Enhancement of eddy-Ekman pumping inside anticyclonic eddies with wind-parallel extension: Satellite observations and numerical studies in the South China Sea

Eddy-Ekman pumping was recently proposed as a potential mechanism to explain the high chlorophyll (Chl) concentrations inside anticyclonic eddies (AEs). In this paper, the influence of eddy-shape changes on the strength of eddy-Ekman pumping is investigated for AEs in the South China Sea (SCS) through multi-satellite data and numerical experiments. The results mainly indicate that wind-parallel (-perpendicular) extensions may enhance (reduce) eddy-Ekman pumping inside AEs. First, an AE detected in northern SCS winter shows significant Chl-wind relation and uplifted near-surface isopycnals when its shape extension becomes parallel with the wind direction. In addition, satellite observations indicate significant eddy-Ekman-pumping signals appear in the central-northern SCS during the winter monsoon and in the central-eastern SCS during the winter and summer monsoons, whenever and wherever AEs' shapes show wind-parallel extensions. This regional-seasonal characteristic is further confirmed by the numerical experiments, which display a positive linear relationship between wind-parallel projective length and the strength of eddy-Ekman pumping. This study may contribute to extend the theory of eddy-Ekman pumping with emphasizing the importance of eddy shapes and wind directions, especially in the situations that high spatial-temporal variability appears in eddies and/or the wind field.