Rozprawy doktorskie na temat „Equatorial Indian Ocean”

Climatological winds over the equatorial Indian Ocean (EqlO) are westerly most of the year. Twice a year, in April-May ("spring") and October-December ("fall"), strong, sustained westerly winds generate eastward equatorial jets in the ocean. There are several unresolved issues related to the equatorial jets. They accelerate rapidly to speeds over lms"1 when westerly wind stress increases to about 0.7 dyne cm"2 in spring and fall, but decelerate while the wind stress continues to be westerly; each jet is followed by westward flow in the upper ocean lasting a month or longer. In addition to the semi-annual cycle, the equatorial winds and currents have strong in-traseasonal fluctuations. Observations show strong 30-60 day variability of zonal flow, and suggest that there might be variability with periods shorter than 20 days in the central EqlO. Observations from moored current meter arrays along 80.5°E south of Sri Lanka showed a distinct 15 day oscillation of equatorial meridional velocity (v) and off-equatorial zonal velocity (u). Recent observations from current meter moorings at the equator in the eastern EqlO show continuous 10-20 day, or biweekly, oscillations of v. The main motivation for the present study is to understand the dynamics of intraseasonal variability in the Indian Ocean that has been documented in the observational literature. What physical processes are responsible for the peculiar behavior of the equatorial jets? What are the relative roles of wind stress and large scale ocean dynamics? Does intraseasonal variability of wind stress force intraseasonal jets? What is the structure and origin of the biweekly variability? The intraseasonal and longer timescale variability of the equatorial Indian Ocean circulation is studied using an ocean general circulation model (OGCM) and recent in Abstract ii situ observations. The OGCM simulations are validated against other available observations. In this thesis, we document the space-time structure of the variability of equatorial Indian Ocean circulation, and attempt to find answers to some of the questions raised above. The main results are based on OGCM simulations forced by high frequency reanalysis and satellite scatterometer (QuikSCAT) winds. Several model experiments with idealized winds are used to interpret the results of the simulations. In addition to the OGCM simulations, the origin of observed intraseasonal anomalies of sea surface temperature (SST) in the eastern EqlO and Bay of Bengal, and related air-sea interaction, are investigated using validated satellite data. The main findings of the thesis can be summarized as: • High frequency accurate winds are required for accurate simulation of equatorial Indian Ocean currents, which have strong variability on intraseasonal to interannual time scales. • The variability in the equatorial waveguide is mainly driven by variability of the winds; there is some intraseasonal variability near the western boundary and in the equatorial waveguide due to dynamic instability of seasonal "mean" flows. • The fall equatorial jet is generally stronger and longer lived than the spring jet; the fall jet is modulated on intraseasonal time scales. Westerly wind bursts can drive strong intraseasonal equatorial jets in the eastern EqlO during the summer monsoon. • Eastward equatorial jets create a westward zonal pressure gradient force by raising sea level, and deepening the thermocline, in the east relative to the west. The zonal pressure force relaxes via Rossby wave radiation from the eastern boundary. • The zonal pressure force exerts strong control on the evolution of zonal flow; the decel eration of the eastward jets, and the subsequent westward flow in the upper ocean in the presence of westerly wind stress, is due to the zonal pressure force. • Neither westward currents in the upper ocean nor subsurface eastward flow (the ob served spring and summer "undercurrent") requires easterly winds; they can be gener ated by equatorial adjustment due to Kelvin (Rossby) waves generated at the western (eastern) boundary. • The biweekly variability in the EqlO is associated with forced mixed Rossby-gravity (MRG) waves generated by intraseasonal variability of winds. The biweekly MRG wave in has westward and upward phase propagation, zonal wavelength of 3000-4500 km and phase speed of 4 m s"1; it is associated with deep off equatorial upwelling/downwelling. Intraseasonal SST anomalies are forced mainly by net heat flux anomalies in the central and eastern EqlO; the large northward propagating SST anomalies in summer in the Bay of Bengal are due to net heat flux anomalies associated with the monsoon active-break cycle. Coherent variability in the atmosphere and ocean suggests air-sea interaction.

