Temperature finestructure and microstructure observations in a coastal upwelling region during a period of variable winds (Oregon, summer 1974)

Abstract

Profiles of temperature and temperature gradient made during two 4-day cruises with a freely-falling probe, in conjunction with conductivity-temperature-depth (CTD) and current meter records, are used to examine a 100-m water column on the Oregon shelf for evidence of finestructure-microstructure correlation, shear-induced mixing, and double-diffusive convecting layers. The rate of dissipation of temperature fluctuations, ε ν , is founnd to be as large as 2 × 10 −5 °C 2 s −1 in a well-stirred surface layer, and as low as 7 × 10 −11 °C 2 s −1 in a region undisturbed by intrusions or boundary stirring. For the surface mixed layer, a decade-long, + 1 slope was observed in the temperature gradient spectrum for vertical wavelengths greater than about 5 mm. If this is assumed to be a viscous-convective subrange, the rate of dissipation of mechanical energy, ε, is calculated to be 0.06 cm 2 s −3 . Transient patches of intense microstructure, 1 to 2 m in extent, can occur in the thermocline. Variations in temperature variance of 10 3 can occur over small vertical separations in several intrusions, but when normalized by the local finestructure gradient (yielding the Cox number) the range is considerably less. A 4-m thick ‘hole’ in the microstructure appears in a shallow, cold intrusion. A warm intrusion has a complicated, time-dependent vertical structure and a low Cox number (less than 10). An interleaving at about 60 m produces a 0.10°C, 10-m thick inversion with many small-scale features indicative of overturning; the Cox number is about 600 in this case. Although all the temperature inversions have T-S gradients favorable for double-diffusive convection, no systems of layers are found. In general, the results are similar to those from other observations of oceanic microstructure associated with the different types of finestructure.