Seasonal dynamics of the surface circulation in the Southern California Current System

Abstract

Abstract The seasonal dynamics of the Southern California Current (SCC) is investigated using a primitive equation ocean model with real coastlines and topography. The model is tested with different wind forcing, and the resulting flow fields are compared to the mean and seasonal circulation inferred from long-term in situ observations (California Cooperative Oceanic Fisheries Investigation (CalCOFI)). The model integration forced with the output winds of a regional atmospheric model (RSM) best captures the statistics of the observed circulation, with a 0.9 correlation coefficient for the streamlines and 0.5 for the velocity fields. The model integrations reveal a pronounced linear response of the flow field to changes in winds on the shelf region. A dynamical feature inferred from CalCOFI hydrography, also suggested in TOPEX/ERS maps, is an annually recurrent westward propagation of SSH anomalies originated in the Southern California Bight (SCB) during the upwelling season. The RSM integration is the only one to capture the correct timing and spatial evolution of this process. We therefore use this model integration for guidance in constructing a dynamical framework to interpret the observed circulation and its variability. During the upwelling season in spring, there is an upward tilt of the isopycnals along the coast directly forced by the winds in the Bight. As the spring transitions to the summer the upwelling winds relax in the Bight but are still strong in the region offshore, approximately in correspondence of the continental slope (positive wind-stress curl condition). Anomalous denser waters in the location of the Southern California Eddy are maintained and reinforced by the combined interaction of the coastal/islands geometry and the wind-stress curl (through Ekman dynamics). The adjustment process to the denser water initiates a westward propagation of ocean density anomaly through Rossby waves, and reinforces the cyclonic gyre-like circulation of the SCE (increasing positive vorticity). Surface poleward flow, maintained by the positive wind-stress curl, is also reinforced in proximity of Point Conception as a consequence of the adjustment. During the summer the cyclonic gyre becomes increasingly unstable as the core of the ocean anomalies crosses the continental slope. Instability processes within the cyclonic region, characterized by a sharp increase in EKE, shed eddies that leave the region either drifting to the west or interacting with existing eddies in the region offshore. The EKE reaches a seasonal maximum at the end the summer in the cyclonic region, and in late fall further offshore where the eddies are fully developed. The shedding of eddies cannot be directly seen in the CalCOFI observations because of the sampling aliasing. For this point we rely on the strong suggestion of the model, which we assume is able to capture the leading order dynamics. Additional integrations with a linearized version of the model are also presented to reinforce our interpretation of the westward propagation of the isopycnal anomalous displacement associated with Rossby wave dynamics.