Climatological winds over the equatorial Indian Ocean (EqlO) are westerly most of the year. Twice a year, in April-May ("spring") and October-December ("fall"), strong, sustained westerly winds generate eastward equatorial jets in the ocean. There are several unresolved issues related to the equatorial jets. They accelerate rapidly to speeds over lms"1 when westerly wind stress increases to about 0.7 dyne cm"2 in spring and fall, but decelerate while the wind stress continues to be westerly; each jet is followed by westward flow in the upper ocean lasting a month or longer. In addition to the semi-annual cycle, the equatorial winds and currents have strong in-traseasonal fluctuations. Observations show strong 30-60 day variability of zonal flow, and suggest that there might be variability with periods shorter than 20 days in the central EqlO. Observations from moored current meter arrays along 80.5°E south of Sri Lanka showed a distinct 15 day oscillation of equatorial meridional velocity (v) and off-equatorial zonal velocity (u). Recent observations from current meter moorings at the equator in the eastern EqlO show continuous 10-20 day, or biweekly, oscillations of v. The main motivation for the present study is to understand the dynamics of intraseasonal variability in the Indian Ocean that has been documented in the observational literature. What physical processes are responsible for the peculiar behavior of the equatorial jets? What are the relative roles of wind stress and large scale ocean dynamics? Does intraseasonal variability of wind stress force intraseasonal jets? What is the structure and origin of the biweekly variability? The intraseasonal and longer timescale variability of the equatorial Indian Ocean circulation is studied using an ocean general circulation model (OGCM) and recent in Abstract ii situ observations. The OGCM simulations are validated against other available observations. In this thesis, we document the space-time structure of the variability of equatorial Indian Ocean circulation, and attempt to find answers to some of the questions raised above. The main results are based on OGCM simulations forced by high frequency reanalysis and satellite scatterometer (QuikSCAT) winds. Several model experiments with idealized winds are used to interpret the results of the simulations. In addition to the OGCM simulations, the origin of observed intraseasonal anomalies of sea surface temperature (SST) in the eastern EqlO and Bay of Bengal, and related air-sea interaction, are investigated using validated satellite data. The main findings of the thesis can be summarized as: • High frequency accurate winds are required for accurate simulation of equatorial Indian Ocean currents, which have strong variability on intraseasonal to interannual time scales. • The variability in the equatorial waveguide is mainly driven by variability of the winds; there is some intraseasonal variability near the western boundary and in the equatorial waveguide due to dynamic instability of seasonal "mean" flows. • The fall equatorial jet is generally stronger and longer lived than the spring jet; the fall jet is modulated on intraseasonal time scales. Westerly wind bursts can drive strong intraseasonal equatorial jets in the eastern EqlO during the summer monsoon. • Eastward equatorial jets create a westward zonal pressure gradient force by raising sea level, and deepening the thermocline, in the east relative to the west. The zonal pressure force relaxes via Rossby wave radiation from the eastern boundary. • The zonal pressure force exerts strong control on the evolution of zonal flow; the decel eration of the eastward jets, and the subsequent westward flow in the upper ocean in the presence of westerly wind stress, is due to the zonal pressure force. • Neither westward currents in the upper ocean nor subsurface eastward flow (the ob served spring and summer "undercurrent") requires easterly winds; they can be gener ated by equatorial adjustment due to Kelvin (Rossby) waves generated at the western (eastern) boundary. • The biweekly variability in the EqlO is associated with forced mixed Rossby-gravity (MRG) waves generated by intraseasonal variability of winds. The biweekly MRG wave in has westward and upward phase propagation, zonal wavelength of 3000-4500 km and phase speed of 4 m s"1; it is associated with deep off equatorial upwelling/downwelling. Intraseasonal SST anomalies are forced mainly by net heat flux anomalies in the central and eastern EqlO; the large northward propagating SST anomalies in summer in the Bay of Bengal are due to net heat flux anomalies associated with the monsoon active-break cycle. Coherent variability in the atmosphere and ocean suggests air-sea interaction.

