[1] A strong upwelling off the continental shelf of the northern South China Sea (SCS) in 1998 summer is reinvestigated using a suite of new satellite measurements and numerical modeling. Previous studies indicate that the upwelling in the western SCS, especially off the Vietnam coast, almost disappears during 1998 summer following the El Niño because of the weakened southwest monsoon. This study identifies that the coastal upwelling in the adjacent northern SCS (NSCS) is significantly strengthened during 1998 summer, and the alongshore wind stress is dramatically enhanced over the region. As a result, the offshore Ekman transport in 1998 summer is the strongest, almost twice the average of the other 17 years during 1997–2007. The Chl a concentrations in the representative upwelling regions are much higher than any in other years. Further analysis suggests that two adjacent basin‐scale upwellings in the SCS have different responses and maintaining mechanisms because of the anticyclonic atmospheric circulation anomaly over the SCS and northwest Pacific. The northern flank of the atmospheric circulation anomaly intensifies the monsoonal winds off the NSCS coast, while the southern flank suppresses the southwesterly winds along the Vietnam coast.

Persistent coastal upwelling and upwelling‐induced thermal fronts in the northwestern South China Sea are investigated using satellite measurements, two intensive mesoscale mapping surveys and three bottom‐mounted ADCPs. The results indicate that pronounced surface cooling and upwelling‐related fronts with a width of 20–50 km occur around Hainan Island and persist through the summer upwelling season. Driven by the prevailing southwesterly monsoon, the subsurface cooling band is ∼6°C colder than the water offshore of the East Coast, where the thermal gradients are generally more than 0.1°C/km. The cold and nutrient‐rich coastal water is identified to be derived primarily from the deep water of the outer shelf. At the same time, the spatial structure of the upwelling and thermal front, as well as the upwelling‐related coastal currents, is significantly regulated by wind forcing. A prominent lagged correlation between the moored temperature records and alongshore wind stress is detected in the East Coast. The correlation coefficient is −0.8 with the temperature lagging behind wind stress by 2.2 days, indicating that the cooling band off the East Coast is dominated mostly by the alongshore southwesterly monsoon during the upwelling season.

Submesoscale density fronts and the associated processes of frontogenesis and symmetric instability (SI) are investigated in the northwest Pacific subtropical countercurrent (STCC) system by a high-resolution simulation and diagnostic analysis. Both satellite observations and realistic simulation show active surface fronts with a horizontal scale of ~20 km in the STCC upper ocean. Frontogenesis-induced buoyancy advection is detected to rapidly sharpen these density fronts. The direct straining effect of larger-scale geostrophic flows is a primary influence on the buoyancy-gradient frontogenetic tendency and frontal baroclinic potential vorticity (PV) enhancement. The enhanced lateral buoyancy gradients in conjunction with atmospheric forced surface buoyancy loss can produce a negative Ertel PV and trigger frontal SI in the STCC region. Up to 30% of the mixed layer (ML) inside a typical eddy has negative PV in the high-resolution simulation. As a result, the cross-front ageostrophic secondary circulations tend to restratify the surface boundary layer and induce a large vertical velocity reaching ~100 m day⁻¹, substantially facilitating the vertical communication of the STCC system. At the same time, the SI is identified to be responsible for a forward cascade of geostrophic kinetic energy in the STCC region, despite the coexistence of ML eddies and SI in the deep winter ML. Therefore, these active density fronts and their SI-associated submesoscale processes play important roles in the enhanced vertical exchanges (e.g., heat, nutrients, and carbon) and energy transfer to smaller scales in the eddy-active STCC upper ocean, as well as triggering phytoplankton blooms at the periphery of eddies.