The Impact of Aeolian Iron Deposition on the Indian Ocean Ecosystem

The aeolian deposition fields from two separate atmospheric models of mineral dust transport have been applied as iron flux boundary conditions to a fully coupled, 3-D bio-physical ocean general circulation model of the Indian Ocean. The atmospheric transport models are those of Tegen and Fung (1994, hereafter the GISS model) and the GOCART model of Ginoux et al., (2001). The biological portion of the ocean model is a 9-component pelagic ecosystem, consisting of a large and small size class for phytoplankton, zooplankton and detritus, as well as three phytoplankton nutrients (nitrate, ammonium and iron). The model ecosystem has been extensively validated with data from the US JGOFS Arabian Sea Process Study, the NODC seasonal nitrate climatology, and a monthly climatology of SeaWiFS ocean color. The GISS model shows order of magnitude higher aeolian iron fluxes northwest of Australia, while the GOCART model shows similar differences over the Arabian Sea during the winter monsoon and spring intermonsoon. Furthermore, differences in iron flux distribution patterns over the Arabian Sea during the summer monsoon are profound. One new insight from climatological solutions with both deposition fields is that iron limited phytoplankton growth prevails over much of the southern Indian Ocean. Another is that the Arabian Sea undergoes a seasonal cycle whereby iron limitation develops during the summer monsoon and iron replete conditions are reestablished during the subsequent winter monsoon and spring intermonsoon, though the specific distribution patterns between the two solutions are significantly different. These differences extend to the spatial distribution of phytoplankton blooms in the central Arabian Sea during the summer monsoon. Finally, interannual variability of these summer blooms, in model simulations forced with interannually varying winds, precipitation and aeolian iron flux, is compared to interannual variability in SeaWiFS ocean color observations.