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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: the positive one of central tropical Pacific SSTAs and the 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 1982/1983 and 1997/1998. However, we find that no strong negative PMM-like SSTAs appeared although the PMM indices (PMMIs) were strongly negative in 1983 and 1998 spring. Observation and model experiments show that the simultaneous warming that extended to the dateline in 1983 and 1998 interrupted the potential impacts of ETP warming in promoting negative PMM, inducing wind anomalies unfavorable for establishing the wind-evaporation-SST feedback for negative PMM. To understand the discrepancy between the large PMMIs and weak PMM SSTAs in the subtropics during strong EP El Niño events, we isolate relative contributions of subtropical and tropical SSTAs to the PMMIs by calculating their spatial projections on the PMM. Analysis combinedly using observation and CMIP6 models shows that despite the large contribution from subtropical SSTAs to the PMMIs, the large tropical SSTAs, especially the extreme ETP warming, during strong EP El Niño events could cause large negative PMMIs even without strong negative subtropical SSTAs. Our study clarifies the impact of ETP warming in causing negative PMM and indicates the overstatement of negative PMMIs by tropical SSTAs during strong EP El Niño events.
