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04.14.11: SMLI and the erosion of the seasonal thermocline

From previous analysis, we know that surface mixed layer instabilities (SMLIs) associated with sharp gradient of sea surface temperature (SST) are responsible for the re-stratification, at least temporally, of the surface mixed layer (SML). What we still do not know if these SMLIs are also responsible for the erosion of the seasonal thermocline. Although the timing of the occurence of SMLIs (January through March) coincides with the disappearance of the seasonal thermocline, there is no reason to think that surface buoyancy forcing and convective mixing is the main cause.

To test this, I choose two locations that have the same latitude and close enough to assume that the surface buoyancy forcing is the same for both cases (is this correct? the locations are separated by about 150 km and are shown by the two red dots in Fig. 1). However, the first time during the year the location is being hit by a SST filament differs (compare Figs. 1, 2 and 3). At 203.64°E (156.36°W), the location is hit the first time on Jan. 19 (Fig. 1) which results in the sudden lost of the seasonal thermocline (Fig. 4, lower). At 202°E (158°W), the location is being hit the first time on Feb. 2 by a relatively week SST filament, then on Feb. 16 by a stronger one. In each case, this results in a successive weakening of the seasonal thermocline.

These simple time series, thus, suggest that the SMLIs may, at least in the model, may be the main mechanism for the erosion of the seasonal thermocline.

Could we see the same coincide of the disappearance of the seasonal thermocline and the passage of a FSLE filament in the WHOTS data? It seems that in WHOTS-1 (see the animations here), we could associate indeed that erosion with the passage of several FSLE filament on Dec. 28, 2004, Jan. 17 and Jan. 28, 2005. It is less clear in WHOTS-2 as the seasonal thermocline has larger variability than during WHOTS-1 [notice, however, that, in WHOTS-2, the near-surface restratification observed in could arguably be associated with a strong filament]. It would be nice if same arguments could be made for the other WHOTS time series.


Figure 1: SST gradient and 10-m BVF on Jan. 19, 2010. The two locations are shown with red dots.


Figure 2: Same as Fig. 1 but for Feb. 2, 2010.


Figure 3: Same as Fig. 1 but for Feb. 16, 2010.


Figure 4: Time series of BVF at the two locations shown in Fig. 1 to 3.

computed with link_gradSST_N_ano.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/HYCOM/ on the central disk.