Erosion of a C alifornia U ndercurrent eddy by bottom topography

Subsurface eddies are ubiquitous features in eastern boundary current systems. These phenomena tend to modulate the across‐shore distribution of heat and biogeochemical tracers. A California Undercurrent eddy was detected by shipboard observations in October 2009 off the northern Baja California continental slope. The spatiotemporal variation in the California Undercurrent eddy is investigated by using a mesoscale‐resolving hindcast ocean simulation. A poleward coastal current that is driven by an upwelling‐wind relaxation event and the coastline geometry instigated the separation of the California Undercurrent from the slope, forming a meander‐like structure, which evolved as a mesoscale eddy‐like structure. The latter structure evolved as a subsurface eddy with a warm anomaly core, a distinctive feature of eddies that form from the California Undercurrent. During the initial stage, the subsurface eddy presented a cone‐shape form, with the maximum amplitude of the relative vorticity in the upper section. The inviscid effect of the irregular bottom topography altered both the initial direction of propagation and the initial eddy shape: the propagation direction of the eddy changed from north‐south to southwest, and the shape changed from a cone shape to a subsurface lens shape. The strong eddy‐topography interactions triggered a sign change in the heat flux direction from the environment toward the eddy and vice versa through the horizontally divergent component of the velocity field, which accelerated the eddy's decay. This study shows the relevance of the synoptic wind stress events and the irregularity of the bottom topography on the mesoscale eddy activity in the southern portion of the California Current.