Circulation and Energetics of a Model of the California Current System

Abstract

Data from an eight-layer FNOC wind-driven eddy-resolving limited-area quasi-geostrophic numerical model have been used to study the dynamics of the California Current System (CCS). with particular emphasis on the energetics. The model includes quasi-realistic bottom topography and the true coastline, and is embedded in a coarser resolution model occupying almost all of the North Pacific Ocean. The model simulates the main components of the CCS, such as the California Current (CC), the Davidson Current, the California Undercurrent, and the Southern California Eddy. The simulated CC is both an active eddy-mean flow interaction area and an effective wave source for the North Pacific Ocean. Making use of the mean potential vorticity field of the different layers, four different regimes are described for the mean circulation: ?inertial?, ?intermediate?, ?beta?, and ?bottom topographic?. Overall, the new-surface circulation is energetically dominated by the mean flow whereas the deep simulation is energetically dominated by the eddy field with a relatively weak mean flow, i.e., a turbulent Sverdrup balance. Examination of the energy balance in certain limited areas indicates that energy transmitted by the wind to the mean flow is both fluxed by the ? effect through the open boundaries and transformed into available potential energy and then by baroclinic instability processes into highly energetic eddies. The latter path mainly occurs above the second layer; below it, energy is mainly radiated vertically by the action of eddies. In the core of the model California Current the downgradient eddy fluxes of temperature are of less importance in the production of eddy kinetic energy than the advection of available eddy potential energy. In the upper layer the enstrophy field drives the eddy flux of potential vorticity (V?Q?); in the remaining layer the turbulence is weaker and the flux is controlled by the mean potential vorticity geometry. The mesoscale variability is characterized by periods of order 100 days, and wavelengths of order 200 km. North of about 34°N baroclinic instability seems able to explain the observed long-period variability (>200 days). Long quasi-linear first baroclinic mode annual Rossby waves are found between 25° and 33°N. The bottom topography plays a very important role in the distribution of different deep fields, such as circulation, energy, and enstrophy, and in the qualitative and quantitative determination of the vertically integrated mass transport.