The Intraseasonal Variation of Summer Monsoon Circulation and Rainfall in East Asia
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Abstract
Based on routine meteorological data and the method of the normalized finite temporal variation (NFTV), the evolution features of East Asian summer monsoon (EASM) circulation are analyzed. It is found that there exist two distinct subseasonal abrupt changes in East Asia during the summertime, characterized primarily by two eastward contractions and northward jumps of the western Pacific subtropical high (WPSH), one of which is in middle June and the other is in late July. Because of the close relationship between the WPSH and rainfall distributions in East Asia, the rain belt in East Asia exhibits two obvious northward jumps in the seasonal evolution as well, corresponding to the beginning of the Meiyu period from the Yangtze-Huaihe valley to Japan and the rainy season in North China and Northeast China, respectively. Compared with the first jump, the second jump of the WPSH is much more evident. The first jump is mainly caused by the enhancement of the convective activities in the South China Sea (SCS) while the second jump is influenced by both the convective activities over the western Pacific warm pool (WPWP) and the circulation systems in high latitudes. Through the phase-locking of the northeastward propagation of the Rossby wave trains from the WPWP and the downstream propagation of the Rossby waves in high latitudes, both the convective activities over the WPWP and the circulation systems in high latitudes play a key role in the second northward jump of the WPSH. In addition, the interactions between the WPSH and the release of the latent heat on its western edge lead to the intraseasonal low-frequency oscillation of the WPSH. The analyses of NFTV indicate that the adjustment of the low-level circulation in high latitudes tends to weaken gradually with the seasonal evolution, which is related to the temperature difference between middle and high latitudes. By contrast, the adjustment of the high-level circulation tends to intensify with the altitude during the latter half of the summertime. Besides, the evolution of the similarity also shows that the atmospheric circulation in East Asia exhibits a distinctly different state after the second jump of the WPSH. The Southern Hemisphere circulation plays an important role in the enhancement of the convective activities in the SCS and the WPWP. In middle June, the enhancement of the convective activities in these two regions is due to the intensification and eastward extension of the westerly on the western edge of the SCS, which is deeply involved with the Mascarene high (MH). In middle July, the enhancement of the Australian high (AH) leads to the intensification of the cross-equatorial flow on its northeastern edge, and a large amount of cold air from the Southern Hemisphere invades into the warm pool region, which increases the atmospheric instability and the low-level convergence over this region. As a result, the convective activities in the warm pool are enhanced. In the first half of the summertime, however, the relationship between the intensity of the AH and the cross-equatorial flow on its northeastern edge can be modulated by the convective activities in the WPWP, resulting in an opposite trend between the AH and the associated cross-equatorial flow. During the austral wintertime, the AH tends to weaken with the low-frequency oscillation, which is affected by both the surface temperature in Australia and the energy dispersion of the upstream MH. The weakening trend of the AH is influenced by the former while the MH plays a dominant role in the low-frequency oscillation of the AH.
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