Variability Of Tropical Indo-Pacific Ocean Sea Surface Temperature And East Asian Summer Monsoon Climate Anomalies

Tropical Indo-Pacific sea surface temperature (SST) variability is one of the crucial factors affecting East Asian summer climate anomalies as well as China climate anomalies. With observational analyses and numerical modeling, this dissertation investigates principal modes of the tropical Indo-Pacific SST (TIPSST) variability and their associations with East Asian summer monsoon (EASM) climate anomalies. A sliding EOF decomposition metheod is used to isolate the interannual modes of TIPSST variability and their possible interdecadal change, and then a seasonal joint EOF decomposition is employed to extract the principal modes with seasonally-varying structures of TIPSST variability and their relationship with atmospheric circulation anomalies. We investigate the interannual variations of cross-equatorial currents under influence of TIPSST and their association with EASM. A series of numerical experiments are conducted to identify the role of the Tropical Indian Ocean in ENSO’s impact on the EASM. The main conclusions are as follows.(1) Interdecadal change of principal modes of TIPSST anomaliesWith conventional EOF decompodsition, the tropical Indian Ocean SST anomalies are characterized by a basin mode during winter through summer and by a dipole mode during autumn, while the tropical Pacific SST anomalies are by a classical ENSO mode and a so-callled central Pacific ENSO. The ENSO is closely related to Indian Ocean SST anomalies. The tropical Indian Ocean SST anomalies lag tropical Pacific ENSO by3-6months. When ENSO develops during autumn, this tends to associated with a dipole mode developing in the tropical Indian Ocean. When the ENSO matures during winter, an Indian Ocean Basin mode tends to develop, lasting to next spring. The tropical Indian Ocean warming excites an anomalous anticyclonic cell in the northwestern Pacific with easterly anomaly near the equator which eventually terminates the initial El Nino event. At the same time, the anomalous anticyclonic cell in the northwestern Pacific tends to enhance the EASM. Relatively, the so-called central Pacific ENSO has less relation to the Indian Ocean SST anomalies. A sliding EOF decomposition is used to identify the interdecadal change of the leading EOF modes in TIPSSTA.(2) Principal modes with seasonally-varying structure of TPSSTA and their impacts.Due to convensional EOF decomposition’s drawback, a seasonal joint EOF (SEOF) decomposition method in this study is applied to the tropical Pacific SST anomalies (TPSSTA). Three principal SEOF modes are identified, the traditional ENSO mode with3-5yr period, the trend mode, and the tropical Pacific meridional mode (TPMM) with11yr period.The ENSO mode is teleconnected to the global SST and atmospheric anomalies in which the SSTA in the tropical Indian Ocean significantly lags the ENSO. The trend mode is characterized a cooling in the equatorial eastern Pacific and a warming in the western tropical Pacific. The TPMM features a meridional dipole with SST anomalies between cold tougue and the northern ITCZ region as well as a zonal diple between the cold tougue region and the central equatorial Pacific, in which the meridional dipole is dominant. The TPMM is a decadal mode with roughly11yr period which is similar to the tropical Atlantic meridional mode. The so-called central Pacific ENSO can be considered the zonal component of the TPMM.The three SEOF modes have distinct impact on the EASM climate anomalies. A warm ENSO event also tends to weaken EASM in current and next summer, while the trend mode is associated with a weakening of EASM. However, the TPMM can favor an enhanced EASM when a warm TPMM is at its decaying stage.(3) TPSST-related cross-equatorial flow variability and its imact Observational analyses show that the cross-equatorial flow (CEF) in east protion has a weak negative correlation with Somlia jet. This is majorly due to that the CEF in east protion and Somali jet have two types of relation:in-phase and out-of-phase, in which the out-of-phase relation is a majority. The two relations correspond to two types of SST aomaly pattern in the tropical Pacific. An out-of-phase relation corresponds to a conventional ENSO mode, while an in-phase relation to a central Pacific SST anomaly. Also, the two relations can be associated with different EASM and China climate anomalies.(4) The role of tropical Indian Ocean in ENSO’s impact on the EASM climate anomaliesThere is a lead/lag inter-basin relation in TPSSTA. The tropical Indian Ocean can be warming3-6months after an El Nino event is developing in the tropical Pacific. A composite anlysis with NCEP/NCAR reanalysis data is made to document the seasonal evolution of atmospheric circulation anomalies at different ENSO phases. And a series of numerical experiments with an atmospheric circulation model (CCM3) are conducted to uncover the effect of the tropical Indian Ocean in ENSO’s impact.Accompanying the development of a El Nino event during summer in the tropical Pacific, an anomalous cyclonic cell appears in the South China Sea (SCS)-Phillipines region, inducing the weakening of EASM. This is determined primarily by the ENSO-related tropical Pacific SST anomalies. When the El Nino event further goes to mature, anomalous easterlies appear in equatorial tropical Indian Ocean and migrate eastward. Associated with this, an anomalous anticyclonic cell shows up near the CSC-Phillipines region during autumn, persisting until up to next suumer. The long-lasting anomalous anticyclonic cell is just a response to the tropical Indian Ocean warming that is the lag effect of the El Nino. The anticyconic cell favors increased in southern China during winter-spring, but an enhanced EASM during summer. Thus the tropical Indian Ocena acts as a delayer and amplifier in ENSO’s imact on EASM anomalies.