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

The Pacific meridional mode (PMM) can modulate El Nifio-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 Nifio 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 warm-ing in 1983 and 1998 not only occurred in the ETP but also extended to the date line, thus inducing wind anomalies unfa-vorable 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 Nifio 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 contribu-tion from subtropical SSTAs, the large tropical SSTAs, especially the extreme ETP warming, could cause large negative PMMIs during strong EP El Nifio 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 Nifio events.