Mechanisms for Occurrence of the Frontal Phytoplankton Bloom in the Taiwan Strait in Winter

In-situ observations found offshore phytoplankton blooms appeared at the north-central Taiwan Strait (TWS) in winter. With the use of a coupled physical-biological numerical model, we simulated the offshore bloom scenario in the winter of 1998. Model results illustrated that while northeasterly wind was relaxed, a cross-strait current was induced which carried the diluted Min-Zhe Coastal Water (MZCW) offshore extending in upper layers of the western TWS. Vertical mixing was greatly weakened in the western TWS due to the intensified stratification formed by the fresh MZCW over the saline water. Consequently vertical chlorophyll diffusion was decreased and the bloom occurred in the upper layer of the western TWS. In addition, the cross-strait current carried the high chlorophyll from nearshore to offshore regions forming the offshore maximal chlorophyll. According to the study, we firstly proposed that the relaxation of northeasterly wind played as a trigger for the winter bloom occurrence in the TWS via complicated physical processes, i.e., the diluted MZCW extending offshore, the stratification intensifying and the mixing weakening, which are distinctive with other bloom occurrences in the world.