1. Introduction
An important feature missing from residual-mean models is the representation of the ACC’s complex frontal structure (Hughes and Ash 2001; Marshall et al. 2006; Sokolov and Rintoul 2009). This complexity in the ACC’s frontal anatomy is complemented by a striking heterogeneity in eddy characteristics (Venaille et al. 2011; Dencausse et al. 2011; Zika et al. 2013b). Fu (2009), Lu and Speer (2010), and Sallée et al. (2011) all document an enhancement of EKE and eddy diffusivities in the lee of topography. Thompson and Sallée (2012), using passive particles advected by a satellite-derived surface velocity field, link these regions to sites of enhanced cross-front transport. Naveira Garabato et al. (2011) develop theoretical arguments, first discussed in Ferrari and Nikurashin (2010), to show that ACC jets suppress meridional transport, except in regions where jets interact with topography. Using a kinematic argument, they conjecture that the interaction of transient eddies with small-scale, nonparallel structure in the mean flow, associated with standing meanders, leads to a local intensification of eddy-induced mixing across the ACC jets.
The apparent disconnect between the ACC’s alongstream structure and zonally averaged theories for overturning and stratification has parallels with atmospheric storm tracks (Chang et al. 2002; Bengtsson et al. 2006; Kaspi and Schneider 2011). Adapting these results directly to the ocean is complicated, however, by the dramatic reduction in the deformation radius, which results in a scale separation between meander and eddy length scales that does not occur in the atmosphere. This scale separation was inferred from a scaling analysis by Williams et al. (2007). Williams et al. (2007) also compare atmospheric and oceanic storm tracks and find significant alongstream structure in eddy vorticity fluxes associated with major topographic features that may both accelerate and decelerate jets.
An important consideration is how the ACC achieves the balance in (1) following a change in τ. These dynamics were explored in a study by Meredith and Hogg (2006), in which satellite altimetry data point to an increase in the ACC’s EKE following an intensification of the surface westerlies, as measured by the southern annular mode (SAM). The EKE rise occurs with a lag of about 2 yr, which the authors identify as the time required for baroclinic eddies to extract, via baroclinic instability, available potential energy created by wind-driven isopycnal tilting. Figure 1 revisits and extends the Meredith and Hogg (2006) analysis by calculating the regional time series of EKE anomalies in the Southern Ocean, derived from satellite altimetry data between 1993 and 2010. Anomalies are defined as differences from the 18-yr average EKE. Figure 1a shows a time series of basin characteristics considered by Meredith and Hogg (2006), with the SAM index overlain. A spatial map, however, indicates that anomalies peak in discrete regions associated with persistent meanders (Fig. 1c). Isolating these features (dashed lines) produces more dramatic EKE variability (Fig. 1b), especially compared to similar-sized regions outside of meanders (dashed–dotted line). The extended time series shows that correlation with the global SAM index is tenuous. This result is consistent with Gille and Kelly (1996), who showed that the amplitude of the first EOF of Southern Ocean SSH variability does not have a circumpolar or even a basin-scale trend. More recently, Dufour et al. (2012) showed that the ACC’s transient eddy response to an increase in wind stress is not zonally uniform. Morrow et al. (2010) note that Southern Ocean variability may also respond to ENSO and other climate modes and provide evidence that local variations in wind stress can generate local EKE anomalies.
(a) Time series of EKE anomalies (cm2 s−2) from 1993 to 2010, calculated from satellite altimetry data, for different regions [solid lines in (c)] of the Southern Ocean (cf. Meredith and Hogg 2006, their Fig. 1). Anomalies are the difference from the 18-yr mean EKE in each region. (b) Time series of EKE anomalies (cm2 s−2) for different standing meanders in the Southern Ocean [dashed lines in (c)] and for a region of similar size outside of standing meanders [dashed–dotted line in (c)]. The cyan curve in (a) and (b) shows the SAM index over this period. (c) EKE anomaly (cm2 s−2) for 2000–02. Boxes correspond to regional averages plotted in (a) and (b).
