Evidence of Eddy‐Enhanced Winter Chlorophyll‐<i>a</i> Blooms in Northern Arabian Sea: 2017 Cruise Expedition

Abstract

Abstract The temperature, salinity, and chlorophyll‐ a (Chl‐ a ) profiles of the upper ocean (0–200 m) were measured during the China‐Pakistan Joint Expedition, 2017. The cruise track passed through an identified cyclonic eddy, and the multi‐disciplinary records thus verify the eddy contributions to winter blooms in the northern Arabian Sea (AS). The records show that the strength of upper blooms within the mixed layer is mainly dominated by surface phytoplankton biomass (the Chl‐ a concentration), which increases simultaneously with the shallow mixed layer and uplifted thermocline. A composite analysis of eddy‐tracked sea level anomalies (SLA) and Chl‐ a reveals the pumping and stirring effects that cyclonic eddies make to the ocean surface nutrient supply. Based on the high‐resolution Ocean General Circulation Model for the Earth Simulator (OFES) outputs, a mixed layer nitrogen budget calculation was subsequently conducted to quantify the contributions of mesoscale eddies to 10‐year climatological winter blooms. The results indicate that eddy flows are primary contributors to the increased nutrients in the coastal north AS during the mature period (usually in January and February) of the winter blooms, with a concentration histogram double that of convective mixing. The concentration decreases from the north to south, agreeing well with the seasonal northward nutrition gradient. This confirms that (i) the growth of nutrients over the northern AS, which is known to be triggered by convective mixing, would be further maintained by eddy‐induced advection; and (ii) higher concentration of nutrients would facilitate an easier response of Chl‐ a to eddy dynamics.