Effects of Mean Flow Direction on Energy, Isotropy, and Coherence of Baroclinically Unstable Beta-Plane Geostrophic Turbulence

Abstract

The effects of mean flow direction on statistically steady, baroclinically unstable, beta-plane quasigeostrophic (QG) turbulence are examined in a two-layer numerical model. The turbulence is forced by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated by bottom Ekman friction. The model is meant to be an idealization of the midocean eddy field, which generally has kinetic energies larger than the mean and is isotropic. Energetic eddies can be generated even when planetary beta (?) dominates gradients of mean potential vorticity (PV; also, q), as long as the mean shear has a substantial meridional component. However, eddies are isotropic only when the angle between layer mean PV gradients exceeds approximately 90°. This occurs when planetary and shear-induced gradients are comparable. Maps of PV indicate that these gradients may indeed be comparable over much of the midocean. Coherent jets form when the mean flow has a substantial meridional component and ? is large. When ? is nonzero, but small enough to permit isotropy, and the zonal component of the mean flow is not strongly eastward, lattices of like-signed coherent vortices develop. Like-signed vortex formation from initial and forcing conditions without a vorticity preference has not been observed before in QG systems. The vortex arrays are sensitive to the details of small-scale dissipation. Both cyclonic and anticyclonic fields arise in the simulations, depending on initial conditions, but they have different energies, consistent with broken symmetries in the governing equations.