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
In this study, we attempt to link the observed insensitivity of the ACC’s isopycnal slope to the notable alongstream variability in ACC properties. Using a high-resolution, eddy-resolving ocean GCM, the Ocean General Circulation Model for the Earth Simulator (OFES), described in section 2, we explore the following hypotheses:
Buoyancy surfaces undergo a persistent wind-driven steepening over long stretches of the ACC where eddy fluxes are weak. This steepening is counterbalanced by isopycnal relaxation across a small number of standing meanders.
Deep eddy kinetic energy and bottom speeds are elevated in the meanders and are coupled to surface variability through vorticity dynamics.
Fluctuations in meander size are dynamically related to processes that transfer momentum vertically in the water column, both eddy buoyancy fluxes and horizontal divergence, and modify the baroclinicity of the flow.
Together these results lead us to propose that a major component of the ACC’s response to changes in surface forcing occurs through fluctuations, or flexing, of the ACC’s dominant standing meanders. This behavior has a negative feedback that contributes to the leading-order wind–eddy balance expressed in (1).
Section 3 characterizes eddy properties across a small number of meanders, focusing on the vertical structure of the flow. Temporal variability in the size and structure of the meanders is also considered. Section 4 presents a discussion of this discrete view of ACC dynamics, and our conclusions appear in section 5. This study does not provide a complete picture of the complex role that standing meanders play in global properties of the ACC. In particular, the relative importance of relaxation by standing meanders and transient eddies remains to be quantified. However, the dynamics discussed below highlight the need to better understand variability in the ACC’s energetic standing meanders.
2. Model data and theory
a. Ocean General Circulation Model for the Earth Simulator
This study focuses on the spatial and temporal variability of eddies and standing meanders, with particular attention on their vertical structure. Meanders arise from flow interactions with significant topographic features. Thus, a realistic, high-resolution, primitive equation numerical model is best suited to obtain these diagnostics.
Improvements in Southern Ocean models have resulted in flows populated by many narrow, meandering small-scale jets and coherent mesoscale eddies (Hallberg and Gnanadesikan 2006; Mazloff et al. 2010). Here we use output from the 
b. Vorticity balance
ACC standing meanders are intricately related to the zonal transport, which is sufficiently strong that baroclinic Rossby waves are unable to travel westward (Hughes et al. 1998). This behavior has been linked to enhanced eddy diffusivities by Ferrari and Nikurashin (2010), among others. The strong mean flow also generates an equivalent barotropic velocity structure in which bottom form stress becomes critical for flow equilibration (Hughes and de Cuevas 2001). In regions where the wind stress is weak, the barotropic vorticity budget implies that meridional velocities, related to meanders, must be balanced by a bottom pressure torque, arising from a geostrophic flow across isobaths.
c. Eliassen–Palm flux
The distribution of eddy fluxes in the ACC is also of interest, because momentum input by surface winds is transferred downward, via eddy buoyancy fluxes or interfacial form stress (Johnson and Bryden 1989), before it is removed by bottom form stress across topographic features (Munk and Palmèn 1951; Olbers et al. 2004). We note that Ward and Hogg (2011), analyzing a five-layer primitive equation channel model, challenge this view by showing that a rapid response of the bottom form stress, following a change in surface wind forcing, results in both a downward- (from the surface) and upward-propagating (from the bottom) baroclinic adjustment of the flow via interfacial form stress. This vertical momentum transfer requires an understanding of interior potential vorticity (PV) fluxes encapsulated by the divergence of the Eliassen–Palm (EP) tensor 
3. Results: The intermittent ACC
a. Meanders and stratification
We first examine the evolution of the ACC’s stratification between energetic standing meanders and regions of predominantly zonal flow. This approach is motivated by observations (Smith et al. 2010) of significant modifications in the vertical structure of the ACC across major topographic features, with a tendency toward the barotropization of the mean flow as it navigates topography.
