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South Eastern Arabian Sea (SEAS) is an upwelling dominated region, where the distribution of chlorophyll-a (Chl-a) is mainly controlled by basin scale to mesoscale processes associated with seasonal reversal of winds and currents. The analysis of long-term satellite-derived Chl-a data revealed inter-annual variability in association with extreme climatic events. In the present study, satellite imageries were applied to study the long term Chl-a variability in SEAS followed by ascertaining the influence of extreme climatic events. Chl-a data were obtained from Ocean Colour Climate Change Initiative (OC-CCI) which provides high resolution cloud free data for the Arabian Sea during summer monsoon season (June to September). Satellite measurements of Sea Level Anomaly (SLA), Sea Surface Temperature (SST) and sea surface wind data were also compiled from various sources. The monthly data for the above environmental variables during the years 1998 to 2016 were analysed as representatives of the possible causative physical processes influencing the variability in Chl-a. The time series and Hovmoller analysis revealed that the maximum variability occurred during summer monsoon. This study revealed that the interannual variability in Chl-a was prominently influenced by the underlying processes involved in coastal upwelling. The extreme climatic events such as ENSO and IOD were particularly taken into account to understand the the interannual variability in Chl-a and associated environmental variables. The relative variability in Chl-a and associated environmental variables with these events was prominent during strong El Nino, La Nina and IOD. Cross Correlation Function (CCF) analysis points out that Chl-a content is negatively correlated with IOD to an extent of one month or less whereas, ENSO doesn’t indicate such relations. Hence, the variability associated with IOD showed pronounced influence than ENSO. The study elucidates the interannual variability of Chl-a and the overall influence of associated extreme events in SEAS.

Understanding the forcing mechanisms that determine the coastal ecosystem dynamics has always been a challenging topic of research. The Arabian Sea (AS) coastal ecosystem in the western half of the northern Indian Ocean is influenced by the seasonally reversing atmospheric forcing and structured by distinct water masses along its depth. Although AS differs greatly from its eastern counter-half, Bay of Bengal (BoB) in the thermohaline characteristics, the inter-basin exchanges of the water masses between these ocean basins add complexity to its hydrography. Hence, the present study was attempted to understand the influence of the intrusion of the BoB water in structuring the zooplankton community of the coastal AS. In the mixed layer depth, a conspicuous spatial heterogeneity was noticed in the distribution and abundance of the zooplankton community contributed by the cumulative influence of the higher phytoplankton biomass and the advection of population through BoB water intrusion regulated by the coastal circulation in the southern part. The study also assessed the interaction of the zooplankton community with the specific water masses defined by distinct thermohaline characteristics along the vertical profile of the AS coastal waters. Three distinct water masses were identified within the 200 m depth of AS each categorized by discrete salinity and density features. The distinct groups of the dominant taxon, Copepoda identified through redundancy analysis demonstrate the species-specific preferences of the community to different water mass characteristics in the AS. This study addressing how hydrographic characteristics shape the macroecology of zooplankton in the coastal AS, will add new insight in understanding the dynamic ecosystems of the northern Indian Ocean.