The Mean Across‐Shore Coastal Upwelling Cell in the Southern California Current System

Abstract Equatorward winds off California transport coastal surface waters offshore, which are replaced by deeper onshore flow that upwells. The mean upwelling cell is rarely measured directly because its magnitude is small compared to other variability. From 2019 to 2024 on an alongshore line in the California Current System, gliders measured the across‐shore currents offshore of the Southern California Bight. Acoustic Doppler Current Profilers provide the total current. Thermal‐wind shear from the alongshore density gradient is referenced to the glider‐measured, depth‐mean velocity and provides the absolute across‐shore geostrophic current. The directly wind‐driven across‐shore velocity is the difference between the total and geostrophic velocities. The onshore geostrophic and offshore wind‐driven components are both vertically sheared, which highlights the importance of measuring the alongshore density gradient. The competition between these flows, with wind‐driving stronger at the surface, and geostrophy extending deeper, produces the overturning cell and determines the source depth. The overturning cell exists above 130 m: offshore flow reaches 2.4 cm above 30 m, while onshore flow reaches 0.7 cm at 60 m with depth‐varying errors of 0.2–0.5 cm . The vertical velocity peaks at 27 3 cm at 30 m and is within error of zero at 130 m. The mean offshore heat flux provides cooling comparable to the warming by the net surface heat flux. The mean offshore oxygen flux suggests oxygen is added inshore. These flux measurements constrain heat and oxygen budgets in the Southern California Bight.