300 m deep and 2200 km2 large palaeo-lake due to an increase in precipitation of only 26%. The record explains the synchronous onset of large lakes in the East African Rift System (EARS) with the longitudinal shift of the Congo Air Boundary (CAB) over the East African and Ethiopian Plateaus, as the direct consequence of an enhanced atmospheric pressure gradient between East-Africa and India due to a precessional-forced northern hemisphere insolation maximum. Pronounced, and abrupt lake level fluctuations during the generally wet AHP are explained by small-scale solar irradiation changes weakening this pressure gradient atmospheric moisture availability preventing the CAB from reaching the study area. Instead, the termination of the AHP occurred, in a non-linear manner due to a change towards an equatorial insolation maximum ca. 6.5 ka ago extending the AHP over Ethiopia and West-Africa. 200 years - Lake Naivasha | The second part of the thesis focused on the analysis of a 200 year-old sediment core from Lake Naivasha in the Central Kenya Rift, one of the very few present freshwater lakes in East Africa. The results revealed and confirmed, that the appliance of proxy records for palaeo-climate reconstruction for the last 100 years within a time of increasing industrialisation and therefore human impact to the proxy-record containing sites are broadly limited. Since the middle of the 20th century, intense anthropogenic activity around Lake Naivasha has led to cultural eutrophication, which has overprinted the influence of natural climate variation to the lake usually inferred from proxy records such as diatoms, transfer-functions, geochemical and sedimentological analysis as used in this study. The results clarify the need for proxy records from remote unsettled areas to contribute with pristine data sets to current debates about anthropologic induced global warming since the past 100 years. 14 years - East African Rift | In order to avoid human influenced data sets and validate spatial and temporal heterogeneities of proxy-records from East Africa, the third part of the thesis therefore concentrated on the most recent past 14 years (1996-2010) detecting climate variability by using remotely sensed rainfall data. The advancement in the spatial coverage and temporal resolutions of rainfall data allow a better understanding of influencing climate mechanisms and help to better interpret proxy-records from the EARS in order to reconstruct past climate conditions. The study focuses on the dynamics of intraseasonal rainfall distribution within catchments of eleven lake basins in the EARS that are often used for palaeo-climate studies. We discovered that rainfall in adjacent basins exhibits high complexities in the magnitudes of intraseasonal variability, biennial to triennial precipitation patterns and even are not necessarily correlated often showing opposite trends. The variability among the watersheds is driven by the complex interaction of topography, in particular the shape, length and elevation of the catchment and its relative location to the East African Rift System and predominant influence of the ITCZ or CAB, whose locations and intensities are dependent on the strength of low pressure cells over India, SST variations in the Atlantic, Pacific or Indian Ocean, QBO phases and the 11-year solar cycle. Among all seasons we observed, January-September is the season of highest and most complex rainfall variability, especially for the East African Plateau basins, most likely due to the irregular penetration and sensitivity of the CAB.
Motivation | Die sozialen und ökonomischen Bedürfnisse Ostafrikas sind in erster Linie von der Wasserverfügbarkeit abhängig, welche durch das regelmäßige Einsetzen der Regenzeiten bestimmt wird. Jegliche Veränderungen der Wasserverfügbarkeit innerhalb der Regenzeiten verursachen Hungersnöte, Ausbruch von Krankheiten oder auch Bevölkerungswanderungen. Klärung der Ursachen von Niederschlagsvariabilitäten erfordert die Auswertung von hochauflösenden Kurz- als auch Langzeitanalysen, welche ich in dieser Arbeit durch drei Studien präsentieren werde. 15,000 Jahre - Suguta Valley | Die Hauptstudie dieser Doktorarbeit befasste sich mit dem Verständnis von Feuchtigkeitsschwankungen innerhalb der Afrikanischen Feuchtperiode (AHP, 5.5 - 14.8 ka BP). In dieser Studie präsentiere ich einen hoch-auflösenden Seespiegel Datensatz aus dem abgeschiedenen, unbewohnten Suguta Tal im nördlichen Grabenbruch in Kenia. Das momentan extrem trockene Tal war während der AHP mit einem 300 m tiefen und 2200 km2 großen Paläo-See bedeckt, was aus nur 26% zusätzlichem Niederschlag resultierte. Diese Erhöhung wurde vermutlich aus der Kombination aus erhöhter atmosphärer Feuchteverfügbarkeit infolge erhöhter früh-Holozäner präzessionsgesteuerten Einstrahlung auf der nördlichen Hemisphere sowie der Verschiebung der feuchten Kongo Luftmassengrenze (CAB) ostwärts über das Ostafrikanische und Äthiopische Plateau erreicht als direkte Folge eines erhöhten atmosphärischen Druckgradienten. Abrupte, starkte Seespiegelschwankungen innerhalb der generellen Feuchtphase sind auf geringe Veränderungen in der solaren Ausstrahlung zurückzufühen, welche zu einer Schwächung des Druckgradienten führten und damit den Einfluss der CAB im Untersuchungsgebiet verhinderten zusammen mit einer allgemeinene Reduktion der atmosphärischen Feuchteverfügbarkeit. Das Ende der AHP erfolgte im Gegensatz dazu eher nicht-linear aufgrund des Wechsels zu einem äquatorialen Einstrahlungsmaximum vor 6.5 ka, welches die AHP in Äthiopien und West-Afrika verlängerte. 200 Jahre - Lake Naivasha | Der zweite Teil dieser Arbeit konzentrierte sich auf die Analyse eines Sedimentkern des Naivasha See aus dem zentralen Kenia Rift über die letzten 200 Jahre, einem der wenigen Frischwasserseen in Ostafrika. Die natürliche Klimavariabilität sollte mittels Proxy-Datensätzen von Diatomeen, Transferfunktionen, geochemischen und sedimentologischen Analysen in dieser Studie aufgedeckt werden. Die Ergebnisse zeigten, dass seit Mitte des 20. Jahrhundert der zunehmende Einfluss des Menschen um den Naivasha See zu kultureller Eutrophierung geführt, welche den Einfluss der natürlichen Klimavariabilität auf den See überprägte. Die Gründe liegen in der Zeit, welche von steigender Industrialisierung und deshalb erhöhtem menschlichen Einfluss auf die Proxy-Daten enthaltenden Seen geprägt ist. Die Ergebnisse verdeutlichen die Notwendigkeit von Proxy-Daten aus unbesiedelten Gebieten, wenn man ,reine‘ Daten zur momentanen Debatte über den anthropogen gesteuerten Klimawandel der letzten 100 Jahre beitragen will. 14 Jahre - Ostafrikanisches Rift | Um räumliche Unregelmäßigkeiten in Proxy-Daten von Ostafrika richtig zu verstehen, konzentrierte sich der dritte Teil dieser Arbeit auf die Auswertung von ausschließlich fernerkundlich erworbenen heutigen, täglichen Niederschlagsreihen (1996-2010). Dies erlaubt ein besseres Verständnis über die möglichen klimatischen Einflussmechanismen und die Abschätzung ihres Einflusses auf die Paläo-Variabilität. Die Studie beschäftigt sich mit der Dynamik saisonaler Niederschlagsverteilung innerhalb der Einzugsgebiete von elf Seebecken im Ostafrikanischen Riftsystem, welche oft für Paläo-Klimastudien benutzt werden. Die Studie ergab, dass Niederschläge in angrenzenden Becken tatsächlich höchst unterschiedlich in ihrer Intensität sein können und dabei zwei- bis dreijährigen Niederschlagsmuster folgen oder sogar gegensätzliche Trends zeigen. Die Variabilität der einzelnen Seebecken wird durch die komplexe Wechselwirkung der Topographie, Form, Länge und Höhe des Einzugsgebietes, der relativen Lage im EARS, sowie dem Einfluss und Intensität der ITCZ und CAB bestimmt, welche z.B. abhängig von der Entwicklung besonders starker Tiefdruckgebiet über Indien, Veränderungen der Meeres-oberflächentemperaturen, QBO und dem 11-Jahres Sonnenzyklus sind. Im direkten Vergleich aller untersuchten Monate stellte sich heraus, dass Juli-September die Jahreszeit mit komplexester Niederschlagsvariabilität ist, besonders für die Becken des Ostafrikanischen Plateau, was durch den unregelmäßigen Einfluss der CAB verursacht wird.

