The Role Of Tropical Monsoon And The Sea-land Thermal Difference On The Sub-tropical Monsoon In East Asia

With the ERA-40 monthly averaged reanalysis datasets, the precipitation datasets of 160 land stations in China and the CMAP precipitation data from NOAA, through defining a East-Asia tropical monsoon index, we analyzed the relationship between the summer monsoon from June to July in the South China Sea and the monsoon and precipitation in the lower valley of the Yangtze River from the point of observations. Then we used the NCEP-NCAR reanalysis datasets and the MM5v3 Meso-scale Model, further verifies the influence that the variation of the monsoon in the South China Sea exerts on the monsoon and precipitation in the lower valley of the Yangtze River; Finally using the same model, we investigated the impacts of surface temperature differences between the East Asia land and its adjacent oceans on summer monsoon and precipitation in the lower valley of the Yangtze River and its mechanism. The conclusions are as follows:(1) The analysis of observations shows the tropical monsoon and precipitation in the South China Sea are in out-of -phase with the summer southwestly winds and rainfall in the lower valley of the Yangtze River. When the tropical monsoon and rainfall strengthen, in the lower valley of the Yangtze River the southwestly winds weakens and rainfall decreases. The modeling results show that when tropical monsoon changes, the southwestly winds and rainfall in the lower valley of the Yangtze River have an obvious change, which is consistant with the results of observations. This shows that the tropical monsoon has a significant impact on monsoon and rainfall in the lower valley of the Yangtze River, and it can be supported by physical process. Furthermore, when summer tropical monsoon strengthens , the tropical westly winds and precipitation increases. But it doesn't move northward and the southwestly winds and rainfall in Southern China don't increase neither. On the contrary, when tropical monsoon strengthens ,the west-Pacific subtropical high weakens and moves southward, leading to an weaker southwestly winds and rainfall in the lower valley of the Yangtze River.(2) It can be concluded from the model results that when we change the East Asia continent surface albedo, or the surrounding sea surface temperature, the Yangtze River southwest wind and precipitation also have changed significantly, which implys the land-sea thermal difference plays an important role in the subtropical monsoon. But the mechanisms on the impact of the monsoon are different. When we increase the local surface albedo, the atmospheric circulation anomalies is over the land center, mainly by affecting the thermal pressure to become weaken , as a result,the Yangtze River southwest wind have weakened. It results in increased precipitation Yangtze River to Northen South , but decreased precipitation northern of Yangtze River. When the sea surface temperature increases, the lower abnormal circulation appears over the sea.,and westward tilt with height to move on land. It mainly through weakens the strength of the subtropical high, leading to a decrease of southwestly winds and rainfall from the southern China to the southeast reagions in North China.