The role of the western North Pacific (WNP) as an El Nino-Southern Oscillation (ENSO) precursor in a warmer future climate

Extratropical air-sea interactions have become increasingly involved in promoting the transition to El Nino-Southern Oscillation (ENSO) with climate change. In this study, we break down the effects of future warming on the 1-year lead relationship between a cold western North Pacific (WNP) phase and El Nino development the following winter. We apply a conditional probability approach and sea surface temperature (SST) budget analysis on historical and Shared Socioeconomic Pathway 3-7.0 (SSP370) model runs. With enhanced warming, cold WNP SST anomalies in the boreal winter further strengthen summer westerly anomalies in the western equatorial Pacific, which promote the intensification of surface convergence and anomalous Ekman and geostrophic advection, and positive SST anomalies in the central equatorial Pacific in the seasons prior to the El Nino. Downwelling equatorial Kelvin waves induced by the westerly wind stress facilitate entrainment of subsurface water into the mixed layer during the transition period to trigger stronger thermocline feedback in the central-eastern equatorial Pacific. As a result, the amplitude and frequency of El Nino and its tropical precursors are projected to increase with warming under the WNP influence. ENSO diversity modulated by this relationship depends on the relative strength of advective and thermocline feedbacks, as well as the background state at the time of the event. The intensification of positive Pacific Meridional Mode (PMM) southwesterlies during the WNP-ENSO transition suggests a strengthened three-way link between WNP, PMM and ENSO under enhanced warming that may promote stronger and/or more frequent El Ninos.