Upper water structure and mixed layer depth in tropical waters: The SEATS station in the northern South China Sea

Abstract

The variability of the upper water hydrographic structure, the efficacy of the different schemes for estimating the mixed layer depth (MLD), the inter-comparability estimation of the MLDs and diurnal and intra-annual MLD climatology in the tropical waters in the northern South China Sea were accessed in 702 depth-profiles of potential temperature (θ) and salinity collected in 64 cruises between 17.5 and 18.5°N and 115.3 and 116.3°E from 1997 to 2013. The hydrographic structure may be sub-divided into three principal types: the classical type, with quasi-isopycnal surface mixed layer followed by an abrupt increase in the depth-gradient in θ and potential density (σθ) to mark the MLD; the stepwise type, with one or more small stepwise decreases in θ and/or increases in σθ in the mixed layer; and the graded type, with a general decrease in θ and increases in σθ with depth into the main pycnocline without a clear break to mark the MLD. These three types of upper waters were found in 75, 10, and 15% of the cruises. Out of the 10 schemes for estimating the MLD, only the fixed temperature difference method of 0.5 and 0.8°C from the 10-m temperature yielded consistent results, with root mean square error and mean absolute percentage difference of 2 m and 2%. MLD varied diurnally with an average standard deviation of 4 m from the mean. The monthly average MLD reached a maximum of 80 m in December/January and dropped to a minimum of 25 m in May.