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12.15.10: Notes on literature concerning mixed layer instabilities

Boccaletti et al. (2007)

  • “A winter storm hits the open ocean, mixing the top 100 m of the water column over a patch of a few hundred kilometers squared. Once subsided, the storm leaves behind a homogenized layer in which horizontal variations of salinity and temperature have survived (Price 1981; Ferrari and Rudnick 2000), yet vertical variations have been virtually erased. An adjustment process begins that restratifies the surface layer by slumping the nearly vertical isopycnals.” See Ferrari and Rudnick (2000) for more details on the formation of the homogenized layer with horizonal gradient, especially if they discuss actual observations.
  • “a second class of small-scale baroclinic disturbances appears in the presence of reduced stratification at the boundaries (Blumen 1979). These small-scale instabilities represent the slumping of density fronts within the surface and bottom ML.”
  • “The ML, visible as a layer of weak stratification in the upper 100 m overlying the more stratified interior, is not horizontally homogeneous: there are numerous lateral density gradients down to scales of a few kilometers. Simultaneous ADCP measurements confirm that these lateral gradients are in thermal wind balance.”
  • Their Fig. 8 illustrates the various mechanisms at play. Notice the upwelling of densirt surfaces in panel b) as well as the development of frontogenesis in panel c).
  • Given these results, we can interpretate the short events of deep water intrusion seen in WHOTS time serie as the submesoscale and mesoscale manifestation of the restratification of the water column after the wind has mixed vertically the mixed layer. The growth rate discussed in Boccaletti et al. (2007) appears to be larger than those observed in WHOTS but that might be due to different initial value of the stratification –in the WHOTS data, the stratification is not too weak and the resulting growth rate is much smaller.
  • Compute KE and PE and see if there is a transfer between the two?
  • Look at the skewness of the vorticity (and compare with their Fig. 16)?
  • “The ubiquitous skewness in vorticity at the submesoscale supports the idea that MLIs are a common feature in the surface ML. Notice however that frontogenesis and cyclonic shears can also be generated through wind-driven flows and mesoscale strain (Hoskins and Bretherton 1972). Further analysis is needed to univocally identify MLIs in observations.”