SHEAR AND STRAIN IN SANTA-MONICA BASIN

By injecting a cloud of SF6 into Santa Monica Basin at a depth of 790 m and observing its vertical spread, Ledwell and Watson (1991) estimate the average diapycnal diffusivity between 750 and 850 m as K-rho = 2.5 x 10(-5) m2 s-1. This diffusivity is higher than inferred from microstructure measurements in the open-ocean thermocline for typical internal wave levels (Gregg and Sanford, 1988). To examine shear and strain in the basin, we dropped expendable current profilers (XCPs) and calculated strain from conductivity, temperature, and depth (CTD) casts. Shear and strain are above the GM76 model spectrum (Garrett and Munk, 1975; Cairns and Williams, 1976), and comparisons with published parameterizations indicate more than enough shear variance to account for the high diffusivity reported by Ledwell and Watson. Thus, the eddy diffusivity in Santa Monica Basin is not at odds with lower values estimated from microstructure measurements in the open ocean, but is due to an elevated internal wave field. The spectral shapes, however, differ from typical open-ocean spectra.