Annual ecosystem variability in the tropical Indian Ocean: Results of a coupled bio-physical ocean general circulation model

A coupled, 3-D bio-physical ocean general circulation model has been employed to study biogeochemical variability in the Indian Ocean. The model's ecosystem includes two size classes each of phytoplankton, zooplankton and detritus, and allows iron- and nitrogen-limited phytoplankton growth to develop. Comparison of SeaWiFS and modeled chlorophyll a demonstrates that the model successfully captures seasonal inter-regional contrasts in phytoplankton distribution. Model validation was performed against SeaWiFS ocean color, NODC nitrate, and US JGOFS observations from the Arabian Sea. These data were used to objectively evaluate several ecosystem model modifications that improved the solution. Phytoplankton speciation shifts were generally consistent with those observed in the Arabian Sea. The model shows pronounced basinwide spatio-temporal variability in iron availability; surface waters in the western equatorial and southern tropical regions always tend toward iron limitation, while the Bay of Bengal and the eastern Arabian Sea are largely iron replete. The model also indicates that surface waters in the western Arabian Sea are prone to seasonal iron limitation, despite proximity to regional dust sources. Phytoplankton blooms that manifest as a response to the Somali Current show better agreement with the SeaWiFS data than do previous modeling studies and biogeochemical variability associated with eastward propagation of the semiannual Wyrtki Jet is described for the first time. In the southern tropical region, the model indicates that austral winter Ekman pumping and westward propagating Rossby waves are the primary means of nutrient supply to these iron-limited waters. Finally, inter-regional connections associated with the propagation of the Somali Current and the Wyrtki Jet out of their respective source regions are shown to be a primary influence on seasonal biogeochemical variability in the southern Bay of Bengal. The results presented here establish a need for additional studies that apply such a basinwide approach to further advance our understanding of how bioavailable iron distribution, physical–biological interactions and inter-regional connections affect biogeochemical processes in the Indian Ocean.