27 / 2023-07-24 15:07:53

Multi-factor governed synoptical winter bloom in the South China Sea

submesoscale processes; winter bloom; baroclinic instability; restratification; South China Sea

The winter phytoplankton flourishes in the South China Sea (SCS), an oligotrophic marginal sea, are often attributed to the seasonal deepening of the mixed layer. However, the governing factors of synoptical blooms during winter, which frequently occur to the west of the Luzon Strait, have rarely been elaborated, especially in the background of enhanced Kuroshio intrusion and active frontal in the northern SCS. This study developed a process-oriented submesoscale-permitting physical–biological coupled model and reasonably reproduced a synoptical tremendous phytoplankton bloom captured by Bio-Argo observation during the winter of 2014–2015. Model diagnoses and sensitivity experiments demonstrated the contributions of multi-scale physical processes in triggering and maintaining the synoptical bloom. Two processes mainly contributed to the synoptical bloom initiation before 25 January. Firstly, the modeled nitrate budget and vertical velocity diagnoses suggested that upwelling induced by mesoscale vorticity advection transported nutrient-rich water upward mainly to the subsurface layer of ~ 50 m, with some amount of nutrients reaching the surface. Secondly, analysis of the instability energy budget and sensitivity experiment consistently showed that near-surface restratification induced by submesoscale baroclinic instability retained phytoplankton within the sun-lit surface, facilitated their growth, and resulted in the earlier occurrence of bloom. Subsequently, after the initiation of the bloom, enhanced wind-induced microscale vertical mixing maintained the phytoplankton biomass by persistently entraining nutrients from the subsurface to the surface.