The best definition, which is modified from the optimal linear fitting method, is established for determining the mixed layer depth, , in the northern South China Sea. Using in situ data, temperature and density profiles in the northern South China Sea (17°-23°N, 110°-118°E) were collected in August 2006, October 2012, October 2013, march 2014, and June 2015. We compared the optimal linear fitting method (OLF) with other three methods: 1. A threshold value of temperature (∆T= , reference depth at 2 m); 2. A threshold value of density ( , reference depth at 2 m); 3. Temperature gradient method ( ), and found that the OLF method is the best definition. The method is applied to calculating the MLD of the northern South China Sea to investigate variability and changes in MLD. The seasonal cycle linked with monsoonal variability is well captured. In monsoon transformation period (in the spring), surface mixed layer depth is extremely thin(less than 2m) in some areas. In the early southwest monsoon, the MLD is deepening. With the net surface heat flux decreasing and the increase of rainfall in the October, the MLD covaries with the buoyancy fluxes and deepens to the depth of the water. The results reveal close relationships of wind speed and net surface heat flux with the variability of the MLD in SCS.
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