Thesis (Ph. D.)--Woods Hole Oceanographic Institution and Massachusetts Institute of Technology, 1990.
Funding was provided by the Office of Naval Research and a Secretary of the Navy Graduate Fellowship in Oceanography. References : p. 117-121.

Multi-proxy approach for the reconstruction of paleo-redox conditions is attempted on a radiocarbon (C-14) dated sediment core near the equatorial Indian Ocean. Based on the behavior and distribution of redox sensitive and productivity proxies, study demonstrates prevalence of anoxic bottom water conditions during LGM due to poorly ventilated bottom waters augmented by high surface productivity resulting in better preservation of organic carbon (OC). During early Holocene, the equatorial Indian Ocean witnessed high sedimentation rates resulting in high organic carbon (OC) with depleted redox sensitive elements thereby causing better preservation of OC. The study underscores poor bottom water ventilation during LGM and preservation of OC as a result of high sedimentation rate in early Holocene.

L'objectif principal de cette thèse était de reconstituer l'évolution de la circulation intermédiaire depuis la dernière période glaciaire à partir de carottes de sédiments marins prélevées dans le golfe du Bengale GB, la Mer d’Arabie MA et l’océan Indien équatorial oriental OIEO. La stratégie scientifique mise en œuvre inclut l’étude des assemblages et de la géochimie des foraminifères benthiques, afin de reconstruire les changements de source et de ventilation des masses d’eau. Les résultats obtenus dans le GB ont permis de restituer les changements hydrologiques à profondeur intermédiaire à haute résolution temporelle au cours des derniers 40 ka. Les enregistrements témoignent de changements dans la source des masses d’eau, entre l’Océan austral avec les eaux antarctiques intermédiaires AAIW et les eaux Nord Atlantique NADW, à l’échelle glaciaire-interglaciaire mais aussi lors des événements millénaires. Ce travail a aussi permis de fournir les premiers enregistrements à haute résolution temporelle des rapports élémentaires des foraminifères benthiques (Mg/Ca, Sr/Ca, U/Ca et Li/Ca) dans le GB et en MA. Ces résultats permettent notamment de mieux contraindre la pénétration des AAIW vers le nord depuis la dernière période glaciaire. La reconstruction de la concentration en ion carbonate permet également de discuter des relations entre les variations de la circulation intermédiaire et les changements profonds du cycle du Carbone à l’échelle globale, notamment via les échanges se produisant dans l’Océan Austral. Nous avons également fourni dans ce travail les premiers enregistrements de Cd/Ca et de Ba/Ca continus et à haute résolution dans le nord de l’océan Indien, pour reconstituer les modifications passées de la teneur en éléments nutritifs. Les enregistrements géochimiques dans l’OIEO témoignent de profonds changements des propriétés des masses d'eau intermédiaires, associées aux changements de circulation
The main objective of this study was to reconstruct the evolution of the intermediate water circulation since the last glacial period by the investigation of marine cores collected from the Bay of Bengal (BoB), Arabian Sea (AS) and Eastern Equatorial Indian Ocean (EEIO). The scientific strategy involves benthic foraminiferal assemblages and geochemical proxies to better constrain past changes in the source and ventilation of water masses. Records from the BoB allowed reconstructing hydrological changes at intermediate depth over the last 40 cal kyr. The records highlight changes in the source of water masses, with a balance between the contribution of southern Antarctic Intermediate Water (AAIW) versus North Atlantic Deep Water (NADW) at glacial-interglacial timescale as well as during millennial events. This work also provided the first high-resolution benthic foraminifera elemental ratio records (Mg/Ca, Sr/Ca, U/Ca and Li/Ca) from the BoB and the AS. These records especially help to better constrain the northward penetration of AAIW over the last glacial period. The reconstruction of the carbonate ion concentration allowed to discuss the relationships between the intermediate water circulation and deep changes in the global Carbon cycle, with a special interest for the Southern Ocean. This work also provides the first continuous and high-resolution benthic Cd/Ca and Ba/Ca records in the northern Indian Ocean, could reconstruct past changes in the nutrient content. Geochemical records from the EEIO exhibit strong changes in the chemical properties of the intermediate water masses, related to global circulation changes in the area

