Off-Coast Phytoplankton Bloom in the Taiwan Strait During the Northeasterly Monsoon Wind Relaxation Period

This study applied cruise, model, and satellite data to analyze the off-coast phytoplankton blooming during the late fall to early spring monsoon period in the Taiwan Strait when northeasterly wind prevails. Based on the composite and self-organizing map analyses, the three data sets consistently show high chlorophyll-a concentration near the along-shore front during the down-front northeasterly wind relaxation period while lower concentration when relatively strong wind is persistent. Meanwhile, the off-coast blooming always coincides with intense near-surface stratification when the northeasterly wind relaxes. Diagnoses of balanced Richardson number, Ertel potential vorticity and instability energy budget from high-resolution cruise observations and model results demonstrate that vigorous submesoscale symmetric and baroclinic instabilities can develop near the along-shore front under the down-front NE wind. Diagnoses of modeled buoyancy and chlorophyll-a budget equations further suggest the submesoscale instabilities lead to rapid near-surface restratification and offshore stretching of the along-shore front within the upper 10-m of the mixed layer when the down-front NE wind relaxes, favoring the surface 10-m phytoplankton growth. As comparison, contribution of the larger-scale advection related with geostrophic adjustment and Ekman transport to the chlorophyll-a increment reached beyond the middle layer of ∼20-m depth.