Bimodal behavior of the seasonal upwelling off the northeastern coast of Taiwan

Observations over the outer shelf and shelf break off the northeastern coast of Taiwan indicate a curious seasonal variability of upwelling. At deeper levels 100 m below the surface, upwelling is most intense in summer but weaker in winter. Nearer the surface at approximately 30 m below the surface, the opposite is true and the upwelling is stronger in winter than in summer. Results from a high-resolution numerical model together with observations and simple Ekman models are used to explain the phenomenon. It is shown that the upwelling at deeper levels (similar to 100 m) is primarily induced by offshore (summer) and onshore (winter) migrations of the Kuroshio, while monsoonal change in the wind stress curl, positive in winter and negative in summer, is responsible for the reversal in the seasonal variation of the upwelling near the surface (similar to 30 m). This mechanism reconciles previous upwelling data.