The Asian Summer Monsoon generates intense seasonal precipitation across India, China and Indochina, comprising Earth’s largest monsoonal climate regime, and this vital component of the global energy and water cycles directly impacts the world’s most populous regions. Accurate palaeomonsoon reconstructions are required to investigate natural climate variability beyond the coverage of instrumental records and inform predictions of future monsoon trends. Stable oxygen isotope ratios (δ18O) are an important proxy for hydroclimate variability and stalagmite δ18O is widely used to investigate East Asian palaeoclimate, typically interpreted as a semi-quantitative measure of precipitation amount. However, recent studies suggest δ18O instead reflects multiple hydroclimatic processes, warranting a detailed understanding of precipitation and proxy δ18O systematics. This thesis (i) presents a quantitative Lagrangian study of atmospheric moisture transport and precipitation across central and eastern China, a continental region affected by the East Asian Summer Monsoon (EASM); (ii) investigates the hydroclimatic drivers of precipitation δ18O variability at Wanxiang Cave, an important site for palaeomonsoon reconstruction located near to the northerly EASM limit; (iii) evaluates the extent to which seasonal- to decadal-scale proxy δ18O variability reflects terrestrial moisture fluxes by constructing a pseudoproxy record for Wanxiang. Present-day precipitation across monsoonal China is primarily derived from the northern Indian Ocean and recycled intensely over the East Asian continent; Pacific Ocean moisture export peaks during winter. A geographically variable isotopic source effect is apparent in present-day mean monthly precipitation δ18O data. Wanxiang Cave precipitation δ18O variability during AD 2001-2002 is successfully reproduced by a recently-developed Lagrangian model which simulates air masses’ isotopic evolution along three-dimensional atmospheric trajectories, though winter values are ~2 ‰ over-depleted. Contributions of Tibetan Plateau- and free troposphere-derived moisture are identified as synoptic-scale isotopic depletion processes. Overall, land-derived moisture causes isotopic enrichment during summer, thus moderating depletion due to strengthened EASM circulation or increased precipitation amount. As such, periods of elevated summer surface temperatures may attenuate the seasonal EASM signal in precipitation and proxy δ18O. An idealised Rayleigh-type isotope model is unable to capture these effects accurately, emphasising the importance of regional moisture fluxes. A pseudoproxy record, driven by terrestrial moisture fluxes, replicates seasonal- to decadal-scale stalagmite δ18O variability at Wanxiang. The work presented in this thesis offers new insights into the influence of atmospheric moisture transport dynamics on precipitation and proxy δ18O variability across central and eastern China. This constitutes an important advancement in our ability to use δ18O to reconstruct past climate variability quantitatively in the EASM region.

