A coupled physical–biological modeling study of the offshore phytoplankton bloom in the Taiwan Strait in winter

• Winter offshore bloom in the TWS is reproduced by the numerical model.
• Relaxation of northeasterly wind acts as a trigger for the bloom occurrence.
• Weakened mixing and offshore chlorophyll advection are mechanisms for the bloom.

In-situ observations find that offshore phytoplankton blooms occur occasionally in the north-central Taiwan Strait (TWS) in winter, but the formation mechanisms behind are not yet understood. We simulate the offshore bloom scenario in the winter of 1998 with a coupled physical–biological numerical model. Model results illustrate that when the northeasterly wind is relaxed, a cross-strait current is induced, which carries diluted Min-Zhe Coastal Water (MZCW) offshore, extending into the upper layer of the western TWS. Vertical mixing is weakened in the western TWS due to intensified stratification formed by the location of fresh MZCW over saline water. Consequently, the vertical diffusion of chlorophyll decreases, and the bloom occurs in the upper layer of the western TWS. Additionally, the cross-strait current carries the high chlorophyll concentration from near-shore to offshore regions, forming a maximal offshore chlorophyll concentration. We propose that the relaxation of the northeasterly wind acts as a trigger for the winter bloom occurrence in the TWS through complicated physical processes, i.e., the diluted MZCW extending offshore, the stratification intensifying and mixing weakening, forming distinctive characteristics of winter blooms in the TWS.