The propagating response of coastal circulation due to wind relaxations along the central California coast

Abstract

[1] Following relaxations of prevailing upwelling‐favorable winds, warm waters from the Santa Barbara Channel propagate poleward around Point Conception and along the south central California coast. We examined characteristics of these relaxation flows, including frontal propagation speed and temperature changes during the warm water arrivals, by using multiyear time series of currents and temperatures from four moorings along the ∼15 m isobath, surface current observations from high‐frequency radars, and satellite sea surface temperature images. Propagation speeds of the warm fronts relative to ambient waters ranged from 0.04 to 0.46 m s −1 . As the fronts arrived at the moorings, temperature increases ranged from 0.7°C to 4.2°C. In ensemble averages over many frontal arrivals, alongshore flow speeds increased by 0.1–0.2 m s −1 over the water column during arrivals. Cross‐shore flows were onshore near the surface and offshore near the bottom with speeds of 0.02–0.05 m s −1 . This cross‐shore flow structure persisted as temperature increased during arrivals and ceased when temperatures stopped increasing. Frontal propagation speeds were correlated with temperature increases at the moorings, consistent with forcing by baroclinic pressure gradients. Compared to other buoyant flows such as from the Chesapeake Bay where density contrasts with ambient waters are 2– 3k g m −3 , these relaxation flows are less buoyant with density contrasts of 0.1–0.9 kg m −3 . Consequently, the propagation of these flows is more affected by bottom friction and the speeds are closer to the “slope‐controlled” or “bottom‐advected” limit described in theoretical and laboratory work but not well studied in the ocean.