Towards a new interpretation of upper-ocean dynamics at meso and submesoscales using Surface Quasi-Geostrophy

Recent analysis of altimetric data (Scott andWang2005,LeTraon et al. 2007) and primitive equation simulations at very-high resolution produced on the Earth Simulator (Klein et al. 2007) have shown that surface and interior ocean dynamics behave quite di?erently with shallow kinetic energy spectrum(close to k -5/3) and large-scale energy transfer at the surface. However the standard arguments of baroclinic turbulence are not able to explain these features since they predict steeper kinetic energy spectra associated with small-scale energy transfer.
Surface oceanic layers are characterized by submesoscales (10km) density fronts and filaments with high values of relative enstrophy that may play a critical role for the surface dynamics. We propose that these structures are important for explaining the properties of turbulent baroclinic flows near the surface.
Using the theory of potential vorticity (PV) inversion, we show that surface density anomalies play a role as important as interior PV anomalies in terms of dynamics. Indeed, the mesoscale flow can be decomposed into a part forced by interior PV and a part forced by surface density. This latter corresponds to a surface-trapped mode that decays exponentially with depth and is known as the Surface QG (or SQG) model.
For turbulent flows forced by baroclinic instability, we show that the oceanic currents of the upper 500 meters are close to SQG balance. This result points toward a new interpretation of upper-ocean dynamics. A major output is the possibility to estimate upper-oceanic 3D currents from SST instantaneous snapshots.
We verify this interpretation with data coming from models with different degrees of realism and from comparison of satellite SSH and SST. We also describe some properties of the SQG model that may apply to the real ocean (such as kinetic energy transfers or vertical tracer fluxes).
References:
G. Lapeyre and P. Klein 2006. "Dynamics of the upper oceanic layers in terms of surface quasigeostrophy theory". Journal of Physical Oceanography, 36, 165-176.
P.Klein, B.L.Hua, G.Lapeyre, X.Capet, S LeGentil,and H.Sasaki,2007. "Upper ocean turbulence from high 3-D resolution simulations". Journal of Physical Oceanography, in revision.