The principal aim of this thesis is to reconstruct East Asian Monsoon (EAM) variability during the mid-Holocene, developing a relatively new proxy of bulk organic carbon isotopic signature (δ13C). C/N ratios and trace elements are also employed to conduct a multi-proxy case study from the Pearl River estuary, southern China. Sources of sediments within an estuary include river-derived terrestrial/freshwater input, in situ brackish-water suspended sediment and tide-derived marine input. This study assumes the three proxies can help differentiate sources of sediments relating to monsoon-driven freshwater flux and help reconstruct monsoonal precipitation history during the mid-Holocene when the sea level was relatively stable. To achieve the aims of this thesis a range of modern samples were collected from terrestrial areas, including plants and soil samples, through to estuarine areas, including seasonal estuarine suspended organic matter (SOM) and surface sediment. Results suggest that bulk organic δ13C and C/N ratios can successfully identify sources of the organic component of the estuarine sediment, and thus can be used to infer relative changes in monsoon-driven freshwater flux to the estuary. For example, more negative δ13C values reflect a greater level of contribution of freshwater organic carbon, i.e. stronger monsoonal freshwater discharge. Results also show that a combination of selected metals, such as the terrigenous metals (Fe, Mn, Co and As), can be useful for indicating sediment sources and sedimentary environment. Analysis of an estuarine core (UV1) shows that freshwater discharge from the Pearl River catchment gradually declined from 6400 to 2000 cal. years BP, suggesting a gradual weakening of summer monsoon precipitation, responding to the weakening insolation controlled by the orbital-driven precession cycle. Superimposed on this are wet/dry intervals, ranging from centennial- to millennial- scale, driven by solar activity. Changes in ENSO and high-latitude cooling events might be responsible for dry/wet events at centennial- to decadal- scale, identified during the mid-Holocene. This study also suggests that the coupling of thermal and moist conditions of the EAM might only have become stable after 4500 cal. yr BP. A sudden shift in the geochemical signature indicates agricultural activity in the Pearl River delta intensified from 2000 cal. yr BP.

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

Le but de cette thèse est de reconstruire l’évolution environnementale de la région nord de la mer de Chine du Sud au cours du Quaternaire récent. L'objectif est d'identifier des traceurs des interactions continent-océan, de la mousson est-asiatique et de la circulation océanique. Pour cela, des analyses des minéralogies argileuse et magnétique, de la composition en éléments majeurs, et la taille des grains ont été effectuées sur les sédiments de carotte MD12-3432 couvrant les derniers 400 ka avec un taux de sédimentation variant entre 4.5 et 24 cm/ka. Nous avons tout d'abord amélioré les calibrations de la composition en éléments majeurs obtenue par XRF core-scanning. En effet, celles-ci ne corrigeaient pas des variations de teneur en eau interstitielle observées la séquence sédimentaire étudiée. Nous avons donc proposé une correction polynomiale quadratique qui convertit maintenant correctement les données XRF core-scanning en concentrations précises d’éléments majeurs. La composition en élément majeurs, les minéralogies argileuse et magnétique, et la granulométrie indiquent que les changements climatiques à basse latitude influencent les diverses fractions terrigènes de différentes manières. Considérant d’abord les éléments majeurs et les argiles, nous observons que les rapports smectite/(illite+chlorite) et K₂O/Al₂O₃ présentent tous deux des cyclicités orbitales. Le rapport K₂O/Al₂O₃ qui reflète la contribution relative des apports de Taiwan par dénudation, augmente pendant les interglaciaires quand la mousson d’été asiatique est forte. Le rapport smectite/(illite+chlorite) présente des cyclicités également liées à l’excentricité et à la précession en phase avec les changements d’insolation de l’hémisphère nord en été. Connaissant les régions sources de ces argiles, nous suggérons que l’apport en smectite est étroitement lié à l'intensité de l’altération chimique et à la dénudation fluvial induite par les fortes précipitations de mousson, tandis que l’apport d’illite/chlorite depuis Taiwan répond principalement à la dénudation. Le rapport smectite/(illite+chlorite) reflète ainsi principalement l'intensité de l’altération chimique contemporaine (rapide) et donc l’intensité de la mousson d'été asiatique. Les résultats obtenus confirment que la mousson d'été asiatique est renforcée pendant les périodes interglaciaires et quand l'insolation d'été boréal est forte. Les propriétés magnétiques sédimentaires offrent des informations complémentaires sur les changements environnementaux passés dans cette région. La fraction magnétique de la carotte MD12-3432 est composée de magnétites, sulfures de fer et hématite. Des augmentations de teneur en hématite et des diminutions de la granulométrie sédimentaire sont observées aux minima de précession. Ces évènements qui ont lieu pendant les périodes arides pourraient illustrer des apports éoliens depuis le nord de la Chine, liés à des changements d'intensité et/ou de route des vents, probablement aussi liés à l’intensification de la mousson d'hiver. Outre les changements climatiques de basse latitude, le climat global et l'activité tectonique influencent aussi les apports terrigènes dans cette région. Les augmentations à long terme de la teneur en pyrrhotite par rapport à la magnétite et à l’hématite, de la teneur de illite/chlorite indiquent une contribution croissante de sédiment fine d'origine Taiwanaise au cours des derniers 400 ka. Ceci est très probablement lié à l’intensification de l'orogenèse taïwanais. A l'échelle glaciaire-interglaciaire, les variations synchrones des compositions argileuse et magnétique, de la taille des grains sédimentaires et magnétiques, et des taux de sédimentation sont attribuées aux changements de niveau marin. En effet, les bas niveaux marins pendant les périodes glaciaires exposent l’immense plateau continental et permettent à la Rivière des Perles de livrer au site du sédiment terrigène en plus grande quantité et plus grossier
The aim of this study is to reconstruct late Quaternary environmental changes in the northern South China Sea by applying multi-disciplinary proxies of land-sea interaction, East Asian monsoon, and oceanic circulation. Investigations of clay and magnetic mineralogy, major element composition, and grain size were performed on marine sediment Core MD12-3432 retrieved from the continental slope of the northern South China Sea. The core covers the last 400 ka with a sedimentation rate varying between 4.5 and 24 cm/ka. We examined the accuracy of existing calibration methods on major element composition obtained by XRF core-scanning, because downcore variations in interstitial water content should strongly affect scanned element contents. We proposed a quadratic polynomial correction to account for this effect and implemented it in the calibration methods. Data from Core MD12-3432 show that the improved calibration process now correctly converts XRF core-scanning data into major element concentrations. Our results on high-resolution major element composition, clay and magnetic mineralogy, and grain size data indicate that low-latitude climate changes influence various terrigenous fractions in different ways. In bulk sediment, K₂O/Al₂O₃ ratio mainly reflects the relative contribution of detrital supply from Taiwan, and its variation exhibits eccentricity cycles. The ratio increases during interglacials, showing that strong precipitation and denudation are induced by enhanced East Asian summer monsoon. In clay fractions, smectite/(illite+chlorite) ratio presents both eccentricity and precession periodicities, in phase with the northern hemisphere summer insolation changes and therefore with the East Asian summer monsoon evolution. Based on the knowledge of sediment provenances, these results suggest that high smectite/(illite+chlorite) ratios illustrate contemporaneous chemical weathering intensity in Luzon. It supports the understanding that East Asian summer monsoon is enhanced during interglacial periods and when the boreal summer insolation is strong. Therefore, these two ratios are appropriate sedimentary tracers for East Asian summer monsoon evolution in the South China Sea. The magnetic fraction yields complementary information about environmental changes in the South China Sea. The magnetic mineral assemblage of Core MD12-3432 is mainly composed of magnetite, sulphide, and hematite, and the relative contributions of all these magnetic mineral contents change with time. Variations on precessional band related to the low latitude East Asian monsoon are observed in magnetic properties and grain size values. High magnetic inputs with high hematite proportion, which is part of fine-grain sediment, are observed during the precession minima. These events occurring during arid periods may illustrate enhanced eolian inputs caused by changes in intensity and/or winds pathway of winds, probably related to enhanced winter monsoon. Besides the low-latitude climate changes, global climate and tectonic activity also influence the terrigenous composition at the studied site at different timescales. A long-term increase in pyrrhotite content with respect to magnetite and hematite and in illite/chlorite contents indicates an increasing contribution of fine grained sediments from Taiwan. This is most likely related to the intensification of Taiwanese orogeny over the last 400 ka. On glacial-interglacial scale, coeval increases observed in sedimentation rate, magnetite/pyrrhotite content, kaolinite content, and grain size during glacial periods are attributed to sea-level changes. Low sea-level during glacial periods exposes the vast shelf and allows the Pearl River to deliver more and coarser terrigenous sediments to our site

