A northern Indian Ocean comparison between nutrient availability derived from AVHRR-SST and phytoplankton biomass/productivity based on SeaWiFS (original) (raw)

This paper describes the methods used in making a inter-comparison between nutrient availability inferred from knowledge of the sea surface temperature (SST) and the phytoplankton biomass and productivity in the northern Indian Ocean based on SeaWiFS ocean color observations. In order to do this, a revised set of temperature -nutrient linear regressions for ten degree squares of latitude and longitude .recently were calculated for nitrate (Ni) and phosphate in the Northern Indian Ocean based on hydrographic data obtained from NODC World Ocean Atlas 1994. These relationships support the estimation of nutrient concentration at a geographic location if water temperature is known. In these relationships it is assumed that the intercept of the regression line with y-axis (temperature) is the temperature of the surface waters above which significant concentrations of the nutrient do not remain or the nutrient depletion temperature (NDT). In this analysis, maps of Monthly Sea Surface Temperature (SST) at 54-km resolution were obtained from the AVHRR Oceans Pathfinder project for the Northern Indian Ocean for 1998. These data were thin interpolated to 0.5° x 0.5°~resplution. These temperatures were then compared to the NDT (also interpolated to 0.5° resolution) to identify surface nutrient absence (negative values set to zero) if the SST was higher than the local NDT or surface nutrient concentration if the SST was lower than the local NDT. Sets of SST and the SST-derived nutrient concentrations are compared to maps of SeaWiFS chlorophyll and chlorophyll-derived total (TPP), new (NPP) and regenerated (RPP) primary productivity for the NE and SW monsoons and intermonsoon seasons. The nutrient data show a coherent sequence related to known sources of nutrients in the Northern Indian Ocean including the monsoonal upwelling and the Pacific throughfloW to the Indian Ocean. The two independent satellite data streams suggest relationships between NPP and nitrate and between RPP and phosphate. The NPP-nitrate relationship is improved if a derived nitrate availability index is used.