Our studies suggest that the equatorial Indian Ocean Oscillation, which is the atmospheric component of Indian Ocean Dipole, is as important as El Nino and Southern Oscillation for the interannual variation of Indian Summer Monsoon Rainfall. Characteristic features of equatorial Indian Ocean Oscillation are suppression (enhancement) of convection over the eastern (western) equatorial Indian Ocean and easterly (westerly) anomalies of the zonal component of the surface wind over the equatorial region. We find that there is a strong, statistically significant, relationship between large deficits/excess in Indian summer monsoon rainfall and a composite index based on indices of El Nino and Southern Oscillation and equatorial Indian Ocean Oscillation. We studied the impact of externally introduced atmospheric heating due to the enhanced convection over the western equatorial Indian Ocean, associated with positive equatorial Indian Ocean Oscillation, on the simulation of Indian summer monsoon by an atmospheric general circulation model. We find that convection over the western equatorial Indian Ocean played a critical role in above normal Indian summer monsoon activity in 1994. We have also studied the triggering and evolution of the positive Indian Ocean dipole events. We suggest that severe cyclones over the Bay of Bengal during April-May, trigger these positive Indian Ocean dipole events. We show that all the positive Indian Ocean dipole events during 1958-2003 are preceded by at least one severe cyclone over the Bay of Bengal during April/May. We show that the severe cyclones over the Bay of Bengal can strengthen upwelling favorable southeasterlies along the Sumatra coast by enhancing pressure gradient across the eastern equatorial Indian Ocean and can suppress convection over the eastern equatorial Indian Ocean. Suppression of convection over the eastern equatorial Indian Ocean leads to enhancement of convection over the western equatorial Indian Ocean and hence weakening of westerlies along the central equatorial Indian Ocean. This enhances the convergence over western equatorial Indian Ocean and further strengthening of convection over the western equatorial Indian Ocean. This positive feedback between convection and circulation strengthens the anomalous easterlies over the central equatorial Indian Ocean, until the wind becomes easterlies. These surface easterlies trigger eastward propagating, upwelling favorable Kelvin waves in the equatorial Indian Ocean. Together with the coastal upwelling due to anomalous southeasterlies along the Sumatra coast, these Kelvin waves lead to anomalous cooling in the eastern equatorial Indian Ocean and trigger positive Indian Ocean dipole events.

博士
國立臺灣大學
大氣科學研究所
106
This study examines the westward-propagating convective disturbances with quasi-2-day intervals of occurrence identified over Gan Island in the central Indian Ocean from mid to late October 2011 during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign. Atmospheric sounding, satellite, and radar data are used to develop a composite of seven such disturbances. Composites and spectral analyses reveal that: (1) the quasi-2-day convective events comprise westward-propagating diurnal convective disturbances with phase speeds of 10–12 m/s whose amplitudes are modulated on a quasi-2-day time scale on a zonal scale of ~1000 km near the longitudes of Gan; (2) the cloud life cycle of quasi-2-day convective disturbances shows a distinct pattern of tropical cloud population evolution—from shallow-to-deep-to-stratiform convection; (3) the time scales of mesoscale convective system development and boundary layer modulation play essential roles in determining the periodicity of the quasi-2-day convective events; and (4) in some of the quasi-2-day events there is evidence of counter-propagating (westward and eastward) cloud systems along the lines proposed by Yamada et al. Based on these findings, an interpretation is proposed for the mechanisms for the quasi-2-day disturbances observed during DYNAMO that combines concepts from prior studies of this phenomenon over the western Pacific and the Indian Ocean.

