Topographic Forcing of Coastal Mesoscale Phenomena: Filamentation, Vortex Formation, and Eddy Detachment

Abstract

Using the methodology of contour dynamics on a quasigeostrophic model, the nonlinear evolution of a coastal potential vorticity front over a Gaussian topographic feature in the presence of an overlying linearly stable basic flow is investigated. The simulations show that increasing the amplitude of the forcing leads to four different qualitative regimes: 1) small amplitude wavelike disturbances are formed, 2) a primary (trapped) disturbance breaks and forms filaments, 3) a secondary (moving) disturbance breaks and forms filaments, and 4) the primary filament winds around the topographic feature until an eddy with considerable internal mixing finally detaches. Other parameters such as the topographic width, the position of the undisturbed front relative to the topography, and the potential vorticity in the ocean region are also shown to be important in controlling (either enhancing or inhibiting) the process of filamentation and vortex formation. The main conclusion is that nonlinear dynamics alone may be responsible for the formation of meanders and eddies without the necessary presence of instabilities in the basic flow.