| Based on the NCEP/NCAR reanalysis monthly data from 1951 to 2011 and monthly temperature data at 116 stations in southwest China from 1960 to 2012, the new indices Of Tibetan Plateau Monsoon (TPM) have been defined and its impacts on climate in southwest China are discussed.The plateau winter monsoon (TPWM) intensity continued increasing since middle 1980’s and its inter-annual period transformed from a longer 7-year to a shorter 4-year. The location of TPWM shifted from north to south and included a 4-year prominent period since middle 1980’s.The TPWM changed faster. For summer, the body strength of TPM is also increasing, while the centrobaric intensity of vorticity becomes weak. The mutation both occurred in 1970’s.there was a continuing eastward movement for the centrobaric longitude of vorticity after mid-1990s. The duration time of TP summer monsoon (TPSM) was prolonged for an earlier breakout time. The stronger the TPWM is, the later the TPSM breaks out and the shorter the duration time is. On the other side, a strong TPSM is conductive to a longer duration time and a later ending.In southwest China, temperature has been significantly higher since the mid-1990s, precipitation and air dry more seriously after entering the 21st century. Generally speaking, the annual precipitation in southwest China reducies, but rainfall intensity increases, and precipitation concentration period comes in advance. Local differences also exists in Western Sichuan Plateau, where the rainy season starts in advance and the end time postpones.The southwestern temperature is significantly influenced by TPWM intensity on the 4a cycle in winter. When the plateau high strengthens, the intersection volume between the northern cold air and southwestern warm air becomes less than normal, which causes less cloud. Otherwise, abnormal downdraft center appears in southwest China. The whole southwestern region is consistent with higher temperature. Vice versa. When plateau high is located north, more warm-wet air from the south and cold-dry air from the north join together in the eastern TP. Simultaneously, abnormal northeasterly winds dominate the eastern part of southwest China, and the western part exists abnormal updraft increasing precipitation which might lead to icing weather. The western part of southwest China maintains lower temperature. Vice versa.In summer, there’re two indices, strength and center of gravity of longitude, which have significant effects on southwest China climate change. The stronger TP summer monsoon is favorable for more moisture convergence, more cloudy and rainy days, less sunshine,weaker evaporation and smaller diurnal temperature range in southwest China. Moreover, Meteorological elements vary more significant in Sichuan-Chongqing region with deep updraft development than in Yunnan-Guizhou region with limited ascending flow anormaly to below 600 hPa. When the center of gravity of longitude moves to the east, the whole southwest China is controlled by the abnormal sinking motion, which contributes to a higher temperature.There appears abnormal strong moisture divergence in middle-west Sichuan, Guizhou, Guangxi province, which result in higher temperature, lower humidity and less rainfall over above regions. Since 21st century, the shrinking strength and eastward-moving of TPSM conspire to the drying trend of southwest China. Seasonal process of plateau monsoon corresponds to the zero hour and duration time of northwest Sichuan to Yunnan-Guizhou plateau rainy season. Under decadal background, the TPSM breaks ahead, while the rainy season in the previously mentioned zones also comes ahead and has a longer last. |
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