Abstract
A series of large-scale numerical simulations is presented, which incorporate parameterizations of vertical mixing of temperature and salinity by double-diffusion and by small-scale turbulence. These simulations reveal the tendency of double-diffusion to constrain diapycnal volume transport, both upward and downward. For comparable values of mixing coefficients, the average diapycnal velocity in the double-diffusive thermocline is much less than in the corresponding turbulent regime. The insulating effect of double-diffusion is rationalized using two theoretical models. The first argument is based on the assumed vertical advective-diffusive balance. The second theory uses the Rhines and Young technique to evaluate the net diapycnal transport across regions bounded by closed streamlines at a given density surface. The numerical simulations and associated analytical arguments in this study underscore fundamental differences between double-diffusive mixing and mechanically generated small-scale turbulence. When both double-diffusion and turbulence are taken into account, we find that the constraints on diapycnal velocity loosen (tighten) with the increase (decrease) of the fraction of the overall mixing attributed to turbulence. The range of diapycnal velocities that could be realized in doubly-diffusive fluids is determined by the variation in the heat/salt flux ratio. We hypothesize that the unique ability of double-diffusive mixing to actively control diapycnal volume transport may have significant ramifications for the structure and dynamics of thermohaline circulation in the ocean.
Highlights
Theory of heat and mass transfer by small-scale transient motions constitutes a large and important component of numerous physical sciences, including fields as diverse and seemingly unrelated as oceanography, astrophysics, geology and materials science
In order to rationalize the tendency of double-diffusion to suppress the diapycnal volume transport, we propose two arguments
Because small-scale mixing in the ocean is controlled by a combination of double-diffusion and mechanically generated turbulence, it is of interest to examine the constraints on diapycnal velocity in a model which includes both mixing processes
Summary
Theory of heat and mass transfer by small-scale transient motions constitutes a large and important component of numerous physical sciences, including fields as diverse and seemingly unrelated as oceanography, astrophysics, geology and materials science. We argue that in double-diffusive flows, diapycnal volume transport is dramatically reduced relative to that in turbulent systems with comparable mixing rates of density components. The overall water-mass distribution is set by large-scale advection whereas diapycnal mixing is relatively weak In this regime it is still possible to predict the impact of higher order diabatic processes on cross-isopycnal transfer using a technique originally developed by Rhines and Young [20]. Our starting point (Section 2) is an analysis of a one-dimensional vertical advective-diffusive balance of temperature and salinity of Munk’s [9] type This analysis, albeit highly idealized, suggests that fundamental differences may exist in the way turbulent and double-diffusive mixing affect diapycnal volume transport.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
