Research on a California undercurrent eddy by seismic oceanography method

The California Undercurrent Eddies (Cuddies) can carry heat, salt, and sediments to the interior of the eastern Pacific Ocean, which is an important mechanism for the California Current System to transport material and energy offshore. A subsurface submesoscale anticyclonic eddy was observed based on the multi-channel seismic data from cruise EW9415 in the Pacific Ocean off the southern coast of California, USA, from October 16 to 19, 1994. The overall reflection structure of the eddy is about 20 ~ 25 km in width, 240 ~ 300 m in thickness, and 240 ~ 540 m in depth. There is a weak reflection core area in the reflection structure, with a width of about 8 ~ 12 km and a depth of about 400 ~ 430 m. We interpreted it as a subsurface submesoscale anticyclonic eddy combined with a reanalysis of hydrological data. The reflection events on both sides of the eddy core are inclined in a opposite direction. The eddy core is slightly upturned on the nearshore side, where the reflection events are strong because of the limitation of the adjacent steep terrain. On the offshore side, the eddy core is nearly horizontal, the reflection event is weak, and the range of the high amplitude reflection area is significantly smaller than that of the nearshore side. The vertical asymmetry of the reflection structure of the eddy core indicates that the irregular topography and frictional torque affect the formation and development of the eddy jointly. The reflection seismic image of the cuddy is given for the first time by the seismic oceanography method in this work. The same eddy was captured on the repetitive observation section at a 20-hour interval. During the observation period, the eddy has a horizontal displacement along the offshore direction of the survey line, and the apparent translational speed is 0.042 m·s−1. With further evolution, the eddy core changed from convex lenticular to nearly oval, the inclination of the reflection events in the strong reflection area becomes smaller, and the thickness increases. Dislocations of small events on the nearshore side were merged, with continuity enhanced. Some events on the offshore side are weakened or even disappeared. The morphological changes of the eddy could be caused by a certain degree of differences in the cross sections at different angles, and the further relaxation of its own morphology.