Formation of a subsurface anticyclonic eddy from the California Undercurrent off northwestern Baja California, Mexico

Abstract

Hydrographic observations off northwestern Baja California in March 2017 revealed the presence of a subsurface anticyclonic eddy in a cross-shelf transect at $\sim 32°$N. This eddy’s width was roughly 30 km and it was located about 15 km from the shelf, at depths between 100 m and 400 m. The eddy’s core had hydrographic characteristics consistent with those found in the California Undercurrent (CUC), which suggests that the eddy was originated in the CUC. Numerical-modeling simulations were used to elucidate the origin and evolution of the observed eddy. These simulations also showed a subsurface eddy which crossed the cross-shelf transect’s location with size, depth, and hydrographic characteristics similar to those of the observed eddy (but formed in November 2007, in a simulation for the 2004–2011 period). Thus, this numerical eddy is considered as analogous to the observed one, hence the life cycle is assumed to be the same in both cases. The eddy formation occurred roughly 40 km southeast of the cross-shelf transect, in a zone characterized by a submarine point, where the isobaths change their orientation abruptly and where an enhanced eddy-energy production occurs, which shows that the irregular bottom-topography is a factor that triggered such an eddy formation. The eddy formed in a period of weak along-shelf velocities, 4-10 days after an event of the most intense velocities of the year, associated with the passage of a wind-driven coastal trapped wave generated in remote locations south of the study area. The flow disturbances forced by this wave could be an additional physical factor which predisposes the water column to instability. However, at least in the cases herein analyzed, the eddy separation required an additional momentum/vorticity input by an adjacent mesoscale cyclonic gyre. Comparison with numerical and laboratory–experimental results support the notion that a baroclinic instability process is involved in the eddy generation. The path followed by the subsurface eddy described herein shows a link of the hydrographic characteristics between northwestern Baja California and the Southern California Bight.