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04.02.11: Summary of my recent workΒΆ

  1. FSLE and near-surface stratification in the regional HYCOM simulation:
    1. On Jan. 15, 2010, the submesoscale gradient of sea surface temperature (SST) are well correlated with the unstable manifolds (estimated via finite-size Lyapunov exponents or FSLE; Figs. 1 and 2).
    2. This suggests that we could link the unstable manifolds to the near-surface stratification (that can both be estimated from observations) as an indication of surface mixed layer instabilities (SMLIs). Indeed, all large Brunt-Vaisala frequency (BVF) events are associated locally with large FSLE (Figs. 3 and 4).
  2. Relationship between near-surface stratification and unstable manifold in Winter-Spring using ARGO data:
    1. The histogram of near-surface (upper 50 m) BVF during February-April in a region northeast of station ALOHA has a long tail, suggesting the existence of stratification (large BVF) events during that period (Fig. 2).
    2. There is a positive correlation between the stratification events and the unstable manifold (Fig. 3).
    3. But it is found that this positive correlation is likely to be due to biases in the sampling: BVF and FSLE are both positive quantities and are most of the time low so that the correlation from a randomly-sampled collection is likely to have a positive correlation. This is confirmed with a null hypothesis that is not found to be statistically insignificant (Figs. 4 and 5).
    4. In conclusion, this analysis is not the right one to link unstable manifolds and near-surface stratification.
  3. Histogram of upper-ocean stratification from ARGO dataset and in the HYCOM simulation: The histogram of upper-ocean (upper 50 m) BVF obtained from ARGO are compared for 4 periods of the year to those obtained from the regional HYCOM simulation. Significative qualitative differences are found. The ARGO histograms have the same structure all year long with a lot of small values and a few large values. In the model, in Spring and Summer, the histograms have two distinct peaks at small and large values. I cannot say if these differences indicate a problem or not in the model.
  4. Next, I look at animation of unstable manifolds around the WHOTS site and the stratification at the site:
    1. Although the large BVF events (either shorther than a day or as long as a couple of days) within the mixed layer do not overall coincide locally with the presence or not of an unstable manifold, two of the most significative stratification events (significative in that they involve a large portion of the surface layer and last several days) are associated with a strong filament: days 475-480 in WHOTS-1, days 430-450 in WHOTS-2.
    2. But this statement is rather subjective and given the natural noisiness of both the unstable manifold and stratification signals, it may be wrong.
  5. Similarly, I look at the relationship between FSLE, BVF and SST gradient in the HYCOM simulation:
    1. During the year, not all near-surface BVF events are associated with submesoscale SST gradient (and thus with unstable manifolds).
    2. There are, however, events that are associated with submesoscale SST gradient and unstable manifold and that resemble qualitatively with some event seen in the WHOTS observsations.

In conclusion:

  1. There are events of SMLIs in the model associated with submesoscale SST gradient, unstable manifold, large BVF values in the mixed layer and, as seen before, large vertical velocities.
  2. There may be some events as well in the observations, when an unstable manifold is found near large BVF events in the mixed layer
  3. To go beyond these qualitative conclusions, I am trying to link, using observations, a time series of BVF events with a time series of FSLE in order to give some weight to the fact that the correlation between the two are not just due to happenstance.