How Does the Antarctic Circumpolar Current Affect the Southern Ocean Meridional Overturning Circulation? Christopher C. Chapman∗ LOCEAN-IPSL Université de Pierre et Marie Curie mailto:[email protected] Jean-Baptiste Salléey LOCEAN-IPSL CNRS/Université de Pierre et Marie Curie December 6, 2016 Abstract The Meridional Overturning Circulation (MOC) in the Southern Ocean is investigated using hydrographic observations combined with satellite observations of sea-surface height. A three-dimensional (spatial and vertical) estimate of the isopycnal eddy-diffusivity in the Southern Ocean is obtained using the theory of Ferrari & Nikurashin (2010), that includes the influence of suppression of the diffusivity by the strong, time-mean flows. It is found that the eddy diffusivity is enhanced at depth, reaching a maximum at the “critical layer” near 1000m. The estimate of diffusivity is used with a simple diffusive parameterization to estimate the meridional eddy volume flux. Together with an estimate of the meridional Ekman transport and the time-mean meridional geostrophic transport, the eddy volume flux is used to reconstruct the time-mean overturning circulation. By comparing the reconstruction with, and without, suppression of the eddy diffusivity by the mean flow, the influence of the suppression on the overturning is illuminated. It is shown that the suppression of the eddy diffusivity results in a large reduction of interior eddy transports, and a more realistic eddy induced overturning circulation. Finally, a simple conceptual model is used to show that the MOC is influenced not only by the existence of enhanced diffusivity at depth, but also by the details of the vertical structure of the eddy diffusivity, such as the depth of the critical layer. I. Introduction subduction of atmospheric CO2, has a large influence on the climate system [Talley et al., he Meridional Overturning Circulation 2003, Marshall and Speer, 2012]. In the South- arXiv:1612.01115v1 [physics.ao-ph] 4 Dec 2016 (MOC), is a global scale circulation that, ern Ocean, the overturning is related to the rate Tdue to its important role in the redistri- that deep carbon-rich waters are ventilated at bution of heat, salt and biogeochemical tracers the surface where they can communicate with from warmer to colder latitudes, and in the the atmosphere, and the rate at which surface waters are in turn subducted into the ocean in- ∗ Corresponding author address: C. C. Chapman, terior [Sallée et al., 2013]. Thus, changes in the LOCEAN-IPSL, Université de Pierre et Marie Curie, Paris CEDEX ,France. rate of the overturning have been hypothesized E-mail: [email protected] to lead to a reduction in the Southern Ocean’s yCorresponding author 1 Submitted for publication in the Journal of Physical Oceanography ability to absorb and sequester CO2 [Le Quéré ence on the stirring ability of meso-scale eddies et al., 2007]. Understanding the dynamic con- and hence their ability of move water poleward trols of the MOC in the Southern Ocean, as [Bates et al., 2014]. The capacity of eddies to well as how it will respond to external changes induce a downgradient flux is often measured in the climate system, is therefore a pressing by the eddy diffusivity, K, which relates the question in physical oceanography. eddy-flux of some tracer with concentration C, Motivated by the widely acknowledged im- to the large scale gradient of that tracer: portance of the Southern Ocean for the global 0 0 MOC, intense focus on this region has led to u C = KrC (2) significant advances in our understanding of Although baroclinic eddies are ubiquitous in the structure of the MOC and dominant dy- the Southern Ocean, certain regions, referred namical mechanisms that lead to its formation. to as “hot-spots" or “storm tracks", which In particular, a description of the Southern arise from the interaction of the ACC with Ocean based on the Transformed Eulerian Mean bathymetry [Williams et al., 2007, Chapman (TEM) formulation has shown that, on the large et al., 2015], cause localised increases in K [Sal- scale, the Southern Ocean overturning results lée et al., 2008]. The ACC also modulates the from a competition between a northward wind vertical structure of eddy diffusivity, which is driven Eulerian mean overturning cell, Y, and known to be enhanced at depth, reaching a a southward eddy induced overturning, Y? maxima at the “steering-level" or “critical layer [Johnson and Bryden, 1989, Döös and Webb, depth" [Ferrari and Nikurashin, 2010, Klocker 1994]. The mean and eddy-induced overturn- and Abernathey, 2014], associated with the ing are thought to be of similar magnitude, yet fastest growing linear waves [Smith and Mar- opposite sign, such that only a small residual shall, 2009]. Linear stability analysis places transport remains. The resulting overturning, this level at around 1000 m depth [Smith and commonly expressed as an overturning stream- Marshall, 2009]. Although it has been shown function, is written: that including a spatially varying K can reduce Yres = Y + Y?. (1) bias in coarse resolution climate models [Fer- reira et al., 2005, Danabasoglu and Marshall, Because of the delicate balance between the 2007], and while the implications of a three- eddy and mean overturning, the residual over- dimensional K on the broad scale flow have turning is sensitive to even small changes in been briefly discussed in several studies [Mar- one or the other component resulting from shall et al., 2006, Shuckburgh et al., 2009, Smith changes in surface forcing [Viebahn and Eden, and Marshall, 2009, Naveira Garabato et al., 2010, Abernathey et al., 2011, Meredith et al., 2011, Bates et al., 2014], a detailed understand- 2012, Downes and Hogg, 2013]. ing of the physical implications of spatially The Southern Ocean also hosts the Antarc- varying K for the large-scale overturning circu- tic Circumpolar Current (ACC), a system of lation is still lacking. currents that are among strongest on Earth. Al- In this study, we seek to characterize and though the ACC is primarily zonally oriented, quantify the influence of “storm-tracks" and it has direct and indirect roles in shaping the the suppression of the eddy diffusivity by the Southern Ocean MOC. For instance, the inter- mean-flow on the Southern Ocean overturn- action of the ACC with bathymetry results in a ing circulation. We will explore the impact significant, but frequently ignored, geostrophic of geostrophic mean-flow on the MOC, and interior overturning circulation that is distinct in particular the impact of the strong currents from the mean ageostrophic overturning asso- of the ACC in modulating the eddy overturn- ciated with Ekman currents [MacCready and ing. To achieve our goals, we reconstruct Rhines, 2001, Mazloff, 2008, Mazloff et al., the overturning circulation from a large obser- 2013]. In addition, the ACC has a strong influ- vational dataset that combines hydrographic 2 Submitted for publication in the Journal of Physical Oceanography data from Argo floats, oceanographic cruises II. The Southern Ocean and instrumented elephant seals, with sea Meridional Overturning surface height altimetry. The observational Circulation datasets are used to develop a direct estimate of the Eulerian mean overturing that includes Here we briefly revise the basic theory of the both ageostrophic Ekman currents and the im- MOC in the Southern Ocean, the theory of portant deep geostrophic currents that arise mean-flow suppression of eddy diffusivity, and from the interaction of ACC with the bottom the formulation of the TEM model equations. bathymetry. In addition, we produce a three- On an isopycnal layer, g, with thickness dimensional estimate of K based on the theory h = −¶z/¶g, the time-mean meridional vol- of Ferrari and Nikurashin [2010] that allows a ume flux is given by: reconstruction of an eddy overturning stream- function from the downgradient diffusion of hv = hv + h0v0, (3) potential vorticity [Treguier et al., 1997]. We will investigate the influence of spatial varia- where v is the meridional velocity, (·) is the tion of K and the suppressing influence of the time-averaging operator, and the flow has been background flow on the reconstructed eddy decomposed into time-mean and eddy com- and residual streamfunctions. By reconstruct- ponents. The primed quantities are perturba- 0 ing Y and Y? from observations, we show that tions from the time-mean, such that v = v + v 0 the ACC can impact significantly both of these and v = 0. We can further decompose v into terms, and has therefore a key role in shaping geostrophic, vg, and ageostrophic, vag compo- the residual overturning circulation. In tandem nents: with this reconstruction, we employ a simple = + + 0 0 + 0 0 conceptual model, based on the TEM approach hv hvg hvag h vg h vag. (4) of Marshall and Radko [2003] and Marshall The meridional transport can then be vertically and Radko [2006] to guide our interpretation integrated on across isopycnal layers to de- of the observational-based reconstruction. termine the time-mean isopycnal overturning streamfunction [Döös and Webb, 1994]: Z g res 0 ? ? Y (x, y, g) = hv dg = Yag + Yg + Yg + Yag . The remainder of this paper is organized 0 | {z } | {z } as follows: the theoretical framework used mean eddy for building the reconstruction of the MOC (5) from observations, including the procedure for estimating the horizontal (isopycnal) dif- i. The ageostrophic transport fusivity K, will be presented in Section II. The observational data set we employ will be de- In the Southern Ocean, the overwhelming ma- scribed in Section III. Our estimate of the three- jority of the ageostrophic transport, hvag oc- dimensional eddy diffusivity will be presented curs due to surface Ekman currents. The 0 0 in Section IV and our estimated MOC recon- ageostrophic eddy transport, h vag, although struction, along with a comparison of the the not completely negligible, is much smaller than results obtained with and without the influence the time-mean Ekman transport [Mazloff et al., of the background flow in Section V.