02.11.11: Surface mixed layer instabilities in HYCOM

Conclusions

1. The explosion of submesoscale features in the HYCOM model is associated with sharp horizontal gradient of density. These gradients are formed by the southward penetration (by the Ekman flow) of heavier water mass into the light water mass of the subtropics.
2. The instabilities are thus frontogenetic in nature.
3. They are associated with anomalies in stratification N, which restratify the surface mixed layer (SML), and values of the vertical component of velocity reaching 10-40 m/day.
4. Large anomalies in N can be used as proxy for the occurence of these frontogenetic instabilities.
5. Similar anomalies in N are observed in the WHOTS dataset suggesting that the HYCOM instabilities are real.
6. We cannot use kinetic energy as a proxy for the instabilties –kinetic energy also shows eddies that are not related to these instabilities.

Results

I look here at the relationship between the Rossby number R, the potential density field σ, the stratification N, the vertical component of velocity w and the kinetic energy at or near the surface on Jan. 15, 2010 in the HYCOM output. We focus on the instabilities that occur north-east of Hawaii on that day. The field of σ is plotted in Fig. 1 and it is contoured in black in Fig. 2 on top of R. We see that the instabilities, seen in σ and large values of R, occur at the gradient in σ. This is important as it suggests that the instabilities are due to frontogenesis. An animation of σ shows that this gradient has been generated due to the drift of a heavier water mass from the north, probably due to Ekman flow.

Figure 1: σ at the surface in HYCOM on Jan. 15, 2010.

Figure 2: σ and R at the surface in HYCOM on Jan. 15, 2010.

Elevated values of N are also found along these gradients of σ (Fig. 3) and are, thus, also correlated with large values of R (Fig. 4). This is an important result for two reasons: 1) this confirms that the submesoscale features are associated with frontogenetic surface mixed layer instabilities (SMLI) that restratifies the surface mixed layer** and 2) we can use N as a proxy for SMLI and their associated submesoscale features in the observations.

Another important consequence can be drawn from the comparison of the evolution of N in HYCOM with that observed by WHOTS (see Figs. 4 to 6 in this note). Because the large anomalies in N observed in HYCOM during Spring are qualitatively similar to those in WHOTS, we can deduce that the HYCOM instabilities are likely to be real.

Figure 3: N at 10 m depth in HYCOM on Jan. 15, 2010.

Figure 4: N at 10 m depth and absolute R at the surface in HYCOM on Jan. 15, 2010.

Fig. 5 shows that large values of w (> 01-40 m/day) are associated with the instabilities and could potentially affect the import of nutrient into the euphotic zone (remember that the surface mixed layer in HYCOM seems to be shallower than in the WHOTS observations so that the w of the instabilities may reach the nutricline).

Figure 5: N at 10 m depth and absolute w at the surface in HYCOM on Jan. 15, 2010 (w is in m/day).

Fig. 6 shows that kinetic energy does not locate only SMLI so that it cannot be used as an index for SMLI.

Figure 6: Kinetic energy at 10 m depth in HYCOM on Jan. 15, 2010.

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Computed using rel_vort_HOT_track.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/HYCOM/analysis/rel_vort_HOT_track/ on ipu1.