The animation of the Brunt-Vaisala frequency (BVF) vertically averaged between the surface and 30 m is given here:
It shows two processes for the changes in near-surface stratification: either inside a SST filament via surface mixed-layer instabilities or a large-scale change in the order of days maybe due to surface buoyancy forcing. Because during that period the background stratification is nearly zero everywhere, the stirring by the mesoscale eddy field has little or no effect on BVF.
This is not the case in the lower part of the surface mixed layer where the BVF field does vary horizontally and is being stirred by the mesoscale eddy field as shown in these animations. It is this stirring that is the main cause of the rapid change of stratification and the isopycnal depths in the 50-100 m depth range.
Below is the same animation but with the BVF time series at the WHOTS location:
computed with anim_upper_BVF_gradSST.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/HYCOM/anim_BVF_100m_gradSST on the central disk.
Below is an animation of 30-m w and SST gradient (upper panel) and a time series of w at a locaiton off the west side of Oahu:
Below is the same animation than the previous one except that the w field has been smoothed with a 5-day running mean:
