Asymmetric impact of the boreal spring Pacific Meridional Mode on the following winter El Niño‐Southern Oscillation

Abstract

A number of previous studies have indicated that boreal spring Pacific Meridional Mode (SPMM) can exert notable impacts on the occurrence of El Niño‐Southern Oscillation (ENSO) event during the subsequent winter. Here, we reveal that the impact of the SPMM on the following winter ENSO is asymmetric. During positive SPMM (+SPMM) years, strong westerly wind anomalies are induced over the tropical western Pacific via wind‐evaporation‐SST (WES) feedback. These westerly wind anomalies lead to pronounced sea surface temperature warming in the tropical central and eastern Pacific, which further develop to an El Niño‐like pattern via positive atmosphere–ocean interaction. However, easterly wind anomalies over the tropical western Pacific during negative SPMM (−SPMM) years are much weaker. The weak easterly wind anomalies cannot lead to a clear La Niña‐like pattern in the following winter. We suggest that the asymmetric strength of the westerly and easterly wind anomalies over the tropical western Pacific is due to the asymmetric intensity of the WES feedback over the tropical north Pacific, with stronger intensity in the +SPMM years. The asymmetric trade wind charging process also partly contributes to the asymmetric SPMM–ENSO relation. Atmospheric general circulation model simulations confirm that the intensity of the +SPMM‐generated westerly wind anomalies over the tropical western Pacific is stronger than that of the easterly wind anomalies related to the −SPMM. The present study indicates that it is necessary to consider the phases of the SPMM when predicting winter ENSO events based on the preceding SPMM.

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