The Maldives atolls, the very top of one of the largest modern carbonate platforms, occupy the central and largest part of the Chagos-Laccadives ridge located in the equatorial Indian Ocean. In the central part of the archipelago, the large atolls form two parallel north-south relatively continuous chains surrounding an internal basin, the Inner Sea, with water depths not exceeding 550 m. The Maldives carbonate system, uniquely evolved through a combination of global sea level fluctuations, subsiding history, and more regional seasonally varying monsoon circulation. Although the long-term evolution of this system is relatively well-established, the understanding of the detailed evolution of the Maldives carbonate edifice in the last 5 million years has remained limited. The latest phase of its stratigraphic evolution is explained by a shift from a well-developed Miocene-Pliocene progradational pattern to a mostly late Pliocene-Quaternary aggradational depositional signature. This last aggradation phase, forming the atolls the way we know them today, consists of stacked inner neritic limestone sequences, separated by a series of exposure horizons. The succesive periods of atoll exposure and re-flooding are recorded in the Inner Sea by late Pliocene-Quaternary glacial/interglacial clearly cyclic deposition of periplatform oozes. This cyclic sedimentary pattern also appears in the internal prograding geometry of carbonate drifts in the Inner Sea. A200 m-thick deep carbonate sediment drift was first observed on a Shell E-W seismic line north of Gaafaru Falhu atoll in the NE corner of the Maldives Inner Sea, in a range of water depths from ~300 to 500 m. During the NEOMA 2007 research cruise on the RV Meteor lead by Universität Hamburg, the deep water sandy drift in the area north of Gaafaru Falhu atoll and an adjacent deeper muddy drift was extensively surveyed via 12 kHz multibeam bathymetry, a 4 kHz sub bottom profiler (Atlas Hydrographics), multi channel high resolution seismics, and three box and piston cores. My study focuses on understanding the Plio-Quaternary overall evolution of the set of adjacent sandy and muddy drifts, just north of Gaafaru Falhu Atoll. The sandy and muddy drift interconnected internal geometries observed in the available seismic data sets are integrated into a sequence stratigraphic framework. Analyses of two piston cores collected from the upper part of the muddy drift and a box core from the top of the sandy drift determine the overall downcore lithology variations and made possible the development of high-resolution chrono and cyclo-stratigraphies. In the muddy drift periplatform sequence, downcore cyclic variations in, (1) sediment coarse fraction, (2) Sr counts as proxy for atoll-derived fine aragonite, (3) planktic foraminifer oxygen stable isotope composition, in addition to carbonate preservation and biostratigraphic markers, were determined. These downcore lithologic and geochemical variations in the muddy drift were tied to the seismic lines imaging the sandy-muddy drifts to resolve the timing of the carbonate sandy drift establishment and its overall evolution. Based on this aforementioned interpretation, the results of my research document the nature and timing of the longer-term evolution of the sandy and muddy drifts over multiple glacial-interglacial sea level cycles in the last 3 million years. Once the timing of the drift was determined, the prograding internal architecture of the sandy drift was examined and interpreted in the context of the relatively well-established Plio-Pleistocene sea level fluctuations and the bottom current variations

In the recent scenario of climate change, the natural variability and uncertainty associated with the hydrologic variables is of great concern to the community. This thesis opens up a new area of multi-disciplinary research. It is a promising field of research in hydrology and water resources that uses the information from the field of atmospheric science. A new way to identify and capture the variability and uncertainty associated with the hydrologic variables is established through this thesis. Assessment of hydroclimatic teleconnection for Indian subcontinent and its use in basin-scale hydrologic time series analysis and forecasting is the broad aim of this PhD thesis. The initial part of the thesis is devoted to investigate and establish the dependence of Indian summer monsoon rainfall (ISMR) on large-scale Oceanic-atmospheric circulation phenomena from tropical Pacific Ocean and Indian Ocean regions. El Niño-Southern Oscillation (ENSO) is the well established coupled Ocean-atmosphere mode of tropical Pacific Ocean whereas Indian Ocean Dipole (IOD) mode is the recently identified coupled Ocean-atmosphere mode of tropical Indian Ocean. Equatorial Indian Ocean Oscillation (EQUINOO) is known as the atmospheric component of IOD mode. The potential of ENSO and EQUINOO for predicting ISMR is investigated by Bayesian dynamic linear model (BDLM). A major advantage of this method is that, it is able to capture the dynamic nature of the cause-effect relationship