碩士
國立臺灣大學
大氣科學研究所
102
Asia has the strongest monsoon system in the world and has the most population continent. In addition to Indian monsoon, East Asia is another area with strong monsoon system, high population and high aerosol pollution. The relevant studies show that aerosol causes decrease in surface temperature, reduction of precipitation and the weakness of monsoon system. However, less research was studied further into the dynamical feedback. This study focuses on the how aerosol will change the East Asian summer monsoon. We apply the NCAR/CESM global model with slab ocean model to get a better ocean feedback, and simulate the year 1850 and 2000 emission scenarios in order to estimate the influence of anthropogenic aerosol forcing and to approach the problem from different perspectives, including dynamical feedback. The result shows that in total aerosol forcing, the direct radiative forcing is main-ly on the high aerosol distribution area. Aerosol indirect effect follow the cloud movement of the synoptic system and influence the whole Asia. The mean regional surface temperature and precipitation decreased by -1.07°C and -0.46 mm/day, respec-tively, over East Asia , but the responses vary significantly from region to region. The dynamical feedback of aerosol forcing, which affect the sea surface temperature and the strength of Pacific high ridge by indirect effect, shows that the wind-evaporation-SST feedback causes the inconsistency distribution between surface temperature and radiation. The main aerosol emission region shows the reduction of temperature, but cause the updraft at the mei-yu front and has more precipitation in the result. This may due to trigger the Pacific-Japan pattern (P-J pattern) feedback, while we see the similar structure in the result. Overall, aerosol particles cause large regional subsidence anomaly, resulting in weakened summer monsoon. When considering only absorbing (black carbon) or non-absorbing (sulfate) aero-sols, the changes in surface temperature are +0.18°C and -0.53°C, respectively; where-as the changes in precipitation are -0.35 mm/day and -0.34 mm/day, respectively. Non-absorbing aerosol can scatter the solar radiation, causing the temperature decrease and stabilize atmosphere. Absorbing aerosol shows heating atmosphere can com-pensate the radiation reduction at surface, while the combined effect shows stabilizing atmosphere. The dynamical feedback of WES feedback and P-J pattern are show in the absorbing and non-absorbing aerosol result, only with a weaker phenomenon.

