Seasonal thermal fronts on the northern South China Sea shelf: Satellite measurements and three repeated field surveys

Abstract

Seasonal thermal fronts associated with wind‐driven coastal downwelling/upwelling in the northern South China Sea are investigated using satellite measurements and three repeated fine‐resolution mapping surveys in winter, spring, and summer. The results show that vigorous thermal fronts develop over the broad shelf with variable widths and intensities in different seasons, which tend to be approximately aligned with the 20–100 m isobaths. Driven by the prevailing winter/summer monsoon, the band‐shaped fronts were observed with a magnitude exceeding 0.1°C/km in the subsurface, and accompanied by energetic coastal downwelling/upwelling due to shoreward/offshore Ekman transport. The downward/upward tilting of seasonal thermoclines across the shelf exceeds 20 m, significantly contributing to the development of thermal fronts over the shelf. In addition, the diagnostic analysis of Potential Vorticity (PV) suggests that the summer frontal activities induced by the coastal upwelling are more stable to convection and symmetric instabilities in comparison to the winter fronts associated with downwelling‐favorable monsoon forcing. This is primarily due to their essential differences in the upper ocean stratification and horizontal buoyancy gradients arising from wind forcing. At the same time, the coastal currents are substantially regulated by the seasonal winds. An expected lag correlation between the velocity from mooring measurements and alongshore wind stress is detected near the frontal region. These results indicate that seasonal wind forcing plays an important role in the frontal activities and coastal water transport over the shelf.

Composite of Typhoon‐Induced Sea Surface Temperature and Chlorophyll‐a Responses in the South China Sea

“…However, the thermocline and nutricline near the coast are shallow (Wang et al, 2020), and the water column can be easily mixed by typhoons such that even a weak and fast‐moving typhoon can induce a prominent change at the ocean surface (Zhao et al, 2017). In coastal upwelling regions, such as the coast of Vietnam (Yu et al, 2019) and the northern SCS (Jing et al, 2016), the upwelling is enhanced by typhoons (Chang et al, 2008), resulting in more prominent changes at the surface (Figures 8b and 8e). A similar scenario has been identified in cyclonic eddies such that the upwelling can be enhanced by typhoons and stronger responses are observed (Chen & Tang, 2012).…”

Enhanced Chlorophyll Concentrations Induced by Kuroshio Intrusion Fronts in the Northern South China Sea

“…Ocean fronts, transitions between different waters with high gradients of density, temperature, and nutrients, contribute significantly to regional ecosystem productivity through generating local upwelling and uplifting nutrients from subsurface to the upper layer (Kahru et al, ; Mahadevan, ; Stegmann & Ullman, ). Detected from satellite images, vigorous thermal fronts exist between Luzon Island and Taiwan Island and stretch several hundred kilometers northwest of Luzon Island (Jing et al, ). Two previous studies (Peñaflor et al, ; Shang et al, ) suggested that the LZBs might be associated with Kuroshio intrusion fronts and the interaction with the SCS water, but their work was based on case studies and did not provide further analysis.…”

Mapping Diurnal Variability of the Wintertime Pearl River Plume Front from Himawari-8 Geostationary Satellite Observations

“…There are various fronts in the northern South China Sea (NSCS), under the influence of complex topography, tides, the East Asian monsoon (strong northeasterly winter winds and weak southwesterly summer winds), Kuroshio intrusion, freshwater outflow, and coastal currents [8,17,18,[21][22][23][24][25]. The frontal position and intensity in the NSCS at monthly to interannual time scales were well documented from remotely sensed data since 1980s.…”

“…Since direct measurements of oceanic fronts are costly and time-consuming, visible and infrared satellite imagery proved to be a powerful tool for remotely mapping and characterizing large-scale fronts around the world [6,9,10]. The most important fronts include estuarine plume [11,12], coastal current fronts [13,14], tidal mixing fronts [15,16], shelf-slope fronts [17,18], and fronts associated with western and eastern boundary currents [8,19,20].…”

“…Chang, Shieh, Lee, Chan, Lan and Weng [23] presented finescale structures and interseasonal evolution of wintertime NSCS thermal fronts from 7-year satellite SST images. Jing, Qi, Fox-Kemper, Du and Lian [17] described the NSCS seasonal thermal fronts associated with wind-driven coastal downwelling/upwelling based on multiyear daily SST satellite observations and in situ measurements. Wang, Yu, Zhang, Zhang, and Chai [18] investigated seasonal and interannual front variability in the entire SCS using 15-year MODIS (Moderate Resolution Imaging Spectroradiometer) SST imagery.…”