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Diagnosing the relation between ocean circulation, mixing and water-mass transformation from an ocean hydrography and air-sea fluxes


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Groeskamp, S 2015 , 'Diagnosing the relation between ocean circulation, mixing and water-mass transformation from an ocean hydrography and air-sea fluxes', PhD thesis, University of Tasmania.

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The aim of this thesis is to understand the relationship between surface freshwater and
heat fluxes, (interior) ocean mixing and the resulting changes in the ocean circulation
and distribution of water-masses.
The ocean circulation is analysed in Absolute Salinity (SA) and Conservative Temperature
(SA) coordinates. It is separated into 1) an advective component related to geographical
displacements in the direction normal to SA and ⊖ iso-surfaces, and quantified
by the advective thermohaline streamfunction Ψadv/SA⊖, and 2) into a local component, related
to local changes in SA and ⊖ values, without a geographical displacement, and
quantified by the local (temporal) thermohaline streamfunction Ψloc/SA⊖. In this decomposition,
the sum of the advective and local components of the circulation is given by the
diathermohaline streamfunction Ψdia/SA⊖ and is directly related to the salt and heat
fluxes of the surface forcing and ocean mixing. Interpretations of the streamfunctions is given
and it is argued that the diathermohaline streamfunction provides a powerful tool for the
analysis of and comparison amongst numerical ocean models and observational-based
gridded climatologies.
The relation between Ψdia/SA⊖ and fluxes of salt and heat is expressed as the Thermohaline
Inverse Method (THIM). The THIM uses conservation statements for volume, salt and
heat in (SA, ⊖) coordinates to express the unknown Ψdia/SA⊖ as surface freshwater and heat fluxes, and mixing parameterised by a down-gradient epineutral diffusion coefficient, and an isotropic down-gradient turbulent diffusion coeffcient of small scale mixing processes.
The resulting system of equations that is solved in the THIM is tested against a numerical
model and shown to provide accurate estimates of the unknowns ( Ψdia/SA⊖, and the
epineutral and small-scale diffusion coefficients).
The THIM has been applied to observations to obtain constrained estimates of the
epineutral and small-scale diffusion coefficients and Ψdia/SA⊖. New insights in Ψdia/SA⊖ are
revealed and the estimate of small-scale diffusion coefficient compares well with previous
estimates. The estimates of the epineutral diffusion coefficient is about 50 times smaller
than those typically used in coarse resolution climate models, suggesting that either the
surfaces fluxes that are used under-estimate the production of epineutral anomalies of
SA and ⊖) or the epineutral diffusion coefficients commonly used in climate models are
too large.
The geometry of interior ocean mixing is analysed and it is found that under the smallslope
approximation there is a small gradient of tracer in a direction in which there is
no actual epineutral gradient of tracer. The difference between the correct epineutral tracer gradient and the small-slope approximation to it, is quantified and it is shown that it points in the direction of the thermal wind. The fraction of the epineutral flux in this direction is very small and is negligible for all foreseeable applications. Smallscale
mixing processes act to diffuse tracers isotropically (i.e. directionally uniformly in
space), hence it is a misnomer to call this process 'dianeutral diffusion'. Both realisations
affect the diffusion tensor, and a more concise diffusion tensor is derived for use in ocean
The techniques, diagnostics and insights presented in this thesis lead to increased understanding
of the relationship between ocean circulation and water-mass transformation due to ocean mixing and surface
uxes, and result in enhanced ability to model the ocean and its role in the climate system.

Item Type: Thesis - PhD
Authors/Creators:Groeskamp, S
Keywords: Circulation, Water-mass transfromation, Mixing, ocean hydrogrpahy, air-sea fluxes
Copyright Holders: The Author
Copyright Information:

Copyright 2015 the author

Additional Information:

Chapter 2 appears to be the equivalent of the post-print version of an article published as: McDougall, T.J., Groeskamp, S., Griffes, S.M., 2014, On geometrical aspects of interior ocean mixing, Journal of physical oceanography, 44(7), 2164-2175

Chapter 3 appears to be the equivalent of the post-print version of an article published as: Groeskamp, S., Zika, J.D., McDougall, T.J., Sloyan, B.M., Laliberté, F. 2014, The Representation of ocean circulation and variability in thermodynamic coordinates, Journal of physical oceanography, 44(7), 1735-1750

Chapter 4 appears to be the equivalent of the post-print version of an article published as: Groeskamp, S. Zika, J.D., Sloyan, B.M., McDougall, T.J., McIntosh, P.C., 2014, A Thermohaline inverse method for estimating diathermohaline circulation and mixing, ournal of physical oceanography, 44(10), 2681–2697

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