Physical Drivers of Surface Water Retention in the Santa Barbara Channel

Abstract Retention of surface water parcels can benefit coastal ecosystems by increasing the residence times of nutrient‐rich waters and marine larvae but can also negatively impact marine life and human health by concentrating oil and other pollutants. To investigate the spatial patterns, temporal variability, and drivers of retention, surface water parcel retention is quantified using particle simulations forced by high‐frequency radar surface current observations in the Santa Barbara Channel (SBC), California, USA. Retention is defined here as the time a particle remains within 20 km of its starting location and typical retention times are ∼4 days. However, the mechanisms driving retention differ across the SBC. In the central SBC, high retention times are driven by a persistent cyclonic eddy, while in the eastern SBC, high retention times are due to weak oscillatory flow. Areas in the western SBC outside of the cyclonic eddy exhibit the shortest mean retention times (<2 days) due to sustained horizontal advection. Stepwise regression is used to assess the drivers of interannual retention anomalies at sub‐SBC scales and is able to predict the greatest (least) amount of retention variability in the western (eastern) SBC. The presence of coherent eddies, both cyclonic and anticyclonic, is a dominant driver of high retention channel‐wide, while some factors such as wind stress and along‐channel flow have counteracting effects on retention at sub‐channel scales.