碩士
臺灣大學
大氣科學研究所
98
The coupled atmosphere-ocean seasonal prediction models have been extensively used nowadays. Compared with the stand-alone atmospheric models or ocean models, the coupled atmosphere and ocean models have higher skills in seasonal prediction, and especially their multi-model ensemble (MME), which more accurately make the 1-month lead seasonal forecast. On the other hand, the coupled atmosphere-ocean models still have deficiencies in reproducing realistic climatology. The year-to-year variation of the Asian-Australia monsoon exhibits enormous regional differences and depends strongly on the phase of the annual cycle. This poses a great challenge for the coupled atmosphere-ocean model in making realistic seasonal simulation. This problem was clearly demonstrated in Wang et al. (2007), which analyzed the seasonal hindcast by the DEMETER and APCC/CliPAS. This study is aimed to evaluate the capability of the DEMETER and NCEP/CFS in hindcasting the interannual variability of the East Asian monsoon for the 1980-2003 period. A preliminary analysis of the 2-meter temperature indicates that the hindcast skill of these coupled models is significantly higher in the ocean than over the continent. Especially, the simulated interannual variance in continent is significantly underestimated. Compared with the simulation skill between high latitude area (20N~90N) and Tropical ocean area (20S~20N), most of the models are able to simulate the variance in low latitude but high latitude. An EOF and SVD analysis of the East Asian monsoon indicates that while the models are able to simulate the leading circulation/precipitation coupled mode, they tend to put too much weight on the leading mode by exaggerating the fractional variance explained by the leading mode. It suggests that the models tend to produce lower variability than the observed and is likely to misrepresent the variability that is not associated with the leading mode. In our study, we found that the AOGCMs tend to show the feature of ENSO and lead by Tropical Ocean but ignore the variance in high latitude and land. This result reveals that the air-sea interaction is reasonably simulated, but the dynamical processes over land and the possible effect of air-land interaction are poorly handled in the models. Overall, the AOGCMs may overestimate the impact of Tropical Ocean, underestimate the influence of continent, and high latitude. Further analyses will focus on the simulation skill in high latitude. Besides, we will evaluate the performance of East Asian monsoon in IPCC models and define a set metrics for to evaluate the models’ capability in simulating the interannual variability of the East Asian monsoon. In addition to identify the deficiencies of the models for further improvement.

博士
國立臺灣大學
大氣科學研究所
102
The interannual variability of East Asia-West Pacific summer monsoon (EA-WPSM) is a very complicated phenomenon. Because eddies with various spatial and temporal scales interact with each other, that causes results by statistical analysis to be sensitive in domain and variable. The study shows two dominant modes of the EA-WPSM which passed statistical tests to gain most comprehensive information. The first EOF mode, called the monsoon trough (MT) mode, is directly forced by current El Nino/Southern Oscillation (ENSO) state. The MT mode is characterized by zonally elongated structure stretching from 130°E to date line. The major bands as a dipole-like pattern, lies on two sides of the equator. The band north of the equator, tilted from the Philippine Sea toward the equatorial eastern Pacific, can be identified with the monsoon trough. The band on the Southern Hemisphere is weakened by New Guinea Island, which relates to the low-level cross-equatorial flow that originates from the Banda Sea and Solomon Sea to the monsoon trough, which causes two anomalous vorticity poles. The major moisture source of the monsoon trough is the low-level mositure advection laterally driven by the New Guinea cross-equatorial flow. By decomposing contributions to the cross-equatorial flow based on the linear mixed layer model, the boundary layer pressure gradient in the Maritime Continent plays a major role. The pressure gradient is further found to be related to the densely-packed sea surface temperature (SST) gradient near the equator around New Guinea, which is well correlated with concurrent ENSO state. The second laeding mode (EOF2 13.9%) shows the famous Pacific Japan pattern discovered by Nitta in 1987, called the PJ mode. It also shows a band structure elongated in the east-west direction and more closes the coast of East Asia, especially around the Philippines to Japan. The major feature of PJ mode is the enhanced western Pacific subtropical high ridge (WPSH) under stronger subsidence. Although this mode reveals a positive correlation with the ENSO phase in the previous winter, their correlation (r=0.53) reflects that some influences are still not found. According to the comparsion the spatial structure between El Nino JJA(1) and PJ mode summer, the significant cold SST anomalies appear over the Kuroshio Extension in the PJ mode. It deepens the meridional SST gradient between 25°N and 40°N. The sharp SST gradient needs more active baroclinic adjustment. Over the North Pacific, the southward displacement of the westerly jet and extratropical storm track are borth connected to the enhanced meiyu rainband, which changes the shortwave radiation at surface and cools SST with shallow oceanic mixed layer depth in summer. The enhanced WPSH also supplies the northeastward warm and moist low-level flow to meiyu. In addition, the wave activity flux shows significant upper southward transport around 30°N~45°N and reveals a northward transport around WPSH (15°N~30°N), which implies the interaction between the meiyu front and subtropical ridge. Therefore, the meiyu front, SST gradient and WPSH ridge among the East Asia summer monsoon via the positive feedback to promote the self-maintaining mechanism. On the other hand, the Indian Ocean capacity effect reveals a pre-condition of enhanced WPSH in spring, but it becomes a second forcing of PJ mode in summer. As a result, both MT and PJ mode have their major original forcing related to the SST gradient, through the mechanisms are completely different. The MT mode reveals the influence of near-equatorial Lindzen Nigam mechanism, and the PJ mode is associated with the enhanced baroclinic adjustment.

