Enhancement of Sea Surface Chlorophyll a Concentration by Interaction of Winds and Currents in a Cyclonic Eddy in the Japan/East Sea

Chlorophyll a concentration (Chl-a), wind, and water temperature were measured at a buoy moored near Dok-Do in the Japan/East Sea (JES). Short-term (1–3 day) enhancements of Chl-a associated with the surface layer cooling observed in spring (May–June) and autumn (Oct-Nov) were found to be correlated with the Ekman pumping induced by the interaction of the eddy current and wind, separable into components due to the wind stress and the lateral gradient in vertical vorticity. In the spring, surface Chl-a increases in conjunction with upwelling due to the lateral gradient in vorticity and the curl of the wind stress. However, in fall, positive Chl-a anomalies are only correlated with upwelling due to the curl of relative wind stress, which arises from the difference between the wind and the ocean surface current. The observed response time of the upwelling induced by wind-current interaction was very short (less than a day) compared to most theoretical studies which are driven by steady winds of more than 3 days. Our analysis suggests that as the mixed layer deepens, Chl-a at the surface is enhanced by the turbulent mixing of the underlying mixed layer water with high Chl-a and nutrients. However, the upwelling is a necessary condition for it to occur.