Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations

The central part of the eastern Arabian Sea (CEAS) remains one of the least-resolved regions of the basin, where satellite and ship-based measurements inadequately capture subsurface chlorophyll structure and year-round variability. Using six years of depth-resolved Biogeochemical-Argo observations (2015–2020), this study provides the first detailed characterization of seasonal productivity dynamics in this transitional zone. Results reveal a unique climatological pattern ‒ including the notable September decline and October rebound in surface chlorophyll ‒ a signature of the CEAS. Mixed layer (ML) variability, shaped by monsoonal winds, episodic upwelling, and seasonal stratification, governs the vertical partitioning of phytoplankton and the development of a pronounced subsurface chlorophyll maximum during intermonsoon periods. Enhanced wind stress and deeper mixing during the summer monsoon increase the nutrient supply and enhance ML biomass, whereas weakened forcing in September suppresses surface productivity despite cooling. In addition to local atmospheric and mixing processes, surface ocean currents and freshwater inputs from river discharge modulate stratification, salinity gradients, and nutrient pathways, further contributing to the region’s strong spatial and vertical heterogeneity. These findings highlight the CEAS as a physically complex yet biologically sensitive region and demonstrate the critical value of Biogeochemical-Argo observations for resolving fine-scale biophysical interactions and improving predictions of ecosystem response under future climatic and circulation changes.