碩士
國立中央大學
大氣物理研究所
96
The weather and climate in Taiwan are regulated by East Asian monsoon systems including both summer and winter monsoons. Due to the modulation of intraseasonal oscillation, the seasonal marching of the East Asian summer monsoon posses a clear onset, active, break, revival and withdrawal life cycle. Meanwhile, the cold surge originated from Siberia with cold and dry air mass will penetrate Taiwan during the winter. Furthermore, cold surge also will induce a strong midlatitude – tropical interaction. After launching in April 2006, FORMOSAT-3 could provide relative abundant temperature and moisture atmospheric profile by using radio occulation method (RO). The most advantage with this FORMOSAT-3 RO data is not only offering a better global coverage but also providing more vertical observation in the ocean that was scarcely obtained by traditional radiosonde observation. Therefore, it makes possible for us to apply this observational RO data in the East Asian monsoon study, particularly for the summer active monsoon phase and winter cold surge phenomena. During active summer monsoon period, FORMOSAT-3 GPS RO observation depicts a more reasonable atmospheric feature in the tropical cooler ocean area if compared to the NCEP analysis data. Through the water budget analysis, the GPS RO data also demonstrates a better water supply in the hydrological process of the summer rainstorm events. Particularly around the heavy rainfall area with strong convergence of moisture flux, the GPS RO observation shows more agreement than that of NCEP analysis data. Two heavy rainfall events occur during early summer: June 3-9 and June 10-16, 2007. In order to further investigate the possible maintenance mechanisms of these two rainstorms, the high resolution NCEP/GFS analysis data was adopted to perform the detail analysis. It shows that the rainstorm, formed at the southern China, propagates eastward toward Taiwan. During its redeveloping stage, the latent heat release by precipitation together with the coupling of lower boundary moisture convergence and mid-tropospheric vorticity will play significant roles in generating heavy shower resulting in floods in Taiwan. For the winter cold surge events, a cases-composite strategy was used to depict the surge daily development during its life cycle. It appears that the GPS RO observations delineate the surge structure both horizontally and vertically much better than NCEP reanalysis II data, especially the cold dry air mass and water vapor transport. In addition, the GPS RO exhibits a relative stronger divergence of moisture flux in the cold surge rainfall events.

Luo, Ming.
Thesis Ph.D. Chinese University of Hong Kong 2014.
Includes bibliographical references (leaves 146-164).
Abstracts also in Chinese.
Title from PDF title page (viewed on 19, September, 2016).

博士
國立臺灣大學
大氣科學研究所
104
The intensity of East Asian winter monsoons (EAWM) is represented by a single EAWM index in many previous works. This study finds a decadal variation, abrupt change and a weakening trend of northerly wind in EAWM in different areas over latitudes along the East Asia Coast. The first mechanism of the northern part of the East Asia Coast is influenced by the forcing from the middle latitudes during 1950s to 2000s and related to the Pacific Decadal Oscillation (PDO). I propose that the decreased EAWM in the recent decades is associated with the change of pressure gradient along the East Asia coast. This mechanism accounts for the change of the westward sea-level pressure (SLP) gradient along the Northeast Asia coast, and is affected by the location of Aleutian low, which is associated with the phases shift of the PDO. As the Aleutian low is influenced by the negative PDO phase and moves westward, the SLP gradient between the Siberian high and the Aleutian low can increase and the northerly wind at 850 hPa will be enhanced. The second mechanism accounts for the phase change of the Arctic Oscillation (AO) after the late 1980s. When the AO is in its positive phase, the low-level northerly wind from South Korea to northern Taiwan and the East Asian trough weaken. The third mechanism of the decadal variation of the northerly wind in the lower latitude is associated with the positive phase of PDO and the location change of the anti-cyclonic circulation center over the southern China. The positive phase of the PDO induces a low-level anti-cyclonic anomalous circulation against the EAWM in the lower latitudes. On the other hand, the forth mechanism accounts for the increased occurrence of Central-Pacific El Niño copes with the phase change of Atlantic Multi-decadal Oscillation (AMO) in early 1990. The anti-cyclonic circulation moves eastward and produces southwesterly wind over the northern part of the South China sea when the Central-Pacific El Niños increase. Therefore the EAWM has weakened in recent decades. This study argues that neither a single EAWM index nor only one mechanism can explain the whole process of the weakening of EAWM over the East Asia Coast from the tropics to the middle latitudes. The four mechanisms provided by this study can explain the decadal variation, abrupt change and the weakening of EAWM at different latitudes.

博士
國立臺灣師範大學
地球科學系
95
In this study, the land surface scheme of the Purdue Regional Model (PRM) was improved. Sun and Chern (2005) had created a new land surface model which had the ability of long term continue simulation to the Sleeper River region. In the new land surface model, including the vegetation, snow and soil parts. The land surface model was tried to improve the land surface process of the PRM. To evaluate the performance of the PRM was simulated to the 1998 East Asian summer monsoon case. The PRM can hold the well major features of the East Asian summer monsoon include the location of summer seasonal mean, the date of the summer onset, the northward propagation of the rain bands and ISOs. The simulated surface temperature is warmer than observed, and the Pacific subtropical high also is too strong. Due to the tropical high is too strong induce the precipitation is less in the south China and Indochina. The location of Mei-Yu front is more northward. The UMD and CNRM land use datasets are more high resolution what datasets were collected. In the vegetation scheme, the LAI data is used to modify the vegetation fraction. In the 1998 summer case, there are better results by using the CNRM dataset and observed LAI. The interaction process is complex between the land and atmosphere. For understand that the impact of land surface process, design some sensitivity test for land surface processes change, such as the warm and cold effect, different land cover and soil moisture cases. In these testes, the land sea heat contrast is more important than the local evaporation release to the influence of monsoon flow change. The precipitation is dominated by the monsoon flow. As the land surface temperature is warm, the monsoon flow is stronger than control run, induce the precipitation increase over East Asian. The monsoon flow is strong because is warm land case, induce the subtropical high decrease and withdraw eastward. One cyclone over Japan Sea is generated because monsoon flow is stronger. In the cold temperature case, the Pacific subtropical high increase and extend westward. The moisture flow of the summer monsoon moves westward, and induce the precipitation decrease over East Asia.

This thesis aims at enhancing our fundamental understanding of the East Asian summer monsoon (EASM), and mechanisms implicated in its climatology in present-day and warmer climates. We focus on the most prominent feature of the EASM, i.e., the so-called Meiyu-Baiu (MB), which is characterized by a well-defined, southwest to northeast elongated quasi-stationary rainfall band, spanning from eastern China to Japan and into the northwestern Pacific Ocean in June and July.

