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.

Asymmetry Between Positive and Negative Phases of the Pacific Meridional Mode: A Contributor to ENSO Transition Complexity

“…Our study reconciles the seemingly contradictory viewpoints about the PMM asymmetry in previous studies (Fang and Yu (2020) suggested that −PMM > +PMM, but Zheng et al. (2021) and Fan et al. (2021) offered that +PMM > −PMM).…”

“…Our study reconciles the seemingly contradictory viewpoints about the PMM asymmetry in previous studies (Fang and Yu (2020) suggested that −PMM > +PMM, but Zheng et al (2021) and Fan et al (2021) offered that +PMM > −PMM). Moreover, the present study declares the contribution of positively asymmetric stochastic PMM events to more frequent episodic El Niño events, which may provide an additional information in understanding the ENSO transition complexity.…”

“…Particularly, Fang and Yu (2020) suggested that the sustaining‐ENSO effect of successive PMM events is more efficient in the negative phase, leading to more multi‐year La Niña events than El Niño events. However, also focusing on the differences between positive and negative phases of the PMM, other studies have drawn a seemly contradictory conclusion that the growth mechanisms for the PMM, including the wind‐SST‐evaporation (WES) feedback (Xie & Philander, 1994) and summer deep convection (SDC) response (Amaya et al., 2019), are more efficient in the positive phase (Fan et al., 2021; Zheng et al., 2021). Thus, one might wonder which is correct, or whether the phase asymmetry of the PMM is negative (−PMM > +PMM) or positive (+PMM > −PMM).…”

Enhanced joint impact of western hemispheric precursors increases extreme El Niño frequency under greenhouse warming

“…Previous studies have shown that the prediction of ENSO evolution in both dynamical and statistical models can be improved when considering the NTA and SNP SST conditions 7 , 10 , 24 , 33 . Since 1980, all the cases of NTA warming (i.e., 1980, 1988, 1998, and 2010) were followed by a La Niña event 6 , and 7 out of the 15 cases of SNP warming (i.e., 1982, 1986, 1991, 1994, 1997, 2014, and 2014) were followed by an El Niño event 34 , 35 . Given that the impact of NTA and SNP SST on ENSO in enhanced under greenhouse warming, the northwestern hemispheric precursors are expected to contribute to increased forecast skill for ENSO.…”

Asymmetric Efficacies Between Warm and Cold Pacific Meridional Modes in Inducing ENSO

“…Recent research has highlighted the differing capacities of warm and cold PMM phases in triggering ENSO (Fan et al., 2021; Thomas & Vimont, 2016; Zheng et al., 2021). These studies assessed PMM capabilities by examining SSTA intensity associated with subsequent ENSO induced by PMM.…”