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Determining Thorpe Scales from Ship-Lowered CTD Density Profiles

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  • 1 Old Dominion University, Norfolk, Virginia
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Abstract

CTD measurements taken as an integral part of oceanographic cruises could provide valuable information on spatial locations and time variability of significant shear-generated mixing in the ocean interior if used routinely to calculate Thorpe scales, that is, estimates of the scales of vertical overturning in an otherwise stably stratified fluid. This paper outlines methods for calculating reliable Thorpe scales from density profiles taken with a shipborne CTD, including removal of questionable instabilities associated with termination of pressure reversals, reduction of the effects of density noise by computation of an intermediate density profile, and overturn verification by a two-parameter (Ro, ΔN) diagnostic. The Ro criterion alone reliably removes overturns that result from salinity spikes at the high gradient boundaries of a weakly stratified layer, a common cause of highly suspect overturns. The ΔN diagnostic is a new water mass test describing the degree of “tightness” of the temperature–salinity (TS) relationship. The present two-parameter diagnostic rejects a significantly larger percentage of suspect overturns than does a previous single-parameter water mass test. Despite developing a more reliable water mass diagnostic, the authors conclude that rejection of overturns based on a water mass test that incorporates expectation of TS tightness is not warranted, given possibilities of TS “looseness” resulting from mixing over regions of nonlinear TS structure and/or from potential effects of differential diffusion.

Corresponding author address: Ann Gargett, Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23508. Email: gargett@ccpo.odu.edu

Abstract

CTD measurements taken as an integral part of oceanographic cruises could provide valuable information on spatial locations and time variability of significant shear-generated mixing in the ocean interior if used routinely to calculate Thorpe scales, that is, estimates of the scales of vertical overturning in an otherwise stably stratified fluid. This paper outlines methods for calculating reliable Thorpe scales from density profiles taken with a shipborne CTD, including removal of questionable instabilities associated with termination of pressure reversals, reduction of the effects of density noise by computation of an intermediate density profile, and overturn verification by a two-parameter (Ro, ΔN) diagnostic. The Ro criterion alone reliably removes overturns that result from salinity spikes at the high gradient boundaries of a weakly stratified layer, a common cause of highly suspect overturns. The ΔN diagnostic is a new water mass test describing the degree of “tightness” of the temperature–salinity (TS) relationship. The present two-parameter diagnostic rejects a significantly larger percentage of suspect overturns than does a previous single-parameter water mass test. Despite developing a more reliable water mass diagnostic, the authors conclude that rejection of overturns based on a water mass test that incorporates expectation of TS tightness is not warranted, given possibilities of TS “looseness” resulting from mixing over regions of nonlinear TS structure and/or from potential effects of differential diffusion.

Corresponding author address: Ann Gargett, Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23508. Email: gargett@ccpo.odu.edu

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