Altimetry (Fu et al. 2010) and models (Hallberg and Gnanadesikan 2006) identify ACC meanders as sites of enhanced EKE. Figure 2 confirms that this relationship holds in OFES (only Eastern Hemisphere shown for clarity). Figure 2a shows mean kinetic energy (MKE) and Fig. 2c shows mean EKE at 100 m. EKE here is a deviation from an 8-yr time mean such that it contains temporal variability from both the standing meanders and transient eddies. Regions of elevated EKE are isolated downstream of significant topographic features (Fu 2009). Figures 2b and 2d show the subsurface distribution of these properties at a depth of 2500 m. The subsurface distribution of EKE is not well constrained by observations. A black contour is drawn in Fig. 2d to indicate regions where both surface and subsurface values of EKE are elevated. These sites are all coincident with standing meanders. One interpretation of this distribution, which we explore next, is that mesoscale eddies make a localized contribution to the wind–eddy balance in (1).
Summary of OFES output. (a) MKE of the ACC’s Eastern Hemisphere at 100-m depth. The 0.05 m2 s−2 contour is shown in white. (b) MKE of the ACC at 2500-m depth. (c) Mean EKE at 100-m depth. The 0.05 m2 s−2 contour is included. (d) Mean EKE at 2500-m depth. An additional black contour indicates where the EKE at 2500 m is at least a tenth of the EKE at 2500 m. Regions in gray indicate bathymetry.
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
(top) Depth (m) of the Southern Ocean in OFES. (bottom) Difference in potential density Δ (kg m−3) [(9)] as a function of depth and dynamic height, referenced to 2000 m, for the regions (a) 28°–38°E, (b)100°–145°E, (c) 120°–70°W, and (d) 60°–40°W. Dynamic height increases from south to north across the ACC. Arrows are provided to show the sense of downstream isopycnal adjustment at 1500-m depth.
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
Figures 3c and 3d show similar behavior in other regions of the ACC. In particular, there is the steepening of isopycnals between 120° and 70°W across the eastern Pacific sector of the ACC. The change in density across Drake Passage is more extreme, with the isolated region now spanning 20° of latitude (60°–40°W). Drake Passage is a more substantial disruption to the ACC and accommodates multiple standing meanders. This region requires further attention than there is scope for here [see review in Meredith et al. (2011)].
The residual-mean framework assumes the ACC’s isopycnal slope arises from approximately equal contributions from the mean wind-driven and eddy-induced circulations. In a zonally symmetric domain, this balance applies locally at every longitude. Figures 2 and 3 show significant variability in eddy characteristics and stratification along the path of the ACC, whereas surface wind and buoyancy forcing are zonally symmetric to leading order (Large and Yeager 2009). Thus, a picture arises in which the ACC is partitioned into regions of wind-driven steepening, compensated by localized isopycnal relaxation. While this is clearly a simplified view, it represents a significant departure from the local balance implied by zonally symmetric theories of the ACC.
b. Meanders and vertical structure
We next consider the vertical structure of a typical standing meander; this feature, found at 30°E, corresponds to the rapid relaxation of density surfaces depicted in Fig. 3a.
(a) Surface buoyancy (m s−2) in the Atlantic sector of the ACC. White curves indicate the 0.0045 and 0.0065 m s−2 contours and provide an envelope for the meander at 30°E. (b) EKE (m2 s−2) along the path of the meander. Values represent a mean value integrated between the contours in the meridional direction; the scale is logarithmic. (c) QG PV (s−1), defined in (11), integrated between the white contours in (a); the scale is logarithmic. Dashed lines are used to compare spatial variability in components of the Eliassen–Palm tensor 
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
Figure 4b shows EKE averaged within the meander envelope. The peak in eddy kinetic energy corresponds with the crest of the meander with a striking two order of magnitude change across the meander. At depths greater than 1500 m, the transition is abrupt. The increase and subsequent decay in deep EKE occurs over 15° of longitude or approximately 1000 km. Along-meander profiles of mean KE look similar to EKE (not shown). This deep signature of EKE is important for two reasons: (i) it suggests an enhancement of subsurface eddy-induced mixing, and (ii) it provides a mechanism to locally enhance bottom form stress, if the velocities have a component directed across isobaths.
