South China Sea Sst Fronts, 2015–2022

Monthly climatology of thermal fronts in the South China Sea was derived from high-resolution (2 km) high-frequency (hourly) SST data of the Advanced Himawari Imager (AHI) onboard the Japanese Himawari-8 geostationary satellite. The Belkin and O’Reilly algorithm (BOA) was applied to SST data from 2015-2022 to generate maps of SST gradient magnitude GM, which were log-transformed to enhance contrasts in digital maps and reveal additional features (fronts). The combination of high-resolution cloud-free four-day-composite SST imagery from AHI, advanced front-preserving gradient algorithm BOA, and digital contrast enhancement with log-transformation of SST gradients allowed us to identify numerous mesoscale/submesoscale fronts (including a few fronts that have never been reported) and document their month-to-month variability and spatial variability on the submesoscale. Spatio-temporal variability of SST fronts was analyzed in detail in five regions: (1) In Taiwan Strait, six fronts were identified: China Coastal Front, Taiwan Bank Front, Northern/Eastern Changyun Ridge fronts, and Eastern/Western Penghu Islands fronts; (2) The Guangdong Shelf is dominated by the China Coastal Front in winter, with the eastern and western Guangdong fronts separated by the Pearl River outflow; (3) Hainan Island is surrounded with fronts formed by wind-induced coastal upwelling in the southeast; tides and topographic upwelling in the northeast; and tidal mixing and upwelling in the west, where fronts are steered by giant sand ridges; in the western Beibu Gulf, the Red River Outflow Front extends southward as the Vietnam Coastal Front, while the northern Beibu Gulf features a tidal mixing front off the Guangxi coast; (4) Off SE Vietnam, the 11°N coastal upwelling gives rise to a summertime front, while the Mekong Outflow and associated front extend seasonally toward Cape Camau, close to the Gulf of Thailand Entrance front; (5) In Luzon Strait, the Kuroshio front manifests as a chain of three submesoscale fronts across the Babuyan Islands, while west of Luzon Island a broad offshore frontal zone persists in winter. The summertime eastward jet (SEJ) off SE Vietnam is documented from five-day mean SST data. The SEJ emerges in June-September off the 11°N coastal upwelling center and extends up to 114°E. The zonally oriented SEJ was observed located between two large gyres, each about 300 km in diameter.