Variability of the bottom mixed layer on the Oregon shelf

Abstract

Fine-scale measurements of temperature gradients near the sea floor on the Oregon shelf revealed rapid changes in the characteristics of the bottom mixed layer. The total thickness of the layer, the thickness of the reverse-gradient region near the bottom, and the magnitude of the reverse gradient all changed greatly within an hour-long data series. Systematic changes over a 5-day period were not much larger than these variations. Cross-shelf variations were also not greater than the 1-h changes at one station, except that in depths of 40 m and less the layer disappeared. Mixed layer thicknesses were comparable to the turbulent Ekman depth. The reversed gradient was slightly superadiabatic on average, with large variations about the mean. Microstructure activity decreased to a low level near the bottom. Vertical wavenumber spectra could not be interpreted in terms of a universal turbulent form.