As discussed in section 2c, acceleration of the alongstream mean flow via eddy-induced fluxes is related to the divergence of the Eliassen–Palm flux ∇ · 
(a) Eddy forcing of the alongstream flow as diagnosed by the divergence of the Eliassen–Palm vector ∇ · 
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
A recurrent pattern found in the ACC standing meanders entails a divergence of 
The distributions of these diagnostics across different standing meanders are remarkably similar. Figure 6 shows the surface buoyancy, EKE, 
Meander characteristics (left) at Macquarie Ridge and (right) over the Udintsev Fracture Zone. (a) Surface buoyancy (m s−2) with contours (white) used to define an envelope of the meander. (b) Vertical profile of mean EKE (m2 s−2; logarithmic scale) along the path of the meander. Values are averaged between the contours in (a). (c) The 
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
c. Meanders and vorticity balance
From (2), the steady vorticity balance in the ocean interior implies a balance between the advection of total vorticity (f + ζ) and the stretching or divergence term −fwz. In the barotropic vorticity balance, an incomplete cancellation between components of the advection of total vorticity results in nonzero bottom velocities (Hughes and de Cuevas 2001). H05 calculated all three terms in the steady vorticity balance [(2)] from altimetry data, but needed to apply a smoothing filter following each differentiation. This resulted in the dominant scale of the relative vorticity advection u · ∇ζ (300–500 km) being comparable to meanders in the dynamic height field, and thus with the advection of planetary vorticity βυ. Here, the 0.1°-resolution OFES allows the calculation of each term in the vorticity balance without the use of smoothing filters.
Figures 7a–c show an 8-yr time average of the three terms in (2) for the standing meander at 150°E at 250-m depth. The advection of the planetary vorticity (Fig. 7c) is the smoothest of the three terms, but is an order of magnitude smaller than the advection of relative vorticity and divergence. The divergence and advection of relative vorticity show smaller-scale variations and are spatially correlated. These terms also cancel to leading order with the residual having the same magnitude as the advection of planetary vorticity, but smaller spatial structure (not shown).
The 8-yr time average of terms in the vorticity balance [(2)] (10−11 s−2) for the standing meander at 150°E and 250-m depth: (a) divergence (stretching) −fwz, (b) advection of relative vorticity u · ∇ζ, and (c) planetary vorticity advection βυ. (d) Bottom pressure torque within the meander (10−5 N m−3). The gray contour indicates the 3500-m isobath; contours of the 4000- (black) and 3000-m (light gray) isobaths are also included in (d).
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
The balance between the advection of relative vorticity and divergence dominates throughout most of the water column, as seen in Fig. 8, which shows the spatial correlation and linear regression coefficients of the different terms in the vorticity budget at various depths. The balance between u · ∇ζ and −fwz also holds in other major standing meanders, as shown in Fig. 9 for meanders at 30°E and 150°W. The regression between these two fields in each meander is close to 1, which is similar to H05, although we also find that correlation and regression coefficients are sensitive to the introduction of smoothing.
(a) Spatial correlation coefficient and (b) linear regression coefficient for terms in the vorticity budget (2) as a function of depth for the standing meander at 155°E (Fig. 7). Colors distinguish correlations/regressions between the different terms given in the legend in (b).
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
(top) The 8-yr average of the (a) divergence (stretching) −fwz and (b) advection of relative vorticity u · ∇ζ at 250 m at the standing meander near 30°E. (bottom) The 8-yr average of the (c) divergence (stretching) −fwz and (d) advection of relative vorticity u · ∇ζ at 250 m at the standing meander near 150°W. Units for all terms are 10−11 s−2.
