The oceanic zone of convergence on the eastern edge of the Pacific warm pool: A synthesis of results and implications for El Niño‐Southern Oscillation and biogeochemical phenomena

Abstract

The eastern edge of the western Pacific warm pool corresponds to the separation between the warm, rainfall‐induced low‐salinity waters of the warm pool and the cold, high‐salinity upwelled waters of the cold tongue in the central‐eastern equatorial Pacific. Although not well defined in sea surface temperature (SST), this eastern edge is characterized by a sharp salinity front that is trapped to the equator. Several studies, using numerous in situ and satellite data and three classes of ocean models, indicate that this front is the result of the zonal convergence of the western and central Pacific water masses into the eastern edge of the warm pool. This occurs through the frequent encounter of the eastward jets in the warm pool and the westward South Equatorial Current in the cold tongue. The notable and alternate variations of these wind‐driven zonal currents are trapped to the equator and are chiefly interannual in the vicinity of the edge. Consequently, the Eastern Warm Pool Convergence Zone (EWPCZ) is subject to eastward or westward displacements over several thousands of kilometers along the equatorial band, in synchrony with the warm phase (El Niño) and the cold phase (La Niña) of the El Niño‐Southern Oscillation (ENSO) phenomenon. Zonal advection appears to be the predominant mechanism for the ENSO displacements of the eastern edge of the warm and fresh pool. The existence of the EWPCZ and its ENSO displacements have significant effects on the physics of the tropical Pacific and on related biogeochemical phenomena. The EWPCZ is important for the formation of the barrier layer in the isothermal layer of the warm pool. Its zonal displacements control SST in the central equatorial Pacific, which in turn drives the surface winds and atmospheric convection (and vice versa). Hence the central equatorial Pacific is a key region for ENSO coupled interactions. All these findings from several studies and additional analyses lead to a revision of the delayed action oscillator theory of ENSO. The existence of the EWPCZ and its zonal displacements are also reasons for the ENSO variations in production and exchange of CO2 with the atmosphere over the equatorial Pacific. The zone of one‐dimensional convergence seems to congregate the world's most important tuna fishery in the western equatorial Pacific, and its displacements are likely the reason for this fishery to move zonally over thousands of kilometers in phase with ENSO.