In the following is given two animations of the same thing. The Brunt-Vaisala frequency (BVF) at 100 m (about the depth of the core of the seasonal thermocline in January-May) and the time series of the upper 200 m BVF at one specific location. In the first animation, the entire domain of the regional HYCOM simulation is given and the time series is that at 203.64E and 24.5N while in the second, the animation focuses on a small region around WHOTS and the time series is that at the WHOTS location:
In each case, the white dot show the location of the time series of the lower panel.
What I conclude is that, effectively, the erosion of the seasonal thermocline seems to be mostly controlled by the southward advection of water that does not have that seasonal thermocline. The transition is abrupt and coincides with the sharp SST gradient (black contours). But, unlike as I stated in this note, the erosion is not due to the SST filaments themselves. On the other hand, the result is still interesting as I don’t think we usually think of horizontal advection to explain the disappearance of the seasonal thermocline per se (in which case, any 1-D model to reproduce the stratification would fail), although we do in the classic LPS theory of the wind-driven circulation.
As a last comment, one can see (especially in the animation for WHOTS) that the SST filaments are nonetheless responsible for the transient re-stratification of the upper mixed layer (0-50 m) via surface mixed layer instabilities. I am still hoping that I can extract some information from the observations to support these ideas.
computed with anim_BVF_100m_gradSST.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/HYCOM/anim_BVF_100m_gradSST on the central disk.
