Revisiting the Impacts of Tropical Pacific SST Anomalies on the Pacific Meridional Mode during the Decay of Strong Eastern Pacific El Niño Events

Abstract

Abstract The Pacific meridional mode (PMM) can modulate El Niño–Southern Oscillation (ENSO) and is also affected by ENSO-related tropical Pacific sea surface temperature anomalies (SSTAs). Two tropical feedbacks on the PMM have been proposed: a positive one of central tropical Pacific SSTAs and a negative one of eastern tropical Pacific (ETP) SSTAs, the latter of which is suggested to be active only during strong eastern Pacific (EP) El Niño events like those in 1982/83 and 1997/98. However, we find that no strong, negative PMM-like SSTAs appeared, although the PMM indices (PMMIs) were strongly negative in spring of 1983 and 1998. Observation and model experiments show that tropical warming in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfavorable for establishing the wind–evaporation–SST feedback for a negative PMM in the subtropics. To understand the discrepancy between the large negative PMMIs and weak PMM-related subtropical cooling during strong EP El Niño events, we isolate the relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combined using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Niño events even without strong, negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing a negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events. Significance Statement This paper aims to reevaluate the previously proposed effect of strong eastern Pacific El Niño events, like those in 1982/83 and 1997/98, on exciting a negative Pacific meridional mode (PMM). We find that although the PMM indices were strongly negative during the decay of strong eastern Pacific El Niño events, the large negative PMM sea surface temperature anomalies (SSTAs) could not be observed in the subtropical Pacific. Further diagnosis indicates that the PMM index can be large if strong SSTAs occur in eastern tropical Pacific even without subtropical SSTAs, implying that one should be careful when using the PMM index.