Study On The Mechanism Of Soil Moisture Affecting Monthly Precipitation In Eastern China In Summe

In the past few decades,researchers have carried out lots of investigations on the summer precipitation variation in eastern China and found the important role played by soil moisture.However,most studies mainly focus on the summer-average precipitation in eastern China.In fact,with the north march of the monsoon rainband,there is an obvious sub-seasonal variation of summer precipitation in eastern China and the effects of soil moisture on the sub-seasonal variation of precipitation in eastern China are not well understood.Therefore,this paper will combine observational analysis and numerical experiments to study the effects of antecedent soil moisture on monthly precipitation in eastern China during summer.The main conclusions are as follows:(1)The observed results show close relationships between antecedent soil moisture in Indochina and southern China and monthly precipitation in eastern China during summer.The May soil moisture in Indochina bears a significant negative correlation with June precipitation in southern China.The June soil moisture in Indochina bears a significant negative correlation with the July precipitation in the Yangtze River Basin,and a significant positive correlation with the July precipitation in the Huang-Huai region(HHR)and southern China,corresponding to a tripolar precipitation anomaly in eastern China.The soil moisture in southern China in July is negatively correlated with precipitation in HHR in August.(2)The Indochina soil moisture anomaly can influence the precipitation in eastern China during early summer and midsummer,through changing the local and non-local atmospheric circulation and the water vapor transport.The positive soil moisture anomaly in Indochina during May can persist to June,and increase air moisture in the lower atmosphere,which is further transported to the upper and middle atmosphere under the background of air updraft.With the release of latent heat due to water vapor condensation,a cyclonic circulation anomaly occurs in the lower troposphere over eastern Indochina.An anomalous anticyclonic circulation occurs in the eastern part of southern China in June.The easterly wind anomaly over southern China inhibits water vapor transport from the Bay of Bengal to southern China and reduces June precipitation in southern China.Additional experiments further found the wet process of soil moisture mainly influences the precipitation near Indochina,while the thermal process can affect the precipitation in both Indochina and southern China.In June,the moister soil in Indochina can also trigger the cyclonic circulation anomaly locally.But due to the differences in background atmospheric circulation,the anomalous anticyclonic circulation appears near the Yangtze River Basin in July,which is conducive to the northward movement of the Western Pacific Subtropical High,resulting in a tripolar distribution of precipitation anomalies in eastern China in July.(3)The soil moisture in southern China can affect the precipitation in HHR during late summer by triggering the anomalous meridional vertical circulation and low-level cyclonic circulation anomaly in eastern China.The higher soil moisture in southern China during July can persist to August and increase the low-level moisture in southern China.The increased moisture is transported upwardly and releases the latent heat of condensation.As a result,there is abnormal upward motion over southern China and anomalous downward motion in HHR.At the same time,there are cyclonic circulation anomalies and convergence of water vapor flux in the local low-level atmosphere.The anomalous cyclonic circulation further increases water vapor transport to southern China and reduces water vapor transport to HHR,leading to less HHR precipitation in August.The additional experiments with fixed soil moisture from July to August further prove that the precipitation anomaly in HHR is mainly caused by the soil moisture anomaly in southern China,rather than the persistent atmospheric circulation anomaly.(4)The sea surface temperature(SST)anomalies that influence the soil moisture in Indochina and southern China are mainly located in the tropical Pacific Ocean and the North Atlantic Ocean.The antecedent SST anomaly in the tropical Pacific can affect the soil moisture in Indochina from May to June,and the North Atlantic and tropical Pacific SST anomalies in June can affect the precipitation and soil moisture in southern China in July.The SST anomaly in the tropical Pacific can affect the precipitation anomaly in eastern China,through changing the Walker Circulation and the Hadley Circulation.The North Atlantic SST anomaly can trigger the Eurasian mid-high latitude wave train and the abnormal westward zonal circulation to affect the precipitation in eastern China.In addition,it is found that the impacts of soil moisture in the key regions are independent of SST signals.SST anomalies in the North Atlantic and the tropical Pacific can act together with the June soil moisture anomalies in Indochina to influence the July precipitation in eastern China.The soil moisture anomaly corresponds to a tripolar precipitation anomaly in eastern China during July,and the SST anomaly corresponds to a dipole precipitation anomaly in eastern China.The results of this paper are helpful to understand the physical mechanism of the soil moisture effects,and are of great significance to improve the sub-seasonal prediction of summer precipitation in eastern China.