Rising Trends in Winter Phytoplankton Blooms in the Northern Arabian Sea Over the Last Two Decades: Drivers and Implications

Abstract

The interannual variability of winter phytoplankton blooms in the northern Arabian Sea (NAS) and the underlying mechanisms are investigated using observations, atmospheric reanalysis, and simulation results. An increasing trend in NAS winter phytoplankton blooms is observed over the past two decades. Scenarios of high-blooms (HBS) and low-blooms (LBS) during boreal winter in the NAS are identified, and the corresponding weekly chlorophyll-a evolutions are analyzed in relation to mixed layer depth (MLD) and atmospheric processes. In the LBS scenario, the southward shift of the westerly subtropical jet (STJ) in winter leads to cyclonic and cold flows in the NAS, which induces a transient deepening of MLD and slow restratification, resulting in prolonged but subdued phytoplankton growth. Conversely, in the HBS scenario, the northward movement of the STJ generates anticyclonic and warm flows and consequent rapid MLD shoaling, promoting nutrient retention and vigorous phytoplankton growth. Climate change over the past two decades favors the latter scenario.