2015/09/02 by Casimir de Lavergne, Gurvan Madec, Julien Le Sommer +2 · 3 citations
Earth and Planetary Sciences · Environmental Science · #Oceanographic and Atmospheric Processes #Marine and coastal ecosystems #Climate variability and models
paper · pdf · doi:10.1175/jpo-d-14-0259.1
Abstract In studies of ocean mixing, it is generally assumed that small-scale turbulent overturns lose 15%–20% of their energy in eroding the background stratification. Accumulating evidence that this energy fraction, or mixing efficiency R f , significantly varies depending on flow properties challenges this assumption, however. Here, the authors examine the implications of a varying mixing efficiency for ocean energetics and deep-water mass transformation. Combining current parameterizations of internal wave-driven mixing with a recent model expressing R f as a function of a turbulence intensity parameter Re b = ε ν / νN 2 , the ratio of dissipation ε ν to stratification N 2 and molecular viscosity ν , it is shown that accounting for reduced mixing efficiencies in regions of weak stratification or energetic turbulence (high Re b ) strongly limits the ability of breaking internal waves to supply oceanic potential energy and drive abyssal upwelling. Moving from a fixed R f = 1/6 to a variable efficiency R f (Re b ) causes Antarctic Bottom Water upwelling induced by locally dissipating internal tides and lee waves to fall from 9 to 4 Sverdrups (Sv; 1 Sv ≡ 10 6 m 3 s −1 ) and the corresponding potential energy source to plunge from 97 to 44 GW. When adding the contribution of remotely dissipating internal tides under idealized distributions of energy dissipation, the total rate of Antarctic Bottom Water upwelling is reduced by about a factor of 2, reaching 5–15 Sv, compared to 10–33 Sv for a fixed efficiency. The results suggest that distributed mixing, overflow-related boundary processes, and geothermal heating are more effective in consuming abyssal waters than topographically enhanced mixing by breaking internal waves. These calculations also point to the importance of accurately constraining R f (Re b ) and including the effect in ocean models.