Fine-scale temperature structure in the bottom mixed layer on the Oregon shelf

Abstract

Temperature and temperature-gradient records made as a freely-falling probe hit bottom show an extremely well-mixed layer on the Oregon continental shelf in September, 1974. Temperatures were uniform to within a few millidegrees in a 10-m deep layer. A reverse gradient of one to four times the adiabatic was found in the last 5 m or so, larger than the geothermal flux could maintain. The layer depth is similar to that of a turbulent Ekman layer. On vertical temperature-gradient spectra several subranges are seen. Estimates of eddy diffusivity by the Cox-Osborn-Hacker method agree within a factor of two with estimates calculated from the level of an assumed inertial sub-range in temperature gradient spectra. Possibly the layer is stabilized by a salinity gradient too small to be observed by the conductivity-temperature-depth probe, or by sediment load. Calculations of heat flux caused by the thermal storage of the sediments yield a flux of the correct order, but the opposite sign, of the calculated flux.