between large-scale circulation information and hydrologic variables, which is quite expected in the climate change scenario. Another new method, proposed to capture the dependence between the teleconnected hydroclimatic variables is based on the theory of copula, which itself is quite new to the field of hydrology. The dependence of ISMR on ENSO and EQUINOO is captured and investigated for its potential use to predict the monthly variation of ISMR using the proposed method. The association of monthly variation of ISMR with the combined information of ENSO and EQUINOO, denoted by monthly composite index (MCI), is also investigated and established. The spatial variability of such association is also investigated. It is observed that MCI is significantly associated with monthly rainfall variation all over India, except over North-East (NE) India, where it is poor. Having established the hydroclimatic teleconnection at a comparatively larger scale, the hydroclimatic teleconnection for basin-scale hydrologic variables is then investigated and established. The association of large-scale atmospheric circulation with inflow during monsoon season into Hirakud reservoir, located in the state of Orissa in India, has been investigated. The strong predictive potential of the composite index of ENSO and EQUINOO is established for extreme inflow conditions. So the methodology of inflow prediction using the information of hydroclimatic teleconnection would be very suitable even for ungauged or poorly gauged watersheds as this approach does not use any information about the rainfall in the catchment. Recognizing the basin-scale hydroclimatic association with both ENSO and EQUINOO at seasonal scale, the information of hydroclimatic teleconnection is used for streamflow forecasting for the Mahanadi River basin in the state of Orissa, India, both at seasonal and monthly scale. It is established that the basin-scale streamflow is influenced by the large-scale atmospheric circulation phenomena. Information of streamflow from previous month(s) alone, as used in most of the traditional modeling approaches, is shown to be inadequate. It is successfully established that incorporation of large-scale atmospheric circulation information significantly improves the performance of prediction at monthly scale. Again, the prevailing conditions/characteristics of watershed are also important. Thus, consideration of both the information of previous streamflow and large-scale atmospheric circulations are important for basin-scale streamflow prediction at monthly time-scale. Adopting the developed approach of using the information of hydroclimatic teleconnection, hydrologic variables can be predicted with better accuracy which will be a very useful input for better management of water resources.

In the recent scenario of climate change, the natural variability and uncertainty associated with the hydrologic variables is of great concern to the community. This thesis opens up a new area of multi-disciplinary research. It is a promising field of research in hydrology and water resources that uses the information from the field of atmospheric science. A new way to identify and capture the variability and uncertainty associated with the hydrologic variables is established through this thesis. Assessment of hydroclimatic teleconnection for Indian subcontinent and its use in basin-scale hydrologic time series analysis and forecasting is the broad aim of this PhD thesis. The initial part of the thesis is devoted to investigate and establish the dependence of Indian summer monsoon rainfall (ISMR) on large-scale Oceanic-atmospheric circulation phenomena from tropical Pacific Ocean and Indian Ocean regions. El Niño-Southern Oscillation (ENSO) is the well established coupled Ocean-atmosphere mode of tropical Pacific Ocean whereas Indian Ocean Dipole (IOD) mode is the recently identified coupled Ocean-atmosphere mode of tropical Indian Ocean. Equatorial Indian Ocean Oscillation (EQUINOO) is known as the atmospheric component of IOD mode. The potential of ENSO and EQUINOO for predicting ISMR is investigated by Bayesian dynamic linear model (BDLM). A major advantage of this method is that, it is able to capture the dynamic nature of the cause-effect relationship between large-scale circulation information and hydrologic variables, which is quite expected in the climate change scenario. Another new method, proposed to capture the dependence between the teleconnected hydroclimatic variables is based on the theory of copula, which itself is quite new to the field of hydrology. The dependence of ISMR on ENSO and EQUINOO is captured and investigated for its potential use to predict the monthly variation of ISMR using the proposed method. The association of monthly variation of ISMR with the combined information of ENSO and EQUINOO, denoted by monthly composite index (MCI), is also investigated and established. The spatial variability of such association is also investigated. It is observed that MCI is significantly associated with monthly rainfall variation all over India, except over North-East (NE) India, where it is poor. Having established the hydroclimatic teleconnection at a comparatively larger scale, the hydroclimatic teleconnection for basin-scale hydrologic variables is then investigated and established. The association of large-scale atmospheric circulation with inflow during monsoon season into Hirakud reservoir, located in the state of Orissa in India, has been investigated. The strong predictive potential of the composite index of ENSO and EQUINOO is established for extreme inflow conditions. So the methodology of inflow prediction using the information of hydroclimatic teleconnection would be very suitable even for ungauged or poorly gauged watersheds as this approach does not use any information about the rainfall in the catchment. Recognizing the basin-scale hydroclimatic association with both ENSO and EQUINOO at seasonal scale, the information of hydroclimatic teleconnection is used for streamflow forecasting for the Mahanadi River basin in the state of Orissa, India, both at seasonal and monthly scale. It is established that the basin-scale streamflow is influenced by the large-scale atmospheric circulation phenomena. Information of streamflow from previous month(s) alone, as used in most of the traditional modeling approaches, is shown to be inadequate. It is successfully established that incorporation of large-scale atmospheric circulation information significantly improves the performance of prediction at monthly scale. Again, the prevailing conditions/characteristics of watershed are also important. Thus, consideration of both the information of previous streamflow and large-scale atmospheric circulations are important for basin-scale streamflow prediction at monthly time-scale. Adopting the developed approach of using the information of hydroclimatic teleconnection, hydrologic variables can be predicted with better accuracy which will be a very useful input for better management of water resources.

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

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

The Western Ghats (WG) orography that runs along the western coast of peninsular India, is a long and narrow mountain chain with an average height of about 1200 meters. The orientation of this orography is approximately perpendicular to the mean low–level winds over the eastern Arabian Sea during boreal summer season (June–September; JJAS). In JJAS, while western sides of this orography receive high intensity of precipitation, a region to the lee side of the mountain, termed as the Bay of Bengal Cold Pool (BoB–CP), receive very less precipitation. In this thesis, we have investigated the role of WG orography in existence of the BoB–CP. In addition, we have shown the influence of WG on climate around the globe as well as the intraseasonal variability over the Indian region and the equatorial Indian Ocean. In the first part of the thesis, we have documented the climatology of BoB–CP and how the region is peculiar compared to other parts of south Asia. In boreal summer (JJAS), most of the Indian land and its surroundings experience rainrates exceeding 6 mm/day with considerable spatial variability. Over southern Bay of Bengal (BoB) along the east coast of the Indian peninsula (BoB–CP), the rain intensity is significantly lower (<2 mm/day ) than its surroundings. This low rainfall occurs despite the fact that the sea surface temperature in this region is well above the threshold for convection and the mean vorticity of the boundary layer is cyclonic with a magnitude comparable to that over the central Indian monsoon trough where the rainrate is about 10 mm/day. It is also noteworthy that the seasonal cycle of convection over the BoB–CP shows a primary peak in November and a secondary peak in May. This is in contrast to the peak in June–July over most of the oceanic locations surrounding the BoB–CP. We use an Atmospheric General Circulation Model (AGCM) to understand this paradox. Decade long simulations of the AGCM were carried out with varying (from 0 to 2 times the present) heights of the WG. We find that the lee waves generated by the strong westerlies in the lower troposphere in the presence of the WG mountains cause descent over the BoB–CP. Thus, an increase in the height of the WG strengthens the lee waves and reduces rainfall over the BoB–CP. More interestingly in the absence of the WG mountains, the BoB–CP shows a rainfall maxima in the boreal summer similar to that over its surrounding oceans. The redistribution of rainfall with the increase in height also resulted in the increase in Indian summer monsoon rainfall (ISMR) by almost 15%. The WG also impacts the climate over the middle and high latitude regions by modifying the upper tropospheric circulation. In the second part of the thesis, we have investigated the role of convection over northern BoB in controlling the rainfall over BoB–CP. Even after the removal of WG, the BoB–CP shows low level divergence, which leads us to speculate the role of acceleration/deceleration of meridional winds by convection over northern BoB. Intraseasonal