Observation Of Front Mixing And Parameterization In The Continental Shelf Sea East Of Hainan Island

The ocean front is a transitional zone between different water systems or water masses in the ocean.It is widely distributed in the areas of the ocean jet,mesoscale eddies with unstable positive pressure and baroclinic pressure,or in the areas such as continental slope and upwelling.When the ocean front is subjected to wind forcing or interaction with environmental factors,the flow shear increases in the frontal area,and the convergence and divergence process become strong,accompanied by sub-mesoscale processes and mixed effects.These processes can significantly improve the exchange of cross-frontal material and energy.It is of great significance for marine dynamics,the balance of ocean energy,heat,and material budgets,as well as the ecological effects of fronts in the continental shelf sea.Based on six-year satellite remote sensing data from 2012 to 2017,we found that the average frontal centerline of the east continental shelf of Hainan Island was located between the 50 m-100 m isobaths in the winter,spring and summer.The front was more stronger in the winter,weakened in the spring and summer,disappeared in autumn.At the end of autumn,a weak front reformed in the northeast of Hainan Island.As the northeast monsoon increased,the SST average value gradually decreased while the average SST gradient increased.With the in-situ measurements from cross-shelf sections data on the continental shelf east of Hainan Island in summer on 2012,the spatial distribution of temperature and salinity during the entire observation period were carried out.By extracting the front,combined with sea surface temperature and wind field data,the different levels and sections of the thermal front in the continental shelf sea east of Hainan Island were analyzed to explore their distribution characteristics,mixing processes and their parameterization.It was indicated that the front on the continental shelf east of Hainan Island in summer was mainly distributed between the 50-m isobath and 100-m isobath,and there was a strong frontal zone along the 100-m isobath.Strong thermal front mainly occurred above 25 m depth,while the more stronger above 14 m depth.The magnitude of the maximum vertical eddy diffusivity in the frontal zone is 10^-3m²s^-1,which is about two orders of magnitude higher than that of the ocean.Moreover,thelarge mixing rate is mainly distributed near the front axis and close to the sea surface.By compared with other parameterized modes and field observations,the eddy diffusivity parameterized mode(K_p=10^-4.427·Ri^-0.9928)proposed in this paper is more closely agreement with the field observation data,and this suggests our parameterized mode can better describe the mixing processes of the front in the east of Hainan Island in summer.