Spatial and temporal changes of the winter bloom in the Arabian Sea during the past two decades

High concentrations of Chlorophyll-a (Chl-a) appear in the Arabian Sea (AS) during the winter and early spring seasons (November–March), known as the winter blooms, primarily due to the reversing monsoon winds and associated changes in the ocean. The onset, duration, intensity and peak period of the seasonal blooms show distinctive regional characteristics in AS. Recent changes in ocean dynamics and plankton composition in AS have adverse effects on the distribution of Chl-a concentration there. Here, we examine the long-term spatio-temporal changes in winter blooms, and assess the impact of wind, mesoscale eddies, surface currents, sea surface temperature (SST), mixed layer depth (MLD) and sea surface salinity on the blooms. We observe a significant decrease in these blooms, which started in the early 2000s and intensified in recent decade (2010–2020), with a prominent decline in the adjascent Gulfs (Gulf of Aden: -1.38 ± 0.7 x 10^-5 mg m^-3 year^-1, Gulf of Oman: -4.71 ± 1.35 x 10^-6 mg m^-3 year^-1), and the West coast of India (-6.71 ± 2.85 x 10^-6 mg m^-3 year^-1). The major factors that control blooms in the Gulf of Oman and open waters are MLD and ocean temperature. On the other hand, in the Gulf of Aden, blooms are largely driven by the coastal upwelling and eddies. The bloom is primarily driven by winter cooling along the north-western Indian coast, but it is inhibited to south by the inter-basin exchange of surface waters carried by the West Indian Coastal Current. This study thus reveals particular mechanisms that trigger and regulate the winter blooms in AS. These seasonal blooms may continue to decline as a result of the ongoing ocean warming, which would be a threat for the regional marine productivity and food security.