Atmospheric Mixed Layers over the South China Sea during SCSMEX

Sounding data from the South China Sea Monsoon Experiment (SCSMEX) have provided a unique opportunity to document the variability of atmospheric mixed layers over the South China Sea (SCS) during the onset of the monsoon in this region. Six-hourly sounding data from two research vessels, deployed over the northern and southern SCS, are 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 48% 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 the monsoon onset, is likely due to northward advection of low-level warm, moist air over cooler waters, which results in very small or negative buoyancy fluxes over the northern SCS, and thus weak upward mixing of heat and moisture from the surface.