variations (ISVs) over BoB–CP also depicts the existence of the see–saw between precipitation over head Bay of Bengal and southern Peninsular India, including BoB–CP. Based on these findings, we performed decade long simulations with varying Sea Surface Temperature (SST) gradients over northern BoB. The SST gradient– experiments reveal that convection over north BoB further reduces rainfall over BoB–CP by intensifying the upper level lee–waves, causing down–draft and accelerating the low level winds causing divergence near the surface. A combined effect of WG and SST gradients shows that even though the SST gradients influence convection over BoB–CP, the effect is overshadowed by the absence of WG indicating that the WG has dominant control on the convection over BoB–CP than the other. In the third part of our study, we analyzed the implications of the perturbations in WG orography on ISVs over India as well as over the equatorial Indian Ocean region. The increase in height of WG leads to the intraseasonal oscillations (ISO) to strengthen over the equatorial region. With the absence of WG, the northward propagations have become stronger compared to the mean state. These variations in ISVs also altered the ISVs over the equatorial Indian Ocean. Madden Julian Oscillation (MJO) is the most important component of ISVs over the equatorial belt, which we have investigated in this study. The model captures the MJO signal reasonably well with slight underestimation in its strength and meridional extent. With the increase in WG height, there is a change in circulation pattern around WG region, increasing the meridional as well as the westerly component of wind over the equatorial region. This provides more moisture as well as an increase in boundary layer convergence, eventually leading to the increase in convective activity associated with MJO over the region. This also suggests that it is essential to represent the orographic features near the equatorial region in order to simulate the MJO reasonably well in a model. In the last part of the thesis, we document the observed changes in the variability of rainfall and outgoing longwave radiation (OLR) associated with the MJO during 1998 to 2015, when reliable satellite derived daily rainfall and OLR are available. Observations show recent weakening of variance of convective activity with MJO across the equatorial Indian Ocean (EQIO) and Maritime Continent (MC) during boreal summer as well as winter seasons. However, during boreal winter MJO variance increased significantly over northern Australia and north–eastern Pacific. Using rain gauge based observations we further show that the decreasing trend of 30–60 day intraseasonal mode over MC is significant for an extended range of period (1958–2007). During northern summer, the MJO variability in the POST (2007–2015) period display remarkable reduction in convection for all the wavenumbers compared to PRE (1998–2006) period. During northern winter, along with reduction of intensity, the maximum variance of MJO related activity is shifted from lower to higher wavenumber in recent years. Thus, during the POST period, the convection associated with an MJO is broken down into smaller scales, reducing the variability in rainfall along the longitudes. The multivariate MJO index (The Wheeler–Hendon Index) exhibits a shift from a higher probability of stronger events to weaker events over EQIO, MC and Western Pacific Ocean in the recent years both during summer and winter seasons. There is a southward shift in the location of maximum variance from northern latitudes towards the southern latitudes either weakening the northern branch of maximum variance or reducing (increasing) the asymmetry along those longitudes during summer (winter). The relationship between OLR and rainfall has also modified from PRE to POST between the Indian Ocean and Maritime Continents possibly due to increase in cloud top height and recent sea surface warming over the Indian Ocean. These variations in MJO strength can have a huge impact on the local and remote climate systems across the globe and can modulate the extreme events across the globe. This study highlights the importance of WG orography in modulating the convection over BoB–CP, redistribution of rainfall over the subcontinent and the climate over the globe. These mountains can impact the ISVs over India as well as the equatorial Indian Ocean. The results of this study underline the importance of narrow mountains like the WG in the tropics in altering the global climate and possibly calls for a better representation of such mountains in climate models. It also provides insights into the recent weakening of MJO and its possible influence on the climate as well as the extreme events across the globe.

Thesis (Ph. D.)--Florida State University, 2007.
Advisor: Sherwood W. Wise, Jr., Florida State University, College of Arts and Sciences, Dept. of Geological Sciences. Title and description from dissertation home page (viewed Mar. 26, 2008). Document formatted into pages; contains xvi, 173 pages. Includes bibliographical references.