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

Abstract

AbstractThis 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.