Dr. Yackar Mauzole
Abstract: The California Current System (CCS) is one of the four major Eastern Boundary Current Systems, where major coastal upwelling is observed in the ocean. Combined satellite observations (altimetry and sea surface temperature -SST) of the CCS however highlight the presence of SST fronts in the southern part of the CCS outside of the coastal upwelling season. The well-known submesoscale-permitting, and tidal-resolving LLC4320-simulation was used to investigate the nature of these features, through the analysis of the density gradients, the vertical velocities and the horizontal strain. An algorithm was also developed to track SST fronts in the model, and the frontogenesis mechanisms present in the CCS were diagnosed through the omega equation. The classical scenario of coastal upwelling was confirmed in the central CCS (off California), with a strain field correlated to vertical velocities associated to the SST fronts, and collocated density fronts. In contrast, in the southern CCS (off Baja California), vertical velocities were much weaker and density gradients were not always observed where SST fronts were detected, leading to the assumption that the strain field related to coastal upwelling was not the main mechanism responsible for the generation of fronts. In fact, vertical mixing of momentum was revealed to be an important contributor to frontogenesis, since vertical velocities due to vertical mixing counterbalance the vertical velocities due to kinematic deformation. These findings demonstrate the existence of SST fronts in the CCS outside of the traditional coastal upwelling framework, with the fronts being passive and advected by the background flow.