The propagating response of inner shelf circulation to wind relaxations along the central California Coast

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Libe Washburn
Chris Melton
Melanie Fewings
Chris Gotschalk

Following relaxations prevailing equatorward winds, warm water from the Santa Barbara Channel flows poleward around Point Conception and along the central California coast. Sequences of satellite sea surface temperature images show the events as bands of warm water extending up to 20 km offshore. Characteristics of these flows were examined using time series of currents and temperature from an array of moorings along the inner shelf (15 m depth), a mooring on the mid-shelf (100 m depth), and surface current observations from high frequency radars. As the warm fronts at the noses of the flows passed the moorings, temperature increases ranged from 1-4 °C and alongshore flow speeds increased by 0.1-0.2 m/s over the water column. Cross-shore flow structure was more complex as the fronts passed: in the upper half of the water column flow was onshore with speeds of about 0.05 m s-1 while in the lower half currents were offshore with similar speeds. This cross-shore flow structure persisted as temperature increased during the warm water arrivals and then ceased when temperature stopped increasing. Propagation speeds along the coast ranged from 0.04-0. 5 m/s and were correlated with temperature increases at the moorings, consistent with buoyancy forcing in the alongshore direction. Compared with buoyant flows such as from the Chesapeake Bay where density contrasts with ambient waters are 2-3 kg m^-3, these flows are less buoyant with density contrasts oft 0.1-0.9 kg m^-3. The ratio cw/ca of the internal wave speed cw to the “slope-controlled” speed ca ranged from 0.8 – 3 which is substantially larger than in the Chesapeake plumes where cw/ca ranges 0.15-0.35. Thus, these observations extend the generality of laboratory results (e.g. Lentz and Helfrich, 2002) to propagating buoyant flows closer to the slope-controlled limit. Similar flows have been observed in the Benguela and Iberian upwelling systems, and we hypothesize that they are common in upwelling systems world-wide.  We speculate that they are important transport mechanisms for connecting kelp forests in coastal upwelling systems such as the Santa Barbara Coastal LTER.