The Influence Of Plateau Summer Monsoon On Summer Precipitation Over Central And East Asia

The plateau summer monsoon not only dominates the plateau climate in summer, but also has an important influence on precipitation over its surrounding area. Using a variety of reanalysis data, this thesis analyzes the basic characteristics of the plateau summer monsoon and its variability, and compares the differences among different reanalysis data. The influences of plateau summer monsoon on summer precipitation in Central and East Asia, as well as the corresponding change of the atmospheric circulation are also discussed. Finally, the relationship of the plateau summer monsoon, Indian summer monsoon and East Asian summer monsoon, as well as the impact of plateau summer monsoon on SST and CGT is investigated in detail. The main results are as follows:The near surface layer of Tibetan plateau in the summer is a thermal low, with cyclonic rotation of wind around plateau and convergence of airflow above. The plateau summer monsoon indexes generated by seven reanalysis data overall, are very consistent, especially for interannual variability. The plateau summer monsoon has remarkable interannual and interdecadal variability. From the early1950s to early1960s, the plateau summer monsoon gradually weakened, but began to strengthen in the subsequent14years. For the period of1974to1986, it fluctuated with small amplitude with no clear trend. It weakened again from1986to1994and then gradually strengthened from1994to2010. The plateau summer monsoon has significant period of70,20and10years in terms of low frequency, and3-4years period in high frequency. Similarly, the climatic variations of dryness and wetness over western area of China also exhibited a significant quasi-70year period, which may be dominated by the plateau summer monsoon.The plateau summer monsoon and summer precipitation show significant positive correlation over the areas from central Asia to eastern plateau across the Tarim basin and over the middle and lower reaches of the Yangtze River. While in Mongolia, Lake Baikal, northern China and northern India, they show a significant negative correlation. The200hPa geopotential height field in central and east Asia show the pattern of "-+-"for strong plateau summer monsoon, but"+-+" for weak plateau summer monsoon. The plateau summer monsoon may have an influence on the teleconnection pattern over the mid-latitude of central and East Asia in summer. When the plateau summer monsoon is stronger, considering the wind field of middle and low layer, the central Asian region is characterized by abnormal cyclone, in favor of updraft convergence of low-level moisture, leading to more rainfall. The southern Xinjiang region are characterized by abnormal eastern flow, which is helpful to guide the warm moist air flow of the east side of the plateau into the region, increasing the moisture conditions in the region and conducive to the occurrence of precipitation. The northeasterly wind over northern China is abnormal, which hinder the south airflow carrying abundant water vapor, resulting in less rainfall. For India region, it shows abnormal east airflow, which hinders the southwest flow of Indian summer monsoon, leading to less rainfall. When plateau summer monsoon was weaker, the central Asian was characterized by abnormal anticyclone, conducive to water vapor divergence, leading to less rainfall; The southern Xinjiang region are characterized by abnormal westerly wind, which hinder the entry of warm moist air flows on the east side of the plateau, not conducive to the occurrence of precipitation. The northern China was characterized by abnormal southwest wind with more water vapor transport, leading to more rainfall. With anomalous westerly in India, southwest flow of Indian summer monsoon is enhanced, leading to more rainfall.The plateau summer monsoon is in negative correlation with Indian summer monsoon and East Asia summer monsoon, with the correlation coefficient of-0.43and-0.24respectively. When the plateau summer monsoon is stronger, the precipitation in north China and northern India is low. However, the Indian summer monsoon and the Indian summer precipitation are in positive correlation, as the East Asian summer monsoon and the summer precipitation in north China. Therefore the plateau summer monsoon may have an influence on the precipitation in northern India and north China by affecting the Indian summer monsoon and the East Asian summer monsoon respectively. The Plateau summer monsoon is in positive correlation with summer sea surface temperature (SST) in Indian Ocean and South China Sea, and in negative correlation with the SST in northwest Pacific. When the plateau summer monsoon is stronger and SST in Indian Ocean is warmer, sea-land thermal difference decreases, so the Indian summer monsoon was weaker. With warmer SST in South China Sea and colder SST in northwest Pacific Ocean, the meridional sea-land thermal difference decreases and zonal sea-land thermal difference increases. Their net effect eventually leads to the weaker East Asian summer monsoon and small correlation between plateau summer monsoon and East Asian summer. The plateau summer monsoon and CGT (Circumglobal Teleconnenction) show significant negative correlation. When the plateau summer monsoon is strong (weak), CGT is in negative (positive) phase, leading to less (more) precipitation in north China. The plateau summer monsoon has an influence on CGT by affecting western plateau sensible heating. When the plateau summer monsoon is stronger, western plateau sensible heating is weaker, leading to cyclonic anomaly in central Asia, so CGT is in negative phase with less rainfall in north China. When the plateau summer monsoon was weaker, western plateau sensible heating is stronger leading to anticyclone anomaly in central Asia, so CGT is in a positive phase, with more precipitation in north China.