We begin with an observational study of the energetics of the MB front in present-day climate. Analyses of the moist static energy (MSE) budget of the MB front indicate that horizontal advection of moist enthalpy, primarily of dry enthalpy, sustains the front in a region of otherwise negative net energy input into the atmospheric column. A decomposition of the horizontal dry enthalpy advection into mean, transient, and stationary eddy fluxes identifies the longitudinal thermal gradient due to zonal asymmetries and the meridional stationary eddy velocity as the most influential factors determining the pattern of horizontal moist enthalpy advection. Numerical simulations in which the Tibetan Plateau (TP) is either retained or removed show that the TP influences the stationary enthalpy flux, and hence the MB front, primarily by changing the meridional stationary eddy velocity, with reinforced southerly wind on the northwestern flank of the north Pacific subtropical high (NPSH) over the MB region and northerly wind to its north. Changes in the longitudinal thermal gradient are mainly confined to the near downstream of the TP, with the resulting changes in zonal warm air advection having a lesser impact on the rainfall in the extended MB region.

Similar mechanisms are shown to be implicated in present climate simulations in the Couple Model Intercomparison Project - Phase 5 (CMIP5) models. We find that the spatial distribution of the EASM precipitation simulated by different models is highly correlated with the meridional stationary eddy velocity. The correlation becomes more robust when energy fluxes into the atmospheric column are considered, consistent with the observational analyses. The spread in the area-averaged rainfall amount can be partially explained by the spread in the simulated globally-averaged precipitation, with the rest primarily due to the lower-level meridional wind convergence. Clear relationships between precipitation and zonal and meridional eddy velocities are observed.

Finally, the response of the EASM to greenhouse gas forcing is investigated at different time scales in CMIP5 model simulations. The reduction of radiative cooling and the increase in continental surface temperature occur much more rapidly than changes in sea surface temperatures (SSTs). Without changes in SSTs, the rainfall in the monsoon region decreases (increases) over ocean (land) in most models. On longer time scales, as SSTs increase, rainfall changes are opposite. The total response to atmospheric CO^2 forcing and subsequent SST warming is a large (modest) increase in rainfall over ocean (land) in the EASM region. Dynamic changes, in spite of significant contributions from the thermodynamic component, play an important role in setting up the spatial pattern of precipitation changes. Rainfall anomalies over East China are a direct consequence of local land-sea contrast, while changes in the larger-scale oceanic rainfall band are closely associated with the displacement of the larger-scale NPSH. Numerical simulations show that topography and SST patterns play an important role in rainfall changes in the EASM region.

碩士
國立高雄師範大學
地理學系
102
This study investigates the circulation patterns of the summer monsoon along with southern Taiwan’s monsoon rainfall. Two types of monsoon patterns are defined: one is characterized by a broad low pressure area extending from a low pressure center near southern China northeastward toward southern Japan. It is found that significantly strong southwesterlies are located over the southern flank of the monsoon trough. For the other type of monsoon pattern, the area north to Taiwan is dominated by a strong westward protruding anticyclone, and a weaker and east-west elongated monsoon trough, which drives the easterly flow to southern Taiwan, is situated south to Taiwan. As the intraseasonal oscillation (ISO) propagates northwestward toward southern China, it will create a favorable environment for the southwesterly flow and convection to develop and become stronger. However due to the latitudinal limit of the northwestward-propagating ISO, the strongest convection over the southern flank of the ISO cyclonic circulation barely reaches southern Taiwan in the westerly phase. Thus, the ISO-westerly pattern seems stronger but brings less rainfall to southern Taiwan. The ISO tends to generate anomalous cyclone/anticyclone that could affect the southwesterlies and northeasterlies near its northern and southern rims through tightening the pressure gradients. Thus, swithing the low-frequency background flow could have significant impacts on the monsoon rainfall and the associated circulation systems.

At present, the East Asian Monsoon (EAM) influences water availability for nearly one third of the global population. The intensity and position of the EAM has varied considerably since its onset, but disagreement still exists related to the precise latitudinal and intensity shifts of the Westerly Jet and associated storm fronts, which mark the northern extent of the monsoon. Paleoclimate research can assist in improved assessment and prediction of EAM intensity, radiative forcing, and biogeochemical cycles in the Japan Sea and North Pacific, especially under the currently changing climate. My research primarily focuses on using major-, trace- and rare earth elements in sediments from International Ocean Drilling Program Expedition 346 in the Japan (Ulleung Basin) and East China Seas (Okinawa Trough) to track variability in the EAM on millennial time scales. Using geochemical and multivariate statistical techniques (Q-Mode Factor analysis and Constrained Least Squares multiple linear regressions), I differentiated compositionally similar terrigenous aluminosilicate materials (continental crust components, eolian dusts, volcanic ash) from these sediment archives. I successfully constructed a robust record of aluminosilicate provenance, which enables more precise determinations of EAM position and intensity than previously possible. Most of my research focused on the interpretation of aluminosilicate records over several different timescales from three sites from Expedition 346. In tandem with this research, I also refined values of the well-known, and widely used, Standard Reference Material (SRM) Hawaiian Volcano Observatory Basalt (BHVO-2). In the Okinawa trough (Sites U1428/U1429), I identified and tracked the increase in flux of five continental crust materials, loesses, and volcanic ashes during glacial cycles, continental shelf exposure, and the migration of paleo-rivers in the last 400 kyr. Additionally, I constructed a 12 Myr record, which identified and quantified the dust fluxes to Ulleung Basin (Site U1430), and emphasized the importance of the Taklimakan and Gobi Deserts as main sources of dust to the Japan Sea and Pacific through the Cenozoic. Collectively, these aluminosilicate flux reconstructions are first to identify multiple specific Asian source regions through the Cenozoic, and highlight the complexity of accurately reconstructing monsoons and other aspects of paleoclimate from sediment in dynamic environments.

