The Role of Eddies in Determining the Structure and Response of the Wind-Driven Southern Hemisphere Overturning: Results from the Modeling Eddies in the Southern Ocean (MESO) Project

Abstract

The Modeling Eddies in the Southern Ocean (MESO) project uses numerical sensitivity studies to examine the role played by Southern Ocean winds and eddies in determining the density structure of the global ocean and the magnitude and structure of the global overturning circulation. A hemispheric isopycnal-coordinate ocean model (which avoids numerical diapycnal diffusion) with realistic geometry is run with idealized forcing at a range of resolutions from coarse (2°) to eddy-permitting ( 1 ⁄6°). A comparison of coarse resolutions with fine resolutions indicates that explicit eddies affect both the structure of the overturning and the response of the overturning to wind stress changes. While the presence of resolved eddies does not greatly affect the prevailing qualitative picture of the ocean circulation, it alters the overturning cells involving the Southern Ocean transformation of dense deep waters and light waters of subtropical origin into intermediate waters. With resolved eddies, the surface-to-intermediate water cell extends farther southward by hundreds of kilometers and the deep-to-intermediate cell draws on comparatively lighter deep waters. The overturning response to changes in the winds is also sensitive to the presence of eddies. In noneddying simulations, changing the Ekman transport produces comparable changes in the overturning, much of it involving transformation of deep waters and resembling the mean circulation. In the eddypermitting simulations, a significant fraction of the Ekman transport changes are compensated by eddyinduced transport drawing from lighter waters than does the mean overturning. This significant difference calls into question the ability of coarse-resolution ocean models to accurately capture the impact of changes in the Southern Ocean on the global ocean circulation.

Lagrangian decomposition of the Deacon Cell

“…However, according to the overturning stream function in density coordinates in Figure 4a, the water that upwells to the surface and is then forced northward by the Ekman flux consists of NADW, and the strength of this upward branch in OCCAM is 13 Sv. This value is comparable to the one reported by Treguier et al [2007], but smaller than the 18 Sv obtained by Hallberg and Gnanadesikan [2006].…”

“…[6] A popular way of understanding the difference between the zonally averaged mean flow (containing the Deacon Cell) and the pathways of actual water particles is to regard it as a result of eddy correlations [Marshall, 1997;Marshall and Radko, 2003;Hallberg and Gnanadesikan, 2006;Treguier et al, 2007]. Obviously, the fact that the diapycnal flow almost vanishes when density coordinates are used means that the diapycnal component of the zonal mean flow must be canceled by the eddy contribution.…”

Barotropic and baroclinic processes in the transport variability of the Antarctic Circumpolar Current

“…Its standard deviation 6.9 Sv (from 5-day mean values) is low, and we have to face that BARBI may have a too large variability. In contrast to OCCAM, the Modeling Eddies in the Southern Ocean (MESO) project (Hallberg and Gnanadesikan 2006) reports a much higher variability, a weekly peak-to-peak range of 30 Sv, although the experiments are forced by constant windstress. The 1-year low-pass-filtered transport in the 1/6 • MESO model still varies by about 5 Sv.…”

Southern Ocean carbon-wind stress feedback

“…However, most CMIP5 models use the same parametrization, so our results and scaling are appropriate for analysing wind stress feedbacks in CMIP5. Nonetheless, studies show that resolved eddies reduce the Southern Ocean carbon response to wind stress perturbations, by means of the effect of eddies on the residual circulation (Hallberg and Gnanadesikan 2006) and their role in setting the stratification of the ocean (Marshall et al 2002;Henning and Vallis 2004), rather than direct carbon transport by the eddy-induced velocity. While further work is needed to determine the impact of eddies on the feedback, additional experiments with a stronger eddy diffusivity parameter show a reduction in the sensitivity of pCO a 2 to SO by affecting the background stratification, an effect that our proposed scaling captures well.…”