Diagnostic Analysis Of Moisture And Energy Balance In Summer Precipitation In Northern China During El Ni(?)o Development And Decline

Based on monthly precipitation data in China and JRA-55 reanalysis data provided by Japan from 1958 to 2019,this study quantitatively analyzes the causes of precipitation anomalies in northern China during El Ni(?)o developing and decaying summer through water vapor and moist static energy(MSE)equation.The main conclusions are summarized as follows:(1)During El Ni(?)o developing summer,the anomalous precipitation in northern China is mainly controlled by the dynamic effect related to the atmospheric circulation.The water vapor budget shows that the reduced precipitation is closely related to the anomalous vertical descending.Through the diagnosis of MSE equation,it is found that this anomalous vertical motion is mainly constrained by the dynamic effect(anomalous wind-climatological moist enthalpy advection),while the thermodynamic effect(climatological wind-anomalous moist enthalpy advection)is counterproductive.The anomalous northerly wind brings low moist energy and low dry enthalpy air into northern China,with a deficit of moist static energy in the region.A descent is then produced to compensate the energy lose and resulting in dryness.Circulation analysis shows that the El Ni(?)o sea temperature anomaly in the Central and Eastern Pacific can cause an anomalous cyclone over Northeast Asia.The northwest wind behind the trough transports the dry and cold air into Northern China,leads to reduced energy in the region,which in turn causes a descending motion.This is consistent with the conclusion of the quantitative diagnoses.In addition,we have further demonstrated that the dynamic and thermodynamic effects of the anomalous precipitation in northern China during the El Ni(?)o developing summer mainly reflect the situation of EP El Ni(?)o.(2)During El Ni(?)o decaying summer,atmospheric circulation dynamic effect is still the main mechanism leading to anomalous precipitation in northern China.From the quantitative diagnostic,the excessive precipitation in northern China during El Ni(?)o decaying summer is related to the anomalous vertical ascending motion.The anomalous southerly wind transports air with high moist energy and high dry enthalpy to the northern China,with an increase of moist static energy in the region.To maintain energy balance,an ascending motion is generated in northern China and eventually leads to anomalous precipitation.Circulation analysis also confirmed the above conclusion,during El Ni(?)o decaying summer,the anomalous anticyclone over Northwest Pacific strengthened the monsoon in eastern China,bringing the warm and wet air from the south into northern China,and the rain belt moved northward.This is also consistent with the quantitative analysis.In addition,further study has shown that,the MP El Ni(?)o plays a relatively weak leading role in dynamic and thermodynamic effects of the anomalous precipitation in northern China during El Ni(?)o decaying summer.(3)Finally,a preliminarily analyze is made of the possible reasons why the anomalous precipitation in northern China is mainly controlled by circulation dynamic effect.Through a comparative analysis with the summer precipitation in the Yangtze River Basin,it is found that the thermodynamic effect has a weak contribution to the summer precipitation anomaly in northern China.For the Yangtze River Basin,thermodynamic effect has a positive feedback on the anomalous precipitation,which promotes the anomalous precipitation further increased.The reason may be related to the local background field in the two regions.The high climatological summer precipitation and low-level wind speed in the Yangtze River Basin make it easy for anomalous precipitation to cause local water vapor changes through evaporation,and then increase the local anomalous precipitation through the positive feedback of the thermodynamic effect.While the climatological summer precipitation and low-level wind speed in northern China are relatively small,and anomalous precipitation is often limited,which causes the local water vapor change is small,so it is difficult to affect local precipitation.Therefore,the anomalous summer precipitation in northern China is mainly affected by circulation dynamic effect.