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

• Published in: International journal of climatology 2021-05, Vol.41 (6), p.3523-3538
• Main Authors: Zheng, Yuqiong, Chen, Wen, Chen, Shangfeng, Yao, Shuailei, Cheng, Conglan
• Format: Article
• Language: English
• Subjects: Anomalies; asymmetric relation; Asymmetry; Atmospheric circulation; Atmospheric circulation models; Easterlies; El Nino; El Nino phenomena; El Nino-Southern Oscillation event; ENSO; Evaporation; Feedback; General circulation models; La Nina; Pacific Meridional Mode; Sea surface; Sea surface temperature; Southern Oscillation; Spring; Spring (season); Surface temperature; trade wind charging; Trade winds; Tropical climate; Wind; wind‐evaporation‐SST feedback; Winter
• ISSN: 0899-8418; 1097-0088
• DOI: 10.1002/joc.7033

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. There exists a significant asymmetric relationship between the spring PMM and the following winter ENSO. Positive spring PMM leads to a significant El Niño‐like SST warming in the following winter. By contrast, impact of the negative spring PMM on the following winter La Niña is fairly weak. Asymmetric impact of the spring PMM on the winter ENSO is due to the asymmetric response of the atmospheric anomalies over the tropical Pacific to the SST changes induced by the spring PMM. The AGCM experiments confirm the observed asymmetric atmospheric responses. Composite anomalies of SST (unit: °C) and surface winds (unit: m·s−1) at (a,b) MAM(−1), (c,d) JJA(−1), (e,f) SON(−1), (g,h) D(−1) and JF 0) for the +SPMM (left column) and the −SPMM (right column). SSTAs and surface wind anomalies that below the 90% confidence level are not shown.