Relationships Between Regional Climate Variations Around Indonesia And Summer Precipitation Anomalies Over China

By using the monthly mean NCEP-NCAR reanalysis, CMAP precipitation and 160 stations data in China during the period 1979-2011, summer climate variations in Indonesia and their relationships with China precipitation changes have been investigated in the present paper. The results are as follows.1. Summer rainfall around Indonesia region exists a weak decreasing trend and apparent decadal variations, and also varies with periodicities of 4-6 yrs. The summer rainfall over Indonesia links significantly with that over China. When there are more(less) precipitation in Indonesia, most of Guangdong, Guangxi and Hunan will receive less (more) precipitation but in the junction part of Gansu, Ningxia, Shaanxi and Sichuan will there will be more(less) rainfall. Precipitation anomalies show anomalously "positive -negative-positive" distribution from the south to north, which is consistent with the distribution of OLR anomalies "negative-positive-negative". The anomalous apparent heat source and the divergence of the water vapor fluxes are in agreement with the distributions of precipitation anomalies. Abnormal diabatic forcing around Indonesia area can lead atmospheric circulation to a response through the southeast-northwest slanted vertical circulation so that in South China the strengthened diabatic heating sink and the corresponding divergence with reduced precipitation are found. On the other hand, in the junction of Gansu, Ningxia, Shaanxi and Sichuan, the stronger upward motions occur, leading to more than normal precipitation there.2. The empirical orthogonal function (EOF) analysis is performed for obtaining the principal modes from ENSO-influence-removed summer precipitation anomalies in Indonesia region. The leading EOF mode (EOF1) of anomalous rainfall around Indonesia archipelagos looks almost anti-symmetric about a slanted line from the northwest of the archipelagos across to the southeast of Indonesia, with the positively signed anomalies in southeastern Indian Ocean (SEIO) and the negatively signed in northwestern tropical Pacific (NWTP). The time-series of coefficients of EOF1 demonstrates that the anomalous rainfall varies with periodicities of 8-10 yrs as well as 2-3 yrs in the late 1990’s. The positive diabatic heating anomalies are found in Indonesia and SEIO whereas the negatives are in NWTP, facilitating the stronger convergence in SEIO and divergence over NWTP. The divergent air flow emanates from NWTP northwestward to coastal region of the southern China, leading to an anomalous anticyclone in lower troposphere over South China Sea (SCS) as a result of atmospheric response to the negative vorticity source in this region according to Gill’s theory. This anomalous anticyclone over SCS is also related to the EAP teleconnection pattern that is excited by the anomalous thermal forcing over NWTP. The southerly winds are strengthened in the northwestern flank of this anticyclone, converging with the northwest winds from mid-latitudes in Yangtze River valley. Due to this anomalous anticyclone, more water vapor is hence to be transported from SCS northward to the Middle Reaches of Yangtze River, where more than normal summer rainfall is hence induced. These results are helpful for us to deeply look into the mechanisms of influence of thermal forcing on the summer rainfall variations in the Middle Reaches of Yangtze River and to look for useful clues to predictions of the rainfall there.3. There occurred a severe flood event in summer of 1998 over Yangtze River basin. Our results demonstrate that the thermal contrasts between the southeastern tropical Indian Ocean and the northwestern tropical Pacific in summer of 1998 were crucial in inducing the convergence in Indian Ocean side and divergence in Pacific side in lower troposphere, which resulted in an anomalous anti-cyclonic circulation over South China Sea and tropical western Pacific, and henceforth weakening the western subtropical high. The atmosphere over Indonesian region was heated anomalously. An anomalous vertical circulation was formed, which connected the anomalous circulation in Pacific side with that in Indian Ocean side. The wave activity fluxes converged over South China Sea, facilitating the maintenance of the anomalous anti-cyclonic circulation there. The anomalous south wind flows in the western flank of the anti-cyclonic circulation brought rich moisture to Yangtze River basin where the convergence of water vapor occurred, inducing strong rainfall there.