Bio-physical interactions in the tropical Pacific

Abstract

Variability of marine phytoplankton associated with El Niño-Southern Oscillation (ENSO) and potential biological feedbacks onto ENSO are investigated by performing coupled ocean/biogeochemical model experiments forced by realistic surface winds from 1951 to 2010. The ocean model used in this study is the MOM4, which is coupled to a biogeochemical model, called TOPAZ (Tracers in the Ocean with Allometric Zooplankton). In general, MOM4-TOPAZ can simulate the observed main features of phytoplankton variability associated with ENSO reasonably well. By comparing the actively coupled MOM4-TOPAZ experiment with the ocean model experiments with prescribed chlorophyll concentrations, potential impacts of phytoplankton on ENSO are evaluated. We found that chlorophyll generally increases mean sea surface temperature (SST) and decreases subsurface temperature by altering the penetration of solar radiation. However, as the chlorophyll concentration increases, the equatorial Pacific SST decreases due to the enhanced upwelling of the cooler subsurface water with shoaling of mixed layer and thermocline. In these experiments, the presence of chlorophyll generally intensifies ENSO amplitude by changing the ocean basic state. On the other hand, interactively varying chlorophyll associated with the ENSO tends to reduce ENSO amplitude. Therefore, the two biological effects on SST are competing aistic surface winds from 1951 to 2010. The ocean model used in this study is the MOM4, which is coupled to a biogeochemical model, called TOPAZ (Tracers in the Ocean with Allometric Zooplankton). In general, MOM4-TOPAZ can simulate the observed main features of phytoplankton variability associated with ENSO reasonably well. By comparing the actively coupled MOM4-TOPAZ experiment with the ocean model experiments with prescribed chlorophyll concentrations, potential impacts of phytoplankton on ENSO are evaluated. We found that chlorophyll g