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“Impact of submesoscales on mesoscale eddy turbulence”, a presentation by P. Klein (12/08/09)

  • submesoscales are energetic in surface layers
  • meso and submesoscales are geostrophically balanced and verify the SQG properties
  • ageostrophic motions have dramatic impact on ocean properties
  • strong phase relationship between dynamics @ surface and dynamics @ depth: the submesoscale near the surface transfers energy to the mesoscale via inverse cascade, which then influences the flow over a larger portion of the water column
  • Importance of gas exchanges between eddies according to Barry Huebert and Ming. To follow.
  • The story goes like this. The large-scale (meso) flow stretches and strains the water due to interacting eddies creating small-scale (submeso) filaments. The resulting imbalance from geostrophy results in frontogenesis that re-established the geostrophic balance via ageostrophic motions involving, inter alia, vertical motions along isopycnals.

Questions

  • Are the two regimes of turbulence (surface, dominated by cyclones and at depth, dominated by anticyclones) coupled? via submesoscale?
  • present SST resolution not enough to deduce the vertical velocity via SQG theory? yes only after a wind burst so that the SST is a good signature of the horizontal temperature distribution bewlo the mixed layer.
  • no clear limit between submesoscale and mesoscale (that is between 30 and 100 km)?
  • Is the vertical velocity diapycnal or not? No. The vertical velocity is along isopycnals that can be slanted with the horizontal. Also be aware that the contours of constant density do not follow the contours of constant nutrients so that even an along-isopycnal velocity can trigger change in the nutrient profile.
  • Is the initial formation of a filament –the unbalanced state– not as important for vertical tracer flux? No. The vertical ageostrophic motion appears only to re-established the thermal wind balance. The initial destabilization from geostrophy is due to the horizontal stretching of filaments by the mesoscale flow.
  • We can also compute the vertical velocity from the mesoscale flow alone (itself deduced from SSH). According to Patrice, this will not give the submesoscale structure of the upwelling/downwelling but it should give you the location where such structure appears.