Submesoscale Eddies in the Southern California Bight Derived from a Decade of High Frequency Radar Observations

Submesoscale eddies form an important component of the circulation of the Southern California Bight (SCB), greatly impacting ecological processes. Despite their acknowledged significance in influencing ocean physics and biology, submesoscale eddies have been exceptionally hard to observe because of the technical challenges posed by both field and remote platforms. Here using a decade of high-frequency radar (HFR) surface current data we address this challenge for the SCB. Over the ten years of data, our research has mapped out the spatial distribution of submesoscale eddies and provided their seasonal and inter-annual variations. Between 2012 and 2021, a total of 235229 eddies were detected, averaging 452 eddies per week. Of these, 56% were cyclonic and 44% were anticyclonic. The contribution is roughly equal if eddies through their life spans are counted as one occurrence. This is because cyclonic eddies lived longer. The spatial distribution of eddies exhibited strong topographically related heterogeneity. Spatially coherent eddies, which reoccurred in certain locations over time, formed hotspots of eddy activity, largely in association with headlands. However, there were hotspots that did not seem to be associated with any typographic feature. Eddy temporal variations were examined at seasonal and interannual scales. On seasonal scales, eddies were found to be more numerous in the summer and early fall than in the spring. In August, the number of eddies was the highest, with 55% more observed eddies than in April, the least active month. The strong equatorward flow in the springtime seems to be linked with the reduced eddy activity at this time, likely due to the flow's suppressing effect on vortices and instabilities. At interannual scales, the eddy activity substantially increased in association with the 2014-2015 Blob event and the 2015-2016 El Niño. Observed eddies rose by 38% in 2014 compared to 2013 and remained high in 2015 and 2016. The results of this study are useful for the validation of numerical modeling studies in the SCB and could be of interest to the biological community to evaluate links between ecosystems and submesoscale activity along the highly productive coasts of the SCB.