Modeling Bottom Mixed Layer Variability on the Mid-Oregon Shelf during Summer Upwelling

This research was supported bythe Office of Naval Research Ocean Modeling and PredictionProgram under Grant N00014-98-1-0043, theNational Science Foundation (NSF) Coastal OceanProcesses program under Grant OCE-9907854, and theNSF GLOBEC Program under Grant OCE-0000733.

Results from a model of wind-driven circulation are analyzed to study spatial and temporal variability inthe bottom mixed layer (BML) on the mid-Oregon shelf in summer 2001. The model assimilates acousticDoppler profiler velocities from two cross-shore lines of moorings 90 km apart to provide improvedaccuracy of near-bottom velocities and turbulence variables in the area between the mooring lines. Modelresults suggest that the response of the BML thickness to upwelling- and downwelling-favorable windsdiffers qualitatively between an area of “simple” bathymetric slope at 45°N and a wider shelf area east ofStonewall Bank (44.5°N). At 45°N, the BML grows in response to downwelling-favorable conditions, inagreement with known theories. East of Stonewall Bank, the BML thickness is increased following upwellingevents. In this area, the southward upwelling jet detaches from the coast and flows over a wider partof the Oregon shelf, creating conditions for Ekman pumping near the bottom. Based on computations ofbottom stress curl, the vertical pumping velocity in this area may reach 15 m day⁻¹ following periods ofintensified upwelling-favorable winds. A column of denser, near-bottom water upwelled over the Ekmanflow convergence area is tilted as a result of vertical shear in horizontal velocities, causing unstable stratificationand convective overturning. As a result of this process, BML thickness values east of StonewallBank can be in excess of 20 m following upwelling, comparable to maximum values at 45°N followingdownwelling.