Seasonal Differences In Precipitation Anomalies In The Equatorial Eastern Pacific In El Ni(?)o Years

The precipitation conditions in the eastern tropical Pacific are an important driver for the complexity of ENSO global atmospheric teleconnections and climate impacts.Previous studies pay more attention to the stronger precipitation over central and western Pacific,the seasonal variation of precipitation in the eastern tropical Pacific during El Ni(?)o events remains further investigation.Based on the observation and atmospheric general circulation model simulations,the seasonally modulated El Ni(?)o precipitation response in the eastern Pacific and its dependence on El Ni(?)o flavors,and the decadal variations of spring and autumn precipitation patterns over the equatorial eastern Pacific are investigated from 1979 to 2019.The main conclusions are as follows:(1)The spatial structure of El Ni(?)o-related precipitation response over eastern tropical Pacific has significant seasonal characteristics.During the El Ni(?)o developing boreal autumn(August-October)and decaying boreal spring(February-April),the El Ni(?)o associated precipitation anomalies display remarkably different patterns over the eastern tropical Pacific despite similar SST anomaly amplitudes.This strong seasonality can be largely attributed to the seasonal cycle of the eastern tropical Pacific SST background state with the cold tongue being strongest in autumn and weakest in spring.Therefore,the El Ni(?)o associated SST in spring is prone to exceeding the convection threshold on both sides of the equator,leading to an approximately symmetric precipitation response about the equator.In contrast,this symmetric precipitation response is absent in autumn since the SST near and south of the equator remains below the threshold and pronounced eastern tropical precipitation anomalies can only be observed in the warm Northern Hemisphere.(2)The seasonality of precipitation response over eastern tropical Pacific is mainly embodied in eastern Pacific(EP)El Ni(?)o events rather than central Pacific(CP)El Ni(?)o events.This is because the center of the warm SST anomaly of EP events is located in the eastern Pacific,and it has a great influence on the local precipitation anomaly when the SST anomaly superimpose on the background SST,which leads to the anomalous precipitation basically symmetric about the equator in spring but asymmetric in autumn.However,the westwardshifted warm SST anomalies for the CP events cannot establish an effective cooperation with the cold tongue SST annual cycle,and thus no apparent seasonality of the precipitation response over the eastern tropical Pacific.(3)The precipitation pattern in spring(FMA)of the eastern tropical Pacific exhibits prominent decadal variation around 1998,while the precipitation pattern in autumn(ASO)shows no decadal change.The first pattern of spring precipitation from 1979 to 1998 is quasisymmetry about the equator.While during 1999-2019,the first pattern of spring precipitation is north-south asymmetry about the equator.The decadal variation of the first pattern of spring precipitation is the result of the occurrence of CP Ni(?)o events more frequently and the westward shift of warm SST anomalies after 1998.In autumn,significant precipitation responses are in the north of the equator during EP and CP events due to the strongest cold tongue,and the decadal increase of CP events has little effect on autumn precipitation modes.