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<p>The eastern offshore area of Hainan Island, an important shelf sea in the northwestern South China Sea (SCS), is characterized by a pronounced thermal front exhibiting distinct seasonal and three-dimensional structures, driven by monsoons, coastal currents, and upwelling/downwelling. Using high-resolution multi-source fusion data and reanalysis data from 2000 to 2024, this study investigates the seasonal cycle, interannual variability, and long-term trends of the East Hainan Coastal Front (EHCF) and explores its three-dimensional structures and formation mechanisms. Results show the EHCF follows a bimodal seasonal cycle, with two prominent intensity peaks occurring in boreal winter (January) and summer (July), respectively. The winter front, aligned with the 50–100&#xa0;m isobaths, attains its maximum intensity at the surface (0.08&#xa0;°C/km). In contrast, the summer front reaches both its peak intensity (0.10&#xa0;°C/km) and maximum spatial extent at a depth of 25&#xa0;m, spanning the 50–150&#xa0;m isobaths. Dynamically, the winter front is primarily forced by the convergence of the cold coastal current and warmer offshore waters, whereas the summer front is driven by wind-induced upwelling. Surface fronts, in both winter and summer, are more prone to symmetric and shear instabilities than their subsurface counterparts, thereby inducing mixing that dissipates the frontal structure. On an interannual scale, the El Niño-Southern Oscillation (ENSO) modulates the EHCF’s intensity and position by altering local wind stress, local advection, and upwelling patterns. Over the past two decades, the winter front has strengthened at a rate of up to 0.02&#xa0;°C/km/decade in response to coastal current intensification, while an onshore intensification of the summer current has caused a shoreward shift of the summer front. These findings provide an improved dynamical understanding of shelf fronts in the South China Sea.</p>

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East Hainan Coastal Front: seasonal cycle, interannual variability, and long-term trend

  • 期刊论文

  • 发表日期：

    2026年5月13日

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  • Teng Liu,
  • Tengfei Xu,
  • Xiao Han,
  • Zexun Wei,
  • Dingqi Wang,
  • Shujiang Li

摘要

The eastern offshore area of Hainan Island, an important shelf sea in the northwestern South China Sea (SCS), is characterized by a pronounced thermal front exhibiting distinct seasonal and three-dimensional structures, driven by monsoons, coastal currents, and upwelling/downwelling. Using high-resolution multi-source fusion data and reanalysis data from 2000 to 2024, this study investigates the seasonal cycle, interannual variability, and long-term trends of the East Hainan Coastal Front (EHCF) and explores its three-dimensional structures and formation mechanisms. Results show the EHCF follows a bimodal seasonal cycle, with two prominent intensity peaks occurring in boreal winter (January) and summer (July), respectively. The winter front, aligned with the 50–100 m isobaths, attains its maximum intensity at the surface (0.08 °C/km). In contrast, the summer front reaches both its peak intensity (0.10 °C/km) and maximum spatial extent at a depth of 25 m, spanning the 50–150 m isobaths. Dynamically, the winter front is primarily forced by the convergence of the cold coastal current and warmer offshore waters, whereas the summer front is driven by wind-induced upwelling. Surface fronts, in both winter and summer, are more prone to symmetric and shear instabilities than their subsurface counterparts, thereby inducing mixing that dissipates the frontal structure. On an interannual scale, the El Niño-Southern Oscillation (ENSO) modulates the EHCF’s intensity and position by altering local wind stress, local advection, and upwelling patterns. Over the past two decades, the winter front has strengthened at a rate of up to 0.02 °C/km/decade in response to coastal current intensification, while an onshore intensification of the summer current has caused a shoreward shift of the summer front. These findings provide an improved dynamical understanding of shelf fronts in the South China Sea.