Pacific Meridional Mode Does Not Induce Strong Positive SST Anomalies in the Central Equatorial Pacific

Abstract

Abstract The positive phase of the Pacific meridional mode (PMM) is closely related to the onset of El Niño. Previous studies have indicated that positive sea surface temperature anomalies (SSTAs) in the central equatorial Pacific (CEP) during the spring and summer of positive PMM years primarily originate from the northeastern tropical Pacific (NETP) via positive wind–evaporation–SST feedback. We review the evolution of PMM and find weak evidence to support such a linkage. Coupled model experiments show that the positive PMM-regressed SSTAs in the NETP only account for ∼24% of those in the CEP from winter to spring, illustrating the principle that correlation does not necessarily mean causality. The strongest positive PMM SSTAs in the NETP and CEP increase El Niño intensity by 1.07°C, whereas that in the NETP alone increase El Niño intensity by 0.69°C. When the composite SSTAs in the NETP during positive PMM years are used, however, the El Niño intensity is increased merely by 0.17°C. The change in the subsurface temperature in the equatorial Pacific is curtailed for the NETP SSTAs to trigger El Niño, while the wind–evaporation–SST feedback plays a less important role. Our results indicate that the impact of PMM on El Niño might be overestimated by ∼55%. Moreover, a comprehensive understanding about the role of the tropical North Pacific on El Niño can be obtained only when the impact from the western North Pacific is considered.