10.3 Potential vorticity dynamics of submesoscale flows in the ocean

Thursday, 11 June 2009: 11:00 AM
Pinnacle BC (Stoweflake Resort and Confernce Center)
Leif N. Thomas, Stanford Univ., Stanford, CA

An overview of theoretical and modeling efforts aimed at understanding the potential vorticity (PV) dynamics of submesoscale flows will be given in this presentation, which will highlight the efficacy of submesoscale processes to cause frictional PV change and advective transfer of PV between the surface mixed layer and the ocean interior. Friction extracts/injects PV out of/into the ocean via a frictional PV flux equal to the cross product of the horizontal buoyancy gradient with the frictional force at the sea surface. Submesoscale flows are characterized by intense lateral density gradients and hence when exposed to frictional forces they drive strong frictional PV fluxes, making them efficient at changing the PV of surface waters. Frictional forces can be caused by an externally imposed wind-stress or by the spin-down of the submesoscale flows themselves. Friction induced during spin-down always injects PV into the surface ocean, while wind-driven frictional forces will decrease or increase the PV in the mixed layer depending on whether the wind-stress has a down or up-front component (i.e. directed with or against the geostrophic shear of the submesoscale flow), respectively. Down-front winds are effective at generating negative PV on the submesoscale, resulting in symmetrically unstable flows. The instabilities that develop under these conditions act as a PV pump, upwelling high PV water from the pycnocline while subducting low PV surface water. The positive correlation between the velocity and PV fields results in an upward eddy PV flux that under certain conditions scales with the frictional PV flux associated with the down-front wind. This PV pump mechanism tends to increase the PV in the surface boundary layer and constrains the extreme low values of the PV from dropping below zero at the expense of reducing the PV in the pycnocline. When the wind alternates orientation between down-front and up-front orientations, PV is both removed from and injected into the fluid. The PV pump mechanism limits the reduction of PV by down-front winds but not the enhancement of PV by up-front winds, causing the skewness of the isopycnal PV distribution to become positive. This asymmetry in PV injection versus removal by frictional processes is further enhanced by the fact that stratification generated by up-front winds confines the turbulent stress to a depth shallower than the Ekman layer, which amplifies the frictional force and hence magnitude of the PV flux relative to that for down-front winds. The implications of the PV dynamics of submesoscale flows on the large scale circulation will be discussed, specifically focusing on the role of frictional removal of PV by down-front winds on the formation of mode waters.
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