Cold Pool Formation Over the Taiwan Bank Driven by Shoaling Internal Tides

Abstract

Cold, saline, and chlorophyll-rich surface waters are frequently observed over the Taiwan Bank in the southern Taiwan Strait, forming a distinct feature referred to as the Green-Cold Pool (GCP). Despite its ecological importance, the physical mechanisms sustaining this cold pool remain unclear. Using an integrated analysis of in situ hydrographic and microstructure observations, geostationary satellite measurements of sea surface temperature (SST) and chlorophyll-a, and high-resolution numerical simulations, we demonstrate that the GCP is primarily driven by shoaling and breaking semidiurnal internal tides. Internal tides generated in the Luzon Strait propagate northwestward and shoal over the continental slope south of the Taiwan Bank. As the internal tide trough encounters the steep slope, it steepens and evolves into internal tidal bores. These bores propagate upslope, inducing cold-water run-up from downslope regions and pushing previously uplifted cold water farther onto the bank. Strong turbulent diffusivity of O(10-3-10-1) m2 s-1 develops near the bore head and within the bottom boundary layer, promoting intense vertical mixing and irreversible upward transport of O(10-3) m s-1. This combined effect forms a bottom-mixed layer (BML) that supplies cold, saline, and nutrient-rich water onto the bank. Depending on the intensity and vertical extent of turbulence, the BML may remain capped by a thin warm surface layer or outcrop at the surface, producing the observed cold SST signal. Barotropic tidal currents modulate the semidiurnal migration of the cold pool but are insufficient to sustain its persistence.