Submesoscale Process Of Ocean Front In The Continental Shelf Sea Of Eastern Hainan Island

Ocean fronts are ubiquitous in the continental margin sea, which is one of the important factors to affect the flow field structures, ocean heat and momentum exchange,air-sea interaction and biochemical processes and plays an important role to maintain efficient operation of the biological pump in the shelf seas. A hydrological investigation was conducted in the shelf of eastern Hainan Island during July 2012. With the in-situ measurements from four cross-shelf sections and satellite data, the submesoscale process of the fronts are discussed in this paper, including the seasonal variation characteristics of thermal front, the three-dimensional structure, dynamic process and mixed characteristics in the shelf sea of eastern Hainan Island.It’s obviously that the thermal front has a seasonal variation: the front is strongest in winter, and decreased gradually in spring and summer. However, it fade and disappear in fall. The core region of the front also changes with the seasons, it moved southward gradually from the northeast cape of Hainan Island to eastern in winter and spring. The weakly front mainly distributed in the upwelling zone and the front center is not obvious in summer. It is a typical upwelling front in Summer, the near shore is compensated with the underlying low-temperature and high-salt water, while the offshore is the high-temperature and low-salinity shelf water. The thermal front distribution is located in the 100 m isobath. The frontal intensity is reduced with increasing depth, and position goes to offshore. Subsurface temperature front is significantly higher than the surface,which may cause by the heating of nearshore sea surface water and lead to the weakening horizontal temperature gradient. Dynamic characteristics of the front has a great difference in both sides. The O(1) Rossby number(Ro) is positive on the dense side and negative on the light side. The along-frontal velocity is large relatively and the stretching is strengthened by strong horizontal shear, also is the potential vorticity(PV), which can trace the cross front Ekman transport. We obtained the vertical velocity with by Quasi-Geostrophic(QG) omega equation and grasped the ageostrophic secondary circulation(ASC). The magnitude of frontal vertical velocity is O(10-5) and causes downwelling on the dense side and upwelling on the light side, which constitute the ASCtogether with the cross-frontal ageostrophic speed. Ther mixed characteristic is weak in summer, but the large turbulent dissipation and mixing rate measured in the frontal region. Heat and buoyancy flux is also strengthen in the frontal region, which show that the front promoted exchange of material and energy in the upper ocean.