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
Any individual snapshot of either fwz or u · ∇ζ shows considerably smaller scales than the time-averaged fields. These fields also vary coherently as the structure of the meander changes, a process suggesting an important role for Rossby waves. From inspection, the two terms are seen to have a strong correlation over short time scales. Figure 10a summarizes this relationship by showing a time series of |fwz| (solid) and |u · ∇ζ| (dashed) averaged over the region shown in Fig. 7. The temporal correlation coefficient between these two time series is 0.905, with no smoothing and increases to 0.956 when a 2-month running-mean smoother is introduced. This strong temporal coherence is indicative of the small, spatial scale agreement that occurs between these terms.
(a) Time series of the magnitudes of different terms in the vorticity balance (2) for the standing meander at 155°E in Fig. 7 at 250-m depth. The spatial average of the absolute magnitude of each term is calculated. The mean is removed, and the time series is normalized by the max value. Time series of the meander-averaged amplitudes of bottom speed and the meridional buoyancy flux at 250 m are also shown. The latter term is smoothed with a 2-month running mean. (b) Time series of the meander-averaged amplitude of bottom pressure torque; the time series is normalized as in (a). An estimate of the meander amplitude is also given (see text for description). These time series have been smoothed using a 2-month running mean.
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
In contrast, the barotropic vorticity balance occurs over larger scales. Figure 7d shows the 8-yr time-mean bottom pressure torque within the same standing meander, and Fig. 10b shows the temporal evolution as described above. A relationship between bottom pressure torque and meander amplitude might be expected because the leading-order balance of the barotropic vorticity budget is between the bottom pressure torque and advection of planetary vorticity (Hughes and de Cuevas 2001). In Fig. 10b, we plot a rough estimate of the meander amplitude, obtained from removing a linear approximation of the mean surface buoyancy across the meander and plotting the root-mean-square amplitude of the buoyancy anomalies over the 8-yr period. The variability of the meander in the context of curvature is explored in section 3d.
In summary, variability in the advection of relative vorticity, related to meander dynamics, is linked to fluctuations in vertical velocities. This relationship is probed further by the inclusion of a third curve in Fig. 10a that shows a regional average of bottom speed (the lowermost complete cell at each longitude and latitude) in the meander. Bottom speed, which shows significant variations over the 8-yr time series, is strongly correlated with both the divergence (correlation coefficient = 0.913) and the advection of relative vorticity (correlation coefficient = 0.892); these vorticity components are calculated at 250 m.
d. Meanders and time variability
The temporal covariance of terms in the vorticity balance motivates a consideration of how changes in the meander structure relate to eddy characteristics and follows earlier studies of standing meanders by H05 and Ochoa and Niiler (2007).
Scale separation between mesoscale eddies and the size of the meanders themselves is a distinguishing feature of ocean storm tracks (Williams et al. 2007). For example, in the vorticity budget, snapshots show a correlation between the advection of relative vorticity and divergence at scales comparable to the deformation radius (~20 km), while the time-averaged balance and the barotropic vorticity balance is related to the meander scale (~400 km).
A comparable scale separation in time is less apparent, however. Figures 11a–c indicate changes to the structure of the standing meander at 155°E over an 8-yr time period. Figure 11a shows the 8-yr time average of the buoyancy contours at 250-m depth for the region shown in Fig. 7—the flow is equivalent barotropic, and thus buoyancy contours have similar curvature at other depths. Figures 11b and 11c are 2-month means of buoyancy during periods of weak and strong curvature, respectively. As a measure of curvature 
Buoyancy contours at a depth of 250 m (a) averaged over a period of 8 yr, (b) during a period of weak curvature in year 4, and (c) during a period of strong curvature. (d) Time series of curvature 
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
Finally, there is also a positive correlation between the time series of 
4. Discussion
The existing framework for understanding the ACC’s equilibrated stratification (isopycnal slope) relies on a near balance between a mean and eddy overturning, where the latter is related to a zonally averaged estimate of an eddy buoyancy diffusivity. Figures 2 and 3 suggest there is a gap between this idealized picture of the ACC and the true spatial distribution of eddy generation and eddy fluxes. The largest EKE values cluster around regions where the curvature of buoyancy surfaces is large and may not be adequately described by traditional eddy parameterizations [e.g., Gent and McWilliams 1990; Ferrari and Nikurashin 2010].
