Observed variability of chlorophyll-a using Argo profiling floats in the southeastern Arabian Sea

Abstract

The time series of temperature, salinity, chlorophyll-a, particle scattering at 700nm, and dissolved oxygen obtained from Argo floats deployed in the southeastern Arabian Sea (AS), were used to investigate the variability of these quantities and the mechanisms that modulate them between March 2010 and March 2011. The observations show a persistent occurrence of a subsurface chlorophyll-a maximum (0.75–1mgm−3) near depths of 40–100m throughout the study period, just above the top of permanent thermocline and euphotic depth. The analysis shows that upward and downward movement of thermocline, which is primarily due to westward movement of low-mode baroclinic Rossby waves, significantly influences the depth of the subsurface chlorophyll maxima and its intensity. Further, the vertical movement of the thermocline significantly influences the depth of the oxycline in the AS. The mixed layer deepening associated with wind induced vertical mixing and convective overturning lead to near surface blooms during the summer and winter monsoons. The analysis clearly shows that, in addition to entrainment of nutrients from rich subsurface water in the near surface layer, vertical fluxes from the subsurface chlorophyll maximum also contribute significantly to mixed layer blooms. The availability of light also plays an active role in the mixed layer bloom, particularly during the summer monsoon season.