The Impacts of Tropical Ocean SST and Tibetan Plateau Snow Depth on Monsoonal Precipitations over Southeastern China

熱帶海洋海温異常與青藏高原積雪對中國東南部季風降水的影響

Student thesis: Doctoral Thesis

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Award date23 Aug 2019

Abstract

In this study, the cross-seasonal relationships and interactions between Indian Ocean basin (IOB), Indian Ocean dipole (IOD), El Niño-Southern Oscillation (ENSO), winter, and summer monsoon activities are investigated.

For the cross-seasonal relationship between winter and summer monsoon activities, this study revisits the forcing of East Pacific (EP) and Indian Ocean (IO) on the seasonal monsoon circulation to propose such a cross-seasonal relationship to improve the seasonal predictability of monsoon activities. The observation results identify the seasonal relationships between monsoon activities, especially over South China (SC) in winter and summer. A time series of this unified monsoon index in summer and winter, and a lead-lag correlation analysis of the unified monsoon index, showed that from summer to winter, monsoon activities tend to have an out-of-phase relationship (weak summer with strong winter monsoons or strong summer with weak winter), while from winter to summer, they tend to be in-phase (weak winter with weak summer or strong winter with strong summer). The composite difference between strong and weak winter monsoons shows that in the preceding summer of strong winter monsoons, monsoon activities tend to be weak, due to the influence of developing La Niña-like events. In the ensuing summer, monsoon activities tend to be strong, modulated by an anomalous cyclone over the western North Pacific (WNP), which is triggered by that La Niña-like pattern over the East Pacific. From summer to winter, the Indian Ocean Dipole (IOD) mode is evident over the Indian Ocean (IO). From winter to summer, the Indian Ocean Basin (IOB) mode is dominant, which confirms the vital role of the IO in these seasonal relationships of monsoon activities. A negative IOD pattern acts together with La Niña-like forcing to enhance northerly anomalies over East Asia, strengthen winter monsoon winds, and thus enhance the out-of-phase relationship. IOB cooling can capture the anomalous signal of ENSO and favor the persistence of an anomalous cyclone over the WNP until the following summer ("capacitor effect"), which is essential for the in-phase relationship.

The delayed effect of IOD on the ensuing summer is also examined. Results prove that there are significant anomalous signals in precipitation during the ensuing summer after the IOD forcing mode has dissipated over the tropical Indian Ocean where there is increasing precipitation over the Yangtze-Huaihe River valley (YHR) region associated with abnormal water vapor convergence contributed mainly by southerly water vapor input along the western flank of an anomalous anticyclone over the subtropical western North Pacific (WNP). The mechanism is summarized as a cross-seasonal relationship between the IOD, Tibetan Plateau (TP) snow, and summer precipitation. IOD can modulate the precipitation over the TP during winter and deepen the snow distribution over the southern TP. Anomalous snow conditions over the TP are a likely capacitor of the delayed effect of the IOD in that they can exert further influence on the ensuing summer monsoon.

The amplification of ENSO on the delayed effect of IOD on the ensuing summer monsoon is further discovered when comparing the most two most active IOD events in 1997 and 2006. The IOD in 1997 occurs with an extreme El Niño at the same time, but the IOD in 2006 independently develops over Indian Ocean (IO) without an El Niño. The anomalous Walker circulation associated with the El Niño leads to stronger easterly anomalies over the equatorial Indian Ocean so that the intensity of IOD in 1997 is strengthened, which triggers a more intensive atmospheric response during autumn and following winter, resulting in deeper snow distribution over southern Tibetan Plateau (TP). When excessive snowpack over TP melts, it would exert an important influence on the ensuing summer climate, through releasing the latent energy favorable for the formation of the lower-level eastward propagating synoptic vortex and through modulating large-scale circulation adjustment. During the ensuing summer of IOD, an anomalous anticyclone forms over western North Pacific (WNP), attributing to the divergence caused by the easterly anomalies over tropical IO, which is induced by the weakened land-sea thermal contrast between TP and tropical IO. The amplification effect of ENSO on the relationship of IOD-summer precipitation also lies in the fact that the ENSO forcing itself can also benefit the establishment and maintenance of the anomalous anticyclone over WNP via local atmosphere-ocean coupling effect and the IO “capacitor” effect.

The asymmetry in the IOD impacts is revealed by composite and regression analysis. Asymmetry in the impacts of the IOD is revealed by composite and regression analysis. During the simultaneous autumn, positive IODs tend to suppress precipitation over the YHR and insignificantly decrease precipitation over SC; negative IODs are also able to decrease precipitation variation over the YHR. During the ensuing summer, only SC has significantly positive responses to positive IODs; negative IODs can weaken precipitation over the YHR but also enhance precipitation over SC. An examination of moisture circulation shows that moisture transport responses are also distinctive for positive and negative IODs. SST evolution in conjunction with IODs in the following seasons is found to be able to account for the persistent influence of the IOD. For the asymmetrical impacts of the IOD, large-scale circulation responses to negative IODs are distinctive. Simultaneously, a barotropic anticyclonic circulation pattern is located in the midlatitudes of East Asia, contributing to the different moisture convergence over eastern China, which is due to anomalous forcing over the central Pacific. The CP ENSO-like forcing can induce a Pacific–North American teleconnection pattern (PNA)-like wave train and consequently transport wave energy to the midlatitudes, influencing East Asia along the wave guide. For delayed effects, negative IODs can further benefit a developing CP ENSO, whose heating maximum is located around the date-line, associated with a cyclonic Gill-type response at the lower level, undermining the influence of the anomalous divergence over the western North Pacific due to the anomalous easterlies over the equatorial Indian Ocean.