Abyssal recipes II: energetics of tidal and wind mixing

Without deep mixing, the ocean would turn, within a few thousand years, int o a stagnant pool of cold salty water with equilibrium maintained locally b y near-surface mixing and with very weak convectively driven surface-intens ified circulation. (This result follows from Sandstrom's theorem for a flui d heated and cooled at the surface.) In this context we revisit the 1966 "A byssal Recipes", which called for a diapycnal diffusivity of 10(-4) m(2)/s (1 cgs) to maintain the abyssal stratification against global upwelling ass ociated with 25 Sverdrups of deep water formation. Subsequent microstructur e measurements gave a pelagic diffusivity (away from topography) of 10(-5) m(2)/s - a low value confirmed by dye release experiments. A new solution (without restriction to constant coefficients) leads to appr oximately the same values of global upwelling and diffusivity, but we reint erpret the computed diffusivity as a surrogate for a small number of concen trated sources of buoyancy flux (regions of intense mixing) from which the water masses (but not the turbulence) are exported into the ocean interior. Using the Levitus climatology we find that 2.1 TW (terawatts) are required to maintain the global abyssal density distribution against 30 Sverdrups o f deep water formation. The winds and tides are the only possible source of mechanical energy to dr ive the interior mixing. Tidal dissipation is known from astronomy to equal 3.7 TW (2.50 +/- 0.05 TW from M-2 alone), but nearly all of this has tradi tionally been allocated to dissipation in the turbulent bottom boundary lay ers of marginal seas. However, two recent TOPEX/POSEIDON altimetric estimat es combined with dynamical models suggest that 0.6-0.9 TW may be available for abyssal mixing. A recent estimate of wind-driving;suggests 1 TW of addi tional mixing power, All values are very uncertain. A surprising conclusion is that the equator-to-pole heat flux of 2000 TW as sociated with the meridional overturning circulation would not exist withou t the comparatively minute mechanical mixing sources. Coupled with the find ings that mixing occurs at a few dominant sites, there is a host of questio ns concerning the maintenance of the present climate state, but also that o f paleoclimates and their relation to detailed continental configurations, the history of the Earth-Moon system, and a possible great sensitivity to d etails of the wind system. (C) 1998 Elsevier Science Ltd. All rights reserv ed.