Influences of a Rough Bottom Topography on Flow Kinematics in an Eddy-Resolving Circulation Model

Abstract

The effects of a rough topography are investigated in a primitive equation, eddy-resolving circulation model of an idealized ocean basin. The topography is chosen as a random field with an isotropic spectrum, specified according to observed abyssal hill topographies. The interactions of the deep current fluctuations with the synoptic-scale irregularities of the ocean floor enhance the baroclinicity of the eddy field; whereas a strong tendency toward barotropization is revealed in a flat-bottom solution, the topographic influence leads to a substantial decrease of eddy kinetic energy below the thermocline and a much more depth-dependent structure, especially in areas of weaker flow intensity. Energy budgets indicate that the adjustment after the introduction of the bottom roughness is dominated by a strong reduction of energy in the external mode. While eddy energy in the thermocline is not significantly altered in the new equilibrium state, energy in the deeper layers is scrambled into smaller, topographic scales and effectively removed by lateral friction. The velocity fluctuations in the thermocline exhibit a tendency toward phase-coherent vortices even in the interior, eastern portion of the gyre. Whereas eddies lose their identity after a few months in the flat bottom case, the presence of topography acts as a stabilizing factor; energetic, preferentially anticyclonic eddies show lifetimes of more than 1.5 years. The scale and propagation characteristics suggest a dynamical identification of these ringlike structures with the vortices of the ?intermediate?geostrophic? (IG) regime. Whereas the low-frequency variability in the flat bottom case is characterized by zonally oriented bands in the external mode, this structure disappears with the introduction of topography. A Lagrangian analysis shows that particle dispersion becomes almost isotropic below the thermocline; in the upper layers a preference of zonal diffusivity remains.