Shipboard observations of a mesoscale eddy pair in the California Current System off the northern Baja California coast

Abstract:

The transition zone of the California Current System is populated with mesoscale eddies. During October 2009 a high-resolution survey was carried out in the transition zone off the northern Baja California coast to investigate the role of the mesoscale features on the circulation. We found that mesoscale eddies dominated the circulation, an eddy pair in particular. In this study, the water mass characteristics of the anticyclonic and cyclonic eddies are analyzed. The anticyclonic eddy had neither surface thermal expression nor mapped sea surface height anomaly signature. It was a subthermocline eddy. In contrast, the cyclonic eddy had a mapped sea height anomaly signature. The mapping of depth, Conservative Temperature, Absolute Salinity, and oxygen concentration on the 26.6 isopycnal surface revealed that the water mass of the core of the anticyclonic eddy is similar to the water mass of the California Undercurrent (warm, saline and with low oxygen concentration) and the water mass of the core of the cyclonic eddy is similar to the water mass of the California Current (cold, fresh and with high oxygen concentration). Based on shipboard measurements, the kinematics and the dynamics of the eddy pair are also analyzed. The radius of the anticyclonic was 27 km and the radius of the cyclonic was 32 km. The cyclonic eddy was larger than the anticyclonic eddy. The oceanographic vessel bisected both eddies which enabled to analyze cross-sections of potential energy, kinetic energy, relative vorticity, and potential vorticity of each eddy.