Mechanism for initiation of the offshore phytoplankton bloom in the Taiwan Strait during winter: a physical–biological coupled modeling study

Abstract. In situ observations showed phytoplankton blooms appear during winter in the Taiwan Strait (TWS), but the mechanism for bloom initiation was unclear. With the use of a coupled physical–biological numerical model, we find the winter bloom is triggered by the relaxation of the northeasterly monsoon. Thus, the aim of this study is to investigate the mechanism for bloom formation using the model. The model results show the weakening of the northeasterly wind generates a current that carries the fresh eutrophic Min-Zhe coastal water (MZCW) off the west coast of the TWS; then a stable stratification is formed in the upper ocean of the western strait, which significantly limits the turbulence. Via diagnostic analysis of the model output, we illustrate that the reduced turbulence allows the phytoplankton to accumulate within the upper layer of the western strait, which leads to an increase in chlorophyll. The analysis is further verified by the critical turbulence theory about the bloom. In addition to reduced turbulence, the lag between zooplankton and phytoplankton responses to the offshore extension of the MZCW is responsible for the formation of the bloom at the front. Therefore, we propose the observed offshore bloom in winter in the TWS is induced by the stable water stratification and the biological processes during the relaxation of the northeasterly wind.