WHOI PO

Title: Mesoscale flow over rough topography

Abstract: The rough ocean bottom has important effects on mesoscale currents, eddies and waves. We study topographic effects on surface vortices and on the stability of surface-intensified currents and planetary Rossby waves.

We find that persistent anticyclones locked to topographic depressions such as the Lofoten, Mann and Rockall Trough eddies can be generated by merger asymmetry in a two-layer quasigeostrophic system with topography. A minimum enstrophy theory predicts a cyclonic, bottom-intensified, isobath-following circulation over a topographic depression, within which deformation-scale anticyclones merge and cyclones are strained out at intermediate energy levels.Topography also affects the evolution of decaying quasigeostrophic turbulence and propagating planetary Rossby waves.

We also study the linear stability properties of surface-intensified, laterally-sheared currents, like the Gulf Stream and the ACC jets. Rough topography facilitates mesoscale energy scattering at depth, and alters both the structure of the most unstable mode and the energetics of mixed baroclinic-barotropic instability. Altimetry and shipboard observations show that these surface jets tend to be asymmetrical, with a narrow (wide) lobe collocated with the shallow (deep) pycnocline side, and half-widths proportional to the local first surface deformation radius. This observed deformation-scaleasymmetry is predicted by simple layered geostrophic adjustment models with a PV jump, suggesting that the jets' scales are set locally by eddy stirring of PV on either side.