Offshore Propagation of Eddy Kinetic Energy in the California Current

Low-pass filtered velocities obtained from surface drifters, and surface geostrophic velocities estimated from TOPEX/POSEIDON altimeter data, have recently revealed a clear and robust seasonal cycle in the surface eddy kinetic energy (EKE) in the California Current (CC). The seasonal cycle begins in spring when a surface intensified baroclinic equatorward jet develops next to the coast in response to strong upwelling favorable winds. This jet, and a developing eddy field, then moves offshore during summer and fall. The EKE maximum associated with the jet progresses only as far as 127W, beyond which it decreases rapidly. This is a robust characteristic of the seasonal cycle that has been previously attributed only to an unspecified dissipation process. To investigate this aspect of the surface EKE, a multi-year simulation of the CC is carried out using the DieCAST regional ocean model. The simulation accurately reproduces many aspects of the observed annual cycle, including the offshore propagation of the EKE at the surface. The model results indicate that the decrease of surface EKE west of 127W in the simulation is not due to dissipation, but rather it is caused by the vertical redistribution of EKE to the deep ocean. This redistribution occurs through the transformation of kinetic energy from the vertical shear flow to the vertical mean flow. The transformation is a nonlinear process that is inherently associated with baroclinically unstable waves. It was previously shown to be important in the atmosphere by Wiin-Nielsen (MWR 1962). In the CC, the transformation effectively energizes the deep ocean at the expense of the upper ocean. Taken together, the recent California Current observations and the new model results strongly suggest that the California Current regularly supplies eddy kinetic energy to the deep waters of the Eastern North Pacific.