Short-Term Response of Chlorophyll-a Concentration to Change in Sea Surface Wind Field over Mesoscale Eddy

Abstract

Short-term biological responses to sea surface wind field over mesoscale eddies were investigated using hourly ocean color chlorophyll-a (chl-a) concentration data from the Geostationary Ocean Color Imager (GOCI), scatterometer wind data, and satellite sea surface temperature (SST) data. Four warm eddies were identified from SST fronts, by subjectively fitting ellipses to selected points, and anticyclonically rotating current vectors were estimated from sequential chl-a images. Scatterometer data confirmed wind speed strengthening by approximately 30% over anticyclonic eddies, regardless of wind direction, caused by stability changes in the marine atmospheric boundary layer. The modified wind speed field produced a characteristic pattern of wind stress curl and Ekman pumping (EKP) over the eddies, showing positive and negative values on the left and right sides of the upwind direction, respectively. The EKP field was divided into three components: eddy current-induced, relative vorticity gradient-induced, and crosswind SST-induced terms. Short-term changes in the chl-a concentration showed a positive relation with EKP variations over most eddies. SST-induced EKP played a significant role in the eddies for 76% of the total variations, which is much higher than that in other oceans. This study emphasizes the significant effect of SST distribution and the biological responses to changes in an EKP field in relation to air–sea interactions and feedback processes in the East Sea (Sea of Japan).