The impact of a variable mixing efficiency on the abyssal overturning

de Lavergne, Casimir; Madec, Gurvan; Le Sommer, Julien; Nurser, A.J. George; Naveira Garabato, Alberto C.. 2016 The impact of a variable mixing efficiency on the abyssal overturning. Journal of Physical Oceanography, 46 (2). 663-681. 10.1175/JPO-D-14-0259.1

Before downloading, please read NORA policies.

Preview

Text
© Copyright 2016 American Meteorological Society (AMS). Permission to use figures, tables, and brief excerpts from this work in scientific and educational works is hereby granted provided that the source is acknowledged. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act September 2010 Page 2 or that satisfies the conditions specified in Section 108 of the U.S. Copyright Act (17 USC §108, as revised by P.L. 94-553) does not require the AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a web site or in a searchable database, or other uses of this material, except as exempted by the above statement, requires written permission or a license from the AMS. Additional details are provided in the AMS Copyright Policy, available on the AMS Web site located at (http://www.ametsoc.org/) or from the AMS at 617-227-2425 or copyrights@ametsoc.org.
jpo-d-14-0259%2E1.pdf - Published Version
Download (3MB) | Preview

Official URL: http://dx.doi.org/10.1175/JPO-D-14-0259.1

Abstract/Summary

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 Rf, significantly varies depending on flow properties challenges this assumption, however. Here, we 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 Rf as a function of a turbulence intensity parameter Reb = εν/νN2, we show that accounting for reduced mixing efficiencies in regions of weak stratification or energetic turbulence (high Reb) strongly limits the ability of breaking internal waves to supply oceanic potential energy and drive abyssal upwelling. Moving from a fixed Rf = 1/6 to a variable efficiency Rf(Reb) causes Antarctic Bottom Water upwelling induced by locally-dissipating internal tides and lee waves to fall from 9 to 4 Sv, 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. Our 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. Our calculations also point to the importance of accurately constraining Rf(Reb) and including the effect in ocean models.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1175/JPO-D-14-0259.1
ISSN:	0022-3670
NORA Subject Terms:	Marine Sciences
Date made live:	28 Jan 2016 13:38 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/512785

Item Type:

Publication - Article

Digital Object Identifier (DOI):

10.1175/JPO-D-14-0259.1

ISSN:

0022-3670

NORA Subject Terms:

Marine Sciences

Date made live:

28 Jan 2016 13:38 +0 (UTC)

URI:

https://nora.nerc.ac.uk/id/eprint/512785