Upper ocean circulation modulation by phytoplankton concentration in the Equatorial Pacific and the Indian Ocean

Abstract

Wind patterns in the equatorial Pacific and Indian oceans are the factors that regulate the chlorophyll pigment distributions in the equatorial region of these oceans. Trade winds and coastline of the Pacific basin supports wave-guide dynamics in the equatorial Pacific, while the spatial asymetricity of wind pattern along with the north-south coastline of the Indian ocean makes upper ocean circulation more complex than that in the Pacific. We propose a feedback mechanisms between phytoplankton and currents in the equatorial Pacific and Indian Ocean, based on numerical experiment using mixed layer-isopycnal ocean general circulation model which includes photosynthesis process parameterizations. In the equatorial Pacific, the quasi-permanent upwelling brings nutrient-rich subsurface waters to the euphotic zone, thus the ocean currents supports perinnial phytoplankton bloom. The phytoplankton inhibits downward solar radiation penetration in the equatorial Pacific, resulting in cross equatorial density gradient in the upper ocean. This strengthens the geostrophically balanced westward currents in both side of the equatorial wave-guide (within 5 degree bands). Once these currents reach the western Pacific coast, they feed the Equatorial undercurrent (EUC). In contrast, in the Indian Ocean upwelling is not well developed due to the asymetric wind pattern about the equator and relatively short duration of the equatorial undercurrent in the Indian Ocean. These along with wind reversal and north-south coastal asymmetry, does not support a chlorophyll pigments distribution in the equatorial wave-guide to bring about a meridional density gradient by altering the penetration of solar radiation. Relevant upper ocean circulations modulated by phytoplanktons in the equatorial regions of the Pacific as well as the Indian Ocean are discussed.