Eddy Formation and Interaction in a Baroclinic Frontal Geostrophic Model

Abstract

The authors investigate the behavior of buoyancy-driven coastal currents in a series of numerical experiments based on a two-layer frontal geostrophic model. The model focuses on baroclinic instability, allows for finite amplitude variations in the upper-layer thickness, and includes a topographic background vorticity gradient. Simulations of isolated fronts demonstrate meandering of the frontal outcropping, filamentation, and the development of both warm core and cold core eddies. Eddies can merge with each other, separate, or be reabsorbed into the current. Despite the assumption of only two layers, it is found that growth rates and length scales of the emergent features are in agreement with results of studies based on more sophisticated primitive equation models. It is determined that the cross-front topographic slope has a significant effect on the instability. In particular, a bottom that slopes in the same sense as the fluid interface hinders the growth of perturbations. Simulations with two outcroppings (i.e., coupled fronts) are also described. The authors found that such currents break up into distinct vortices that propagate very little but exhibit merging and splitting, behavior consistent with previous numerical studies involving similar models as well as laboratory experiments. Finally, an analytical nonlinear wave-packet stability theory for a marginally unstable flow with a simple linearly varying height profile is presented. The authors show that the unstable modes can saturate as solitons.