Abstract
Many previous studies have shown that El Niño exhibits a strong seasonality in its teleconnections and regional climate impacts. Aside from the seasonal synchronization of El Niño anomalous sea surface temperature (SST) itself, seasonal differences in its associated climate impacts could also stem from the local background seasonal cycle. During the El Niño developing boreal autumn (August–October) and decaying boreal spring (February–April), the El Niño–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 Niño–associated SST in spring is likely to exceed 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. This seasonality is mainly embodied in eastern Pacific (EP) El Niño events rather than central Pacific (CP) El Niño events since the westward-shifted warm SST anomalies for the latter cannot establish an effective cooperation with the cold tongue SST annual cycle. This study has important implications for regional climate prediction by involving the different local precipitation responses over the tropical eastern Pacific.
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