Systematic microstructure variability in double‐diffusively stable coastal waters of nonuniform density gradient

Abstract

Conductivity microstructure, water velocity, and stratification were measured during a tow‐yo transect near the New England shelf/slope front in early August 1997. Velocity data were collected with an acoustic Doppler profiler on the ship. The other data were collected with a towed platform. Estimates of χ, the rate of dissipation of temperature variance, were computed from the conductivity data with vertical resolution of 0.3 m. Relationships between χ and shear, temperature gradient, buoyancy frequency (N), and gradient Richardson number (Ri) were explored, with special focus on measurements taken in waters stable to double‐diffusive processes (to avoid ambiguity of interpretation) and exhibiting variable density gradient (N ranging from 5 to 40 cph). For this subset of data, χ computed from data grouped into classes of local mean temperature gradient (/dz) was proportional to /dz to the 0.7 power, which is consistent with diapycnal thermal eddy diffusivity K being proportional to (/dz)−1.3 within the framework of the Osborn‐Cox model that relates χ and the mean temperature gradient to the heat flux. No correlation between K and Ri was observed, with Ri computed at 4 m vertical scale, so that systematic inhomogeneous large‐scale forcing is not responsible for a false correlation of K and /dz. Water mass salinity characteristics in the area caused N2 to be proportional to (/dz)4/5, rather than to /dz as in the isohaline case, giving rise to the steep inverse relation K = 10−10N−3.3, with N in radians/s and diffusivity in m2/s. The fit K = 2 × 10−9N−2.5 results if one questionable data ensemble is disregarded. These relations are comparable to results obtained previously from the near‐bottom tow data. They are not intended to be universal formulae but are meant to describe the conditions we encountered. They are not expected to hold at high and low values of N outside of our measurement range. An interpretation is that under these conditions the less strongly stratified (lower N) layers in this shelf area are more prone to instability of the larger‐scale shear than the intervening interfaces, with the subsequent greater energy dissipation in the layers leading to higher buoyancy flux KN2 in the layers than in the interfaces.