Modulation of the Intraseasonal Chlorophyll-a Concentration in the Tropical Indian Ocean by the Central Indian Ocean Mode

Abstract

This study diagnoses the drivers of intraseasonal variability of mixed layer chlorophyll-a (Chl) concentration in the tropical Indian Ocean during boreal summer (June–September) using a well-tested coupled ocean-ecosystem model. Results show that wind forcing is the primary source for energetic intraseasonal Chl concentration variability in the eastern Arabian Sea and western Bay of Bengal (BoB) modulated by the Central Indian Ocean (CIO) mode. The atmospheric anomalous anticyclone associated with the positive phase of CIO mode drives southeastward currents in the Arabian Sea, which transport nutrient and Chl towards the east. Simultaneously, the wind-induced offshore currents strengthen upwelling in the western boundary of BoB, resulting in enhanced Chl in the upper ocean. Conversely, because of the formation of a barrier layer, the nutrient supply is suppressed along the equator, which causes a negative correlation between the CIO mode and Chl. This study complements the Madden-Julian Oscillation forcing on the intraseasonal Chl concentration anomaly in the tropical Indian Ocean.