碩士
國立中央大學
大氣物理研究所
84
In order to understand the characteristics of the monsoon onset, theevolution of large scale flow patterns during the onset of East Asian summermonsoon has been studied. The data used here are the ECMWF Data. Streamfunctionand velocity potential fields are calculated from the wind field.Center of these fields are identified as an index to illustrate theevolution of the monsoon system. Years chosen for the analyses are 1983, 1985,1991 and 1992. From spring to summer, earth''s surface receives more radiation. Theland heates the atmosphere through sensible heat flux, and the sea transferswater vapor to the atmosphere through air-sea interaction. When the systemwas advected to Indochina Peninsula, the latent heat is released. Upper-levelwarm core will move northwestward from warm pool to Indochina Peninsula,such that meridional temperature gradient reverses. It was also found that theupper-level South Asian anticyclone moves northwestward with easterlies tothe south, and westerly jet jumping northward consequently. The cross-equatorial flow also transfers moisture from equatorial ocean surface. Thiscauses tropical atmosphere environment becomes more unstable. And it wasconcluded that the onset of East Asian summer monsoon will not occur untilthe Subtropical High withdrawed from the South China Sea and moved eastward.After the onset, the southwesterlies intensify and deep convection developesover South China Sea. The large-scale circulation was also found to undergo two distinctstages of abrupt transitions annually. Those changes are associated withupper-level warm core moving northwestward suddenly. If these two changeshappen on middle May and middle June, and Subtropical High withdraws atthe first signal, then the onset of East Asian summer monsoon is earlier thanthose of Indian monsoon. However, if these two changes happen on late Apriland late May, and Subtropical High doesn''t withdraw at the first signal, thenthe onset of the East Asian and Indian monsoons occurs at the same time. After the onset of East Asian summer monsoon, large-scale upwardmotion dominates over South China Sea. The divergence circulation is builtand intensified. The zonal flow is maintained by heat source of Bay of Bengalto warm pool, and the meridional one is maintained by heat source of SouthChina Sea. The importance of South China Sea as the heat source to EastAsian summer monsoon can also be indentified through the structure of thelocal mean meridional circulation.

Global and regional-scale climatic changes caused by volcanic eruptions are difficult to discern conclusively based on limited 20th century climate records. Analyses of paleoclimate records and global climate model (GCM) simulations indicate that a significantly increased volcanic signal occurs in East Asia in response to historical eruptions, many of which were much larger than those experienced in the 20th century. Records of historical floods vs. drought in eastern China suggest that major eruptions over the past millennium typically led to a relatively wet north and a dry south. The GCM simulates a 10% reduction in the strength of tropical Hadley circulation and significantly decreased precipitation throughout the tropics under Tambora-like volcanic forcing conditions. The volcanic-induced weakening of the West Pacific sub-tropical high apparently contributes to a decrease in modeled precipitation throughout northeastern China. Meanwhile a general decrease in tropical precipitation resulting from reduced incoming solar radiation and lower evaporation is hypothesized to have caused observed (and modeled) decreases in summertime precipitation in southeastern China.

碩士
中國文化大學
大氣科學研究所
96
The rainfall characteristic of the East Asian winter monsoon (EAWM) is less emphasized in previous works. This study reveals that the circulation of the EAWM weakened in the recent decades which results in the decreasing of winter rainfalls over several windward coastal areas over the East Asia including the hill region in the northern Taiwan. However, the southern China and the plain region over the northern Taiwan have an abrupt increase of rainfalls in early 1980s. This is due to the increasing of the sea surface temperature and lower troposphere moisture over the South China Sea under the global warming climate and the anomalous northward flow which enhance the moisture transport to the southern China. Because more moisture is provided for the frontal system which moves eastward, the fronts frequently come with abundant moisture and well-developed rain band in winter. Therefore, the plain region over the northern Taiwan can obtain more rainfalls after early 1980s.

碩士
國立臺灣大學
地質科學研究所
95
Monthly-resolution geochemical proxies, such as 18 O, 13 C, Sr/Ca, and Ba/Ca, in a living Porites coral head, collected from Son Tra Island, central Vietnam (162''59.4", 108''57.1"), was used to quantita ?? tively reconstruct the local sea surface temperature (SST), the annual dynamics of the East Asian winter monsoon (EAWM), as well as the monthly change of regional hydrological conditions. By comparing the 1/4-century geochemical data, five features are exhibited. (1) The coral Sr/Ca-inferred summer SSTs correspond well with the 1x 1instrumental data to ?? demonstrate the validity of coral Sr thermometry. (2) The winter surface pressure difference, between the southern South China Sea (SCS) (0-10 癗 , 105-115 癊 ) and the northern SCS (22.5-32.5 癗 , 112-122 癊 ), matches coral winter SST data, indicating that the coral-inferred thermal extremes can be employed to retrieve the EAWM 18 O residual ( 18 O) suggests a 2-4-psu seasonal salinity change between dry and wet seasons. (4) The monthly 18 O data show that the regional rainy season is from late summer to winter which is consistent with the meteorological record. This phenomenon is also observed in seasonal variability of Ba/Ca record. (5) The high Ba/Ca background level of 10 mol/mol in 1992-2004, 2-3 times larger than the averaged value of 4 mol/mol in 1978-1992, suggests an enhanced terrestrial sediment discharge into the marginal sea over the past 10 years. Ba records reveal an economical impact on hydrological change since the Vietnam War. Our results show that the Son Tra coral geochemical records can be used as proxies of climate and environment changes and of human activity.

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