Atmospheric Mixed Layers over the South China Sea during the Summer Monsoon

In this study we examine the behavior of atmosphere mixed layers over South China Sea (SCS) during the summer monsoon season. Analysis of sounding data from two research vessels, deployed over the northern and southern SCS, during South China Sea Monsoon Experiment (SCSMEX) conducted in May and June 1998 were used to determine the mixed-layer depth and its thermodynamic properties. Results from the southern ship show the presence of mixed layers 83% of the time with a mean depth of 459 m, similar to other tropical oceanic locations. On the other hand, the northern ship exhibited mixed layers < 50% of the time with a mean depth of 342 m, considerably less frequent and shallower than other tropical regions.

This anomalous behavior, which was particularly evident after monsoon onset, is likely due to northward advection of low-level warm, moist air over cooler waters, which results in stable boundary layers with very small or negative buoyancy fluxes over the northern SCS, and thus weak upward mixing of heat and moisture from the surface. During these periods in SCSMEX with stable boundary layers, model reanalyses (such as those produced by JMA and ECMWF) produce surface fluxes which are several times larger than observed. Some implications of these model difficulties will be discussed.

To investigate whether such conditions with stable boundary layers are present over the SCS in the monsoon environment of other years, we analyze sounding data taken during TiMREX (Terrain-influenced Monsoon Rainfall EXperiment). This experiment, which was conducted from 15 May to 25 June 2008, took soundings over the SCS from a research vessel and from several dropsonde missions. These soundings are used to examine the boundary layer structure over the SCS during the TiMREX period.