Enhanced phytoplankton bloom triggered by atmospheric high-pressure systems over the Northern Arabian Sea.

During winter, the dry, cool air brought by prevailing northeasterly trade winds leads to surface ocean heat loss and convective mixing in the northern Arabian Sea. The current paradigm is that the convective mixing process leads to the injection of nutrients up into the surface waters and exert a dominant control on winter productivity. By combining a variety of observations, atmospheric reanalysis and model simulations, we unraveled the processes responsible for the observed year-to-year chlorophyll-a variations in the northern Arabian Sea. Our findings suggest that the atmospheric high-pressure systems that traverse the northern Arabian Sea every winter and spring disrupt winter convective mixing and create an array of environmental conditions conducive to trigger phytoplankton blooms. The arrival of an atmospheric high with the anticyclonic flow in the northern Arabia Sea sets the stage for a sequence of events culminating in intermittent mixed-layer restratification due to buoyancy gain aided by increased specific humidity, supplemented with abundant sunlight due to clear skies, and suppressed turbulent mixing owing to weak winds. These combined with the mixed layer that is shallower than the euphotic zone and the influx of nutrients into the euphotic zone brought by convective mixing between the calm periods, caused unprecedented high concentrations of chlorophyll-a in the northern Arabian Sea.