Oceanic and atmospheric influences on the variability of phytoplankton bloom in the Southwestern Indian Ocean

The phytoplankton bloom developed in the southwestern Indian Ocean during austral summer is unique in its occurrence. Interannual and intraannual variability of this large phytoplankton bloom were studied using satellite derived, model and reanalysis data together with hydrographic observations. The study shows that the bloom is not confined to Madagascar basin alone, but also developed in the Mozambique Basin as well as in the southern Mozambique Channel. The strongest bloom event in the Madagascar and Mozambique basin since twelve years (1997–2008) occurred during January 2008. Intraannual variability of the phytoplankton bloom is linked to the upwelling along the south coast of Madagascar, precipitation along the east coast of Madagascar, light limitation and local mesoscale circulation features. Distribution pattern of the mesoscale eddies is found to play an important role in inducing the interannual variability of the bloom. The study reveals the dominance of the mesoscale eddies during the bloom events and the distinct association of chlorophyll maxima with either anti-cyclonic or cyclonic eddies. Flow pattern of the East Madagascar Current also shows interannual variability, which constrains the distribution of the bloom in different basins. ENSO is likely to have a direct and remote impact in inducing the interannual variability of the bloom.