Asymmetric Effects Of ENSO On Precipitation In The Western North Pacific During The Following Summer

Based on the analysis of Nino3.4 index from 1979 to 2018 and the precipitation anomaly of the following summer in the Western North Pacific(WNP),it is found that there was a significant negative correlation between the intensity of El Nino and the WNP precipitation.anomaly in the following summer,while there was no significant correlation between La Nina and WNP precipitation anomaly in the following summer.After dividing El Nino and La Nina into EP type and CP type,it is found that the influence of EP El Nino and La Nina on the WNP summer precipitation is asymmetrical.There is a negative correlation between the intensity of EP El Nino and the following summer WNP precipitation anomaly,while the correlation between EP La Nina and the following summer WNP precipitation anomaly is not significant.The effects of CP El Nino and La Nina on the following summer precipitation of WNP are symmetrical.The intensity of both CP El Nino and La Nina was significantly negative correlated with the WNP precipitation anomaly in the following summer.By studying the circulation field and SST field of each ENSO in the following summer,it was found that the signal of EP El Nino,CP E1 Nino and CP La Nina was still strong in the following summer,while the signal of CP La Nina had basically disappeared.After each type is further differentiated by strength and weakness,the analysis of velocity potential and stream function field shows that the precipitation of strong EP El Nino,strong CP E1 Nino,strong EP La Nina,weak EP La Nina and strong CP La Nina is mainly controlled by the convergence and divergence of wind field,and the precipitation anomaly area corresponds well to the convergence and divergence center.The weak EP El Nino and the weak CP El Nino are mainly influenced by the Gill's symmetrical heating of the central Pacific Ocean,and a pair of cyclones are generated in the west of the convergence center,which leads to the positive anomaly of precipitation in the northwest Pacific Ocean.The weak CP La Nina,influenced by the heating of the subtropical central Pacific Ocean,generated an anticyclone in the eastern part of the WN P,which resulted in the negative precipitation anomaly in this region.As a result,the precipitation of strong CP La Nina in the WNP region is significantly greater than that of weak CP La Nina in the WNP region.Combined with the convergence and divergence field and the SST field,it can be found that the positive SST anomaly center corresponds well to the convergence center,while the negative SST anomaly center corresponds well to the divergence center in the equatorial Middle East Pacific Ocean.But that relationship does not hold in the WNP.Therefore,we conclude that although the effect of CP El Nino and La Nina on the precipitation anomaly of the next summer over the WNP is symmetrical in numerical value,it is asymmetrical in mechanism.Then we study the evolution of the sea surface temperature and circulation fields of various types of ENSO from winter to the next summer.We found that the strong EP El Nino type changed to CP La Nina in the next summer,while the weak EP El' Nino type changed to CP El Nino in the next summer,and the weak EP La Nina type changed to El Nino in the next summer.The intensity of weak CP El Nino remained unchanged.The positive anomaly of subtropical SST of weak CP La Nina has strengthened.The intensity of the remaining types of ENSO was weakened to varying degrees.