Baroclinic instability-induced intensification of phytoplankton blooms at submesoscales in eutrophic frontal regions

Abstract

Off-coast phytoplankton blooms occur frequently in the frontal region of the eutrophic Taiwan Strait during the northeasterly monsoon relaxation period, as consistently revealed by extensive cruise and satellite observations. Realistic model simulations have shown that restratification by frontal baroclinic instability (BCI) plays a crucial role in triggering blooms under nutrient-rich conditions. This study deciphered the distinct contributions of submesoscale and mesoscale BCIs to bloom development using sensitivity tests of an idealized model of the Taiwan Strait featuring an intense alongshore front with ample nutrients. In three-dimensional fine simulations with both submesoscale and mesoscale BCIs present, blooms were triggered by the cessation of a down-front wind. Chlorophyll a was higher in submesoscale front regions than in mesoscale regions, primarily because of the higher upper-ocean stability resulting from more effective restratification by submesoscale BCI. In three-dimensional coarse simulations, mesoscale BCI led to relatively lower upper-ocean stability and weaker blooms following wind relaxation, consistent with those in mesoscale regions in corresponding three-dimensional fine simulations. In two-dimensional simulations without submesoscale and mesoscale BCIs, blooms could not be triggered despite the cessation of a down-front wind, primarily because of the absence of significant near-surface restratification by BCIs. Furthermore, although symmetric instability was present in two-dimensional fine simulations, its contribution to blooms was limited because of its minimal restratification effect. These results show that BCIs play the predominant role in triggering off-coast blooms in eutrophic coastal front regions such as the Taiwan Strait.