Lack of high-resolution data has hampered previous studies of the ACC’s vorticity balance, especially in regions of strong meandering (H05; Chereskin et al. 2010), with Firing et al. (2011) as a notable exception. These studies focus largely on the barotropic vorticity balance and emphasize a balance between the advection of relative vorticity and advection of planetary vorticity. Crucially, these studies only resolve features on the scale of the meanders themselves (300–500 km), while in the GCM data, the prominent spatial scales in the vorticity balance are on the order of a few tens of kilometers. It is only in the time average (Fig. 7) that the structure of the standing meander is recovered. Through the resolution of these scales, we find that at any given level the amplitude of the advection of relative vorticity and the divergence term are roughly an order of magnitude larger than the advection of planetary vorticity, a result that is confirmed in an idealized 5-km resolution Massachusetts Institute of Technology General Circulation Model (MITgcm) channel model with topography (Bischoff and Thompson 2014, manuscript submitted to J. Phys. Oceanogr.). The importance of the divergence term throughout the water column (Fig. 8) suggests a role for ageostrophic vertical velocities in modifying the stratification across a standing meander. Ageostrophic vertical circulations that occur during frontogenesis (Spall 1997) are likely to be important, as Williams et al. (2007) have shown that meanders tend to be regions of frontogenesis.1 Lateral eddy buoyancy fluxes also play a role in the equilibrated stratification.
These results point to variability in the structure and eddy characteristics of standing meanders being critical to the global ACC structure. As discussed in detail in H05, standing meanders arise from the arrest of barotropic Rossby waves by the mean flow. Neglecting vertical and lateral shears, the arrested Rossby wave is expected to exhibit a single characteristic wavelength. Waves with longer and shorter wavelengths will propagate upstream and downstream, respectively. This approach suggests that long-term changes in the shape or curvature (Fig. 11) of standing meanders are representative of changes in the mean flow. The consequences of these fluctuations are twofold.
The first effect of the meander fluctuations is somewhat tenuous, but offers a potentially new way of viewing the equilibration of the ACC. A clear signal in the diagnostics presented here, and consistent with Ward and Hogg (2011), is that changes in surface wind forcing have a rapid deep, or barotropic, response that modifies the patterns of bottom form stress or bottom pressure torque. At the ocean bottom, the pattern of these changes is related to the scale of the standing meander, not the smaller scales that govern the interior baroclinic response. An adjustment of the barotropic vorticity balance thus requires a change in the meander structure. This may occur either through an increase in meander wavelength, consistent with the expectation from the barotropic Rossby wave dispersion relation that a longer wave would be arrested by a stronger zonal-mean flow, or through an increase in meander amplitude related to an adjustment of the barotropic planetary vorticity advection. This rapid bottom response is consistent with diagnosed transports in channel models with bathymetry following a step change in wind stress: the transport rapidly increases before adjusting to a new equilibrated value (Ward and Hogg 2011). Regardless of the mechanism, this adjustment of the meander necessitates the movement of the pressure field across f/h contours, modifying bottom form stress in the meander. In the model, we find a correlation between enhanced meander curvature and increased bottom speed, and we suggest that an enhancement of bottom speed may be a mechanism for enhanced dissipation either through bottom friction or more likely through the generation of lee waves (which are not resolved in the OFES). Mazloff et al. (2013) have also identified the importance of fluctuations in the position of buoyancy outcrops in the Southern Ocean, showing that a geostrophic component of the overturning circulation arises from these variations. Variability, or flexing, of key meanders will make an important contribution to this component.
