A dual-tracer approach to estimate upwelling velocity in coastal Southern California

•Using 7Be and 234Th, we estimate upwelling velocities to be 0.5±0.6 to 2.5±1.3md−1.•These agree to within uncertainty with the pressure field-based Bakun Index.•A heat budget and hydrography suggest that upwelling is driven by wind stress curl.

The distribution of the cosmogenic radionuclide 7Be (t1/2=53d) in the surface ocean has previously been used to estimate upwelling velocity in the open ocean. However, the loss of 7Be to particle export has limited this approach in high particle density environments like the continental margins. In this study, we combine a mass balance of 7Be with a 234Th budget in the surface ocean to constrain the loss of 7Be to particle sinking at the San Pedro Ocean Time-series (SPOT) in the inner Southern California Bight during spring 2013. Upwelling velocities (all in m d−1) determined from the 7Be mass balance were observed to increase from 0.5±0.6 in January to 2.5±1.3 in May, then decrease to 1.2±0.5 in June. These results agree within uncertainty with upwelling velocities derived from the monthly Bakun Upwelling Index, which ranged from 0.1 to 2.8 m d−1, supporting the pressure-field-based approach. Evidence from a heat budget and the nutrient distribution over the course of the study supports that the upwelling signal at SPOT (20 km offshore) is not transported from coastal upwelling near shore, but instead is dominantly a local signal, likely driven by wind-stress curl.