Linkage between Northeast Pacific warm blob and following El Niño: Pivotal role of North Pacific meridional mode

Abstract

Anomalous upper-ocean warming spanning several thousand kilometers and persisting across seasons in the Northeast Pacific (NEP), known as the “warm blob”, has significant ecological and climatic impacts. Previous studies have suggested the statistical linkage between the warm blobs in preceding winter and the El Niño in subsequent winter, however, the involving physical mechanisms remain unclear. Our results show that some warm blobs induce a transition from a high-pressure system to a quasi-barotropic low-pressure system over the NEP in boreal winter through local ocean-atmosphere interaction: diabatic heating and transient eddy forcing jointly promote the low-pressure system in the lower troposphere, while transient eddy vorticity forcing contributes substantially to its upper-tropospheric structure. Such mid-latitude low-pressure anomaly leads to the development of the North Pacific Meridional Mode in boreal spring, with southwesterly wind anomalies reducing oceanic turbulent heat loss over the subtropical NEP, which ultimately triggers the following El Niño. The potential pathway underlying the linkage between the warm blobs and El Niño is further evidenced by numerical experiments with a slab ocean model coupled to the Community Atmosphere Model version 5. Our results advance understanding on the physical connection between extratropical NEP variability and El Niño.