The second effect is clearer and shows that meander curvature is linked to eddy characteristics in the standing meander region, diagnosed through either EKE or more complex diagnostics like the Okubo–Weiss parameter or eddy potential vorticity fluxes. The reason for this is not fully explained here, but is likely related to the enhanced instability of a nonzonal-mean flow (Smith 2007; Boland et al. 2012). This increase in eddy fluxes, however, is directly related to enhanced eddy buoyancy fluxes and the vertical transfer of momentum.2 Thus, standing meanders are key sites where changes in surface momentum forcing can be communicated rapidly throughout the water column. Fluctuations between states of strong and weak curvature are reminiscent of the observed flipping between steered and zonal jets in idealized simulations by Thompson (2010) and may have some relation to the jet jumping of Chapman and Hogg (2013). An analysis of the Okubo–Weiss parameter (not shown) shows that meandering also enhances the strain fields, suggesting that meanders are regions where eddies are not only generated, but also torn apart by strong lateral shears, potentially leading to elevated mixing (Naveira Garabato et al. 2011; Thompson and Sallée 2012).
While this study alone is insufficient to complete the full picture, we put forward a discrete view of the ACC, motivated by the enhanced variability in a small number of standing meanders (Fig. 1). In this view, the equilibrated structure of the ACC is maintained by fluctuations in these standing meanders. The fluctuations are a combined response of arrested Rossby waves and the barotropic vorticity balance to changes in the mean flow. Figure 12 provides a diagram of this feedback mechanism. An intensification of surface westerlies steepens isopycnals and accelerates the ACC, especially upstream of meanders where EKE is relatively weak (Fig. 12a). The increased zonal transport of the ACC then results in a change in the Rossby waves that are arrested by the mean flow, specifically longer wavelengths; increases in curvature are also expected as the flow moves toward a new steady vorticity balance (Fig. 12b). Enhanced curvature leads to increased EKE and lateral eddy buoyancy fluxes, the latter being responsible for vertical momentum transport and the adjustment of the baroclinic structure of the meander (Fig. 12c). Furthermore, the standing meander sites we identify are coincident with regions of enhanced conversion of geostrophic flow into internal lee waves, as identified in recent maps by Nikurashin and Ferrari (2011) and Scott et al. (2011) and observations by Waterman et al. (2013). Most importantly, this work offers a physical mechanism that resonates with recent findings (Dufour et al. 2012; Zika et al. 2013a) that local standing meanders may provide the strongest control over the ACC’s overturning. We acknowledge that a “meander only” equilibration is likely an extreme view of the ACC, and the relative importance of relaxation occurring in standing meanders and that by transient mesoscale eddies distributed throughout the ACC requires further examination.
Diagram of the negative feedback mechanism mediated by Southern Ocean standing meanders. Red curves and text indicate the departure from an initial state (gray curves) in response to an increase in surface wind stress. (a) Isopycnals steepen outside of regions of strong EKE, accelerating the ACC. (b) Increased zonal transport results in a reequilibration of the standing meander due to the arrest of Rossby waves with a different wavelength and change in amplitude due to a modified vorticity balance. (c) Increases in meander curvature enhance EKE, eddy buoyancy fluxes, and vertical momentum transport, which change the baroclinic structure of the meander and decelerates the mean flow. Reduction of the mean flow allows the Rossby wave to travel upstream and modify the stratification.
