Cruise, satellite, and model data consistently reveal that phytoplankton blooms always occur near the along-shore front during the down-front northeasterly monsoon relaxation period, while relatively low Chlorophyll-a concentration presents under persistent strong winds in the eutrophic winter and spring Taiwan Strait. Diagnoses of the high-resolution cruise observations and realistic model results demonstrate the existence of vigorous instabilities in the along-shore front region. The present study deciphers symmetric, submesoscale, and mesoscale baroclinic instabilities and their contributions to phytoplankton growth based on realistic model diagnoses as well as idealized model sensitivity experiments in terms of dimensionality and grid resolution. Despite the presence of restratifying baroclinic instability under persistent down-front wind, the destratification effect of the wind maintains steeply tilted isopycnals. Due to the limited restratification effect and enhancement of turbulent dissipation near the submesoscale front region, symmetric instability is unfavorable to phytoplankton growth. In contrast, with the relaxation of the destratifying down-front wind, submesoscale and mesoscale baroclinic instabilities play a dominant role in driving relatively strong and weak restratification near the submesoscale front and mesoscale eddy regions. This leads to significantly weaker turbulent dissipation and triggers relatively strong and weak near-surface phytoplankton blooms, respectively.

Submesoscale Instabilities; Phytoplankton Bloom; Taiwan Strait