The Arabian Sea is one of the most productive and complex ecosystems among the tropical oceans, and it significantly influences the regional and global carbon and oxygen cycle. The biological productivity of Arabian Sea is driven mainly by seasonal monsoons and the upper ocean dynamical processes. During the boreal summer monsoon, strong southwesterly wind drives coastal upwelling along the coasts of Somali, Oman and west coast of India,  and the upwelling brings high nutrient waters to the surface, leading to strong summer phytoplankton blooms that extend offshore to the Central and Northern Arabian Sea (NAS). In contrast, prevailing northeasterly winter monsoon favors surface ocean heat loss and leads convective mixing by carrying southward continental cold and dry air, which contributes to vertical nutrient mixing and enhances phytoplankton blooms, particularly in the Northern Arabian Sea. Here, by using various datasets including observations, atmospheric reanalysis and model simulated results, we analyzed the interannual variability of sea surface chlorophyll-a in the NAS during the late winter monsoon period (January to March). We found that observed phytoplankton bloom events in the NAS occur during the relaxation of winter monsoon and the year-to-year sea surface chlorophyll-a variation is mainly determined by mixed layer depth (MLD) changes, which is closely related to the air-to-sea heat flux. Moreover, the interannual variability of winter phytoplankton blooms in the NAS appear to be coupled with recent change in atmosphere dynamics from mid-latitude region, not solely by the local processes, and therefore further investigation is warranted to advance our understanding of the ecosystem dynamics in the NAS region.

phytoplankton blooms,winter monsoon,Arabian Sea