Vertical heat fluxes through the Beaufort Sea thermohaline staircase

Abstract

Microstructure profiles of temperature, conductivity, and velocity shear during the Arctic Internal Wave Experiment (AIWEX) in March-April 1985 in the Beaufort Sea are used to investigate the thermodynamic processes in a diffusive thermohaline staircase. The staircase occurs between depths of about 320 and 430 m, above the core of the relatively warm, salty Atlantic water, where the mean temperature and salinity are increasing with depth. Individual isothermal layers can be tracked for at least several hours, suggesting a horizontal length scale of several hundred meters or more, assuming a typical relative velocity of 0.01 m s-x at this time. Over the depth range 320-430 m the mean (average over several steps) density ratio (R,)=/•(S:)/ot(T:) varies between 4 and 6, while the typical temperature difference between layers decreases from 0.012 ø to 0.004øC. The mean thickness of the layers also varies, from 1 m at 320 m depth to 2 m at 430 m. The relationship proposed by Kelley (1984), relating layer height to (N'•), (Rp), and molecular properties of the fluid, overestimates the mean layer thickness by about a factor of 2. The variability of staircase characteristics suggests that oceanic staircases may rarely, if ever, be steady state, but in general be slowly evolving from previous perturbations. Heat fluxes estimated from laboratory-based flux laws, involving Rp and AT, are in the range 0.02 < F n < 0.1 W m -'•, which is in agreement with the molecular heat fluxes through the maximum interfacial temperature gradients. There are no interfaces where the kinetic energy dissipation rate (averaged over 0.5 m) exceeds the lower limit for diapycnal mixing, 24.5vN 2.

The diffusive regime of double-diffusive convection

“…Some measure of confidence may be inferred from the fact that the observations compiled by Fedorov (1988), like those compiled by Kelley (1984), span a wide range of conditions. However, Figure 6 also illustrates observations made in the field study of Padman and Dillon (1987). These results are in systematic disagreement, by a factor of…”

“…As is the case for the Antarctic, Arctic staircase fluxes appear to be weak compared with mixing rates expected in the open ocean. For an example, Padman and Dillon (1987) estimated thermal fluxes to be just & % times larger than molecular fluxes. One might compare this with a factor of & % " % typically found for turbulent mixing in the open sea (Ledwell et al, 1993;Kelley and Van Scoy, 1999).…”

“…Such discrepencies limit the reliability of applications to the ocean to perhaps within a factor of 1 , for both heat and salt. in DL staircases in various ocean and lake regions, as reported by Kelley (1984), Fedorov (1988, and Padman and Dillon (1987). The curve is the prediction of a model of convectively-sheared DL interfaces (Kelley, 1984(Kelley, , 1988.…”

Simulations of a double-diffusive interface in the diffusive convection regime

“…Furthermore, the field observations of Padman & Dillon (1987) and Timmermans et al (2008) have shown a close agreement between F mol T and the F T predicted by different laboratorybased flux laws. However, we are not aware of any study that has definitively shown the fluxes to be molecular, with the exception of the experiments of Shirtcliffe (1973) using a salt-sugar interface (τ ≈ 0.33).…”

Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake

“…In fact, in deep and undisturbed lakes (such as Powell Lake), the layering is very pronounced, while in other systems staircases do not form probably because the environment is too turbulent (e.g., in the boundary region of Lake Kivu). Finally, statistical analyses on layer and interface dynamics can be performed to explore the variability of the process in different environments, where other interesting large data sets could be used such as the Arctic microstructure data [Padman and Dillon, 1987;Dillon and Padman, 1989].…”