Citation: Journal of Physical Oceanography 44, 7; 10.1175/JPO-D-13-0163.1
Using ship-based and Argo hydrographic data, Böning et al. (2008) show that the ACC’s mean isopycnal slope has changed little over the past two decades, despite significant changes in wind stress as measured by the SAM index. Eddy saturation was suggested as a potential explanation for this behavior. However, Munday et al. (2013) have argued that the relevant time scale for the ACC to adjust in the classical residual overturning framework is on the order of many tens of decades, due to the equilibration of the large basins to the north of the Southern Ocean to a new stratification. The localized response to changes in surface forcing, through the adjustment of Rossby waves and curvature discussed here, may provide a mechanism for a faster mode of equilibration to wind forcing, especially if large isopycnal excursions are limited to narrow regions around standing meanders. Finally, much of the interest in understanding the ACC’s response to changes in wind stress is the potential for this mechanism to explain variations in the structure and rate of the meridional overturning circulation, through modification of isopycnal outcropping and surface buoyancy forcing, across major climatic transitions (Lynch-Stieglitz et al. 2007). It is worth noting that these standing meanders are determined by topography, and thus their location is likely to remain fixed over climatic time scales, while their structure may respond to changes in the strength of the ACC or in the spatial distribution of surface forcings. An interesting open question is the degree to which local variations in the wind stress impact Southern Ocean stratification and overturning (Zika et al. 2013a).
5. Conclusions
Spatial variations of the ACC’s EKE distribution reflects significant alongstream changes in the dynamics that set the Southern Ocean stratification. Broadly, anomalies from the zonally averaged stratification of the ACC can be related to wide regions of isopycnal steepening where the wind-driven overturning out competes the eddy-induced overturning. These regions are typically associated with near-zonal flows with established frontal jets. Eddy activity is enhanced within the standing meanders, where isopycnal slopes relax dramatically. This enhancement of EKE, which may arise from the increased instability of the mean flow’s nonzonal orientation, has two consequences. The first is to increase the cross-stream buoyancy flux, whose vertical divergence is related to the vertical transport of alongstream momentum. Enhancement in EKE is also linked to the enhancement of vertical stretching that balances the advection of relative vorticity in the vorticity budget. This balance holds specifically in the standing meanders and differs from previous analyses of the ACC vorticity balance.
While this balance holds over an 8-yr time average of the model output, perhaps more intriguing is the high coherence between temporal fluctuations generated by internal variability and seasonal fluctuations in the climatological winds. Each of the terms in the vorticity balance shows a strong correlation with meander curvature. As the flow has an equivalent barotropic structure, this change in meander structure impacts the advection of planetary vorticity. Changes in the mean flow are also expected to modify the wavelength of arrested barotropic Rossby waves, while a change in meander amplitude is consistent with the flow adjusting toward a barotropic vorticity balance. The vorticity balance suggests that eddy–meander dynamics provide a negative feedback between changes in the ACC zonal transport, patterns of bottom form drag, and eddy-induced vertical momentum fluxes responding to changes in meander curvature. This provides a mechanism by which eddy saturation of the ACC may arise. This mechanism is separate from, but potentially working in tandem with, relaxation by transient eddies.
Appreciation of the importance of standing meanders is found in some of the first studies that probed the dynamics of the ACC (e.g., Stevens and Ivchenko 1997). Here, the emphasis is on transient fluctuations in the meander structure having a strong control over the equilibration of the ACC’s stratification. Our results have motivated a series of idealized numerical simulations (Bischoff and Thompson 2014, manuscript submitted to J. Phys. Oceanogr.) that confirm the importance of the meander response to changes in surface forcing. While this approach presents a potentially more complicated view of the ACC, with an emphasis on local dynamics rather than zonal- or streamwise-averaged properties, it also points to key regions that are critical to our understanding of the Southern Ocean’s evolution over climatic time scales.
Acknowledgments
AFT gratefully acknowledges support from the National Science Foundation (OCE-1235488), and ACNG support from a Philip Leverhulme Prize. Development of the ideas in this study have benefited from conversations with Ryan Abernathey, Toby Bischoff, Stuart Bishop, Chris Hughes, Kurt Polzin, Andrew Stewart, and Jan Zika. We thank Andy Hogg and an anonymous reviewer for comments that significantly improved this manuscript.
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Snapshots of the Rossby number from OFES, defined as ζ/f, intermittently show magnitudes greater than 0.5 in regions of strong strain.
Nearly stationary meanders are apparent throughout the ACC, but regions of enhanced EKE are limited to the main standing meander regions highlighted in this study.
