A very oligotrophic zone observed from space in the equatorial Pacific warm pool

Abstract

The analysis of the SeaWiFS chlorophyll archive shows a quasi-persistent strip of oligotrophic waters (chl<0.1mgm−3) extending over about 20° longitude in the eastern part of the equatorial Pacific warm pool. Other space-borne data sets (scatterometric wind, microwave sea surface temperature (SST), altimetric sea level, and surface currents) were used together with barrier layer thickness derived from Argo floats to investigate the variability of the oligotrophic zone and of its eastern and western boundaries, and to propose processes that could explain why surface chlorophyll is so low in this region. The eastern limit of the oligotrophic waters matches the eastern edge of the warm pool and moves zonally both at seasonal time scale and with the El Niño/La Niña phases whereas the western limit moves mostly at intraseasonal and interannual time scales. On average, about half of the surface of the zone is occupied by very oligotrophic waters (chl<0.07mgm−3) located in the eastern part. The degree of oligotrophy of the zone increases when its width is maximum during boreal fall and winter and during El Niño events. Oligotrophy in the eastern part of the warm pool most likely persists because of the lack of vertical or horizontal penetration of nutrient-rich water due to the following processes. 1/ The equatorial oligotrophic warm pool is bounded poleward by the oligotrophic subtropical gyres. 2/ The deep nutrient pool prevents strong vertical nutrient inputs into the euphotic layer and the barrier layer above it potentially reduces the efficiency of mixing. 3/ During westerly wind events, mesotrophic waters in the far western basin are too distant from the oligotrophic zone to be efficient nutrient and phytoplankton sources, and become nutrient and phytoplankton depleted during their eastward advection. 4/ Nutrient-rich waters from the central basin and nutrient-poor surface waters of the warm pool do not blend because of subduction at the eastern limit of the oligotrophic zone.