Chlorophyll variability induced by mesoscale eddies in the southeastern tropical Indian Ocean

Abstract

Abstract High sea surface chlorophyll-a concentrations (CHL) and intensive mesoscale eddy activity coexist near the coast of Java in the southeastern tropical Indian Ocean (SETIO) in upwelling season (July–November). In this study, the modulation effects of mesoscale eddies on the local CHL variability are investigated by analyzing remote sensed as well as in situ observations together with ocean and atmospheric reanalysis data from 1997 to 2010. Within 400 km off the coast, the mean CHL inside cyclonic eddies (CEs) is 2–3-fold higher than background, and is obviously suppressed inside anticyclonic eddies (AEs). During upwelling season, more and stronger CEs are generated, which significantly contributes to the high mean CHL levels. A composite analysis revealed that CEs induce CHL anomalies through both vertical upwelling that elevates nutrient-rich water from the subsurface to the surface and horizontal advection that carries high-CHL water from the coast to the offshore area. These effects are particularly visible during the upwelling season, when advection of the background high CHL favors large eddy modulation on the CHL. The eddy-induced CHL anomalies also exhibit prominent interannual variability, which has a close relationship with the El Nino-Southern Oscillation and the Indian Ocean Dipole (IOD). The wind forcing of El Nino and positive IOD events can enhance the vertical shear of the background currents and favor energetic CEs off Java coast, which contributes to the interannual variability of the local CHL. These results underscore the fundamental role played by mesoscale eddies in the ocean dynamics, environment, and bio-system of this region.