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

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 an eddy pair dominated the circulation. In this study, the water mass, kinematic, and dynamics characteristics of the anticyclone and cyclone eddy were analyzed. The anticyclone eddy had neither surface thermal expression nor sea level anomaly signature. It was a subthermocline eddy. In contrast, the cyclone eddy exhibited a strong sea level 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 anticyclone eddy was similar to the water mass of the California Undercurrent (warm, salty and with low oxygen concentration) and the water mass of the core of the cyclone eddy is similar to the water mass of the California Current (cold, fresh and with high oxygen concentration). The radius of the anticyclone was 27 km and the radius of the cyclone was 32 km. The Burger and the Rossby numbers were larger for the anticyclone eddy than for the cyclone. The oceanographic vessel bisected both eddies which enabled us to analyze cross-sections of potential energy, kinetic energy, relative vorticity, and potential vorticity of each eddy.