Deacon Cell Research Articles

Roles of Meridional Overturning in Subpolar Southern Ocean SST Trends: Insights from Ensemble Simulations

Abstract One of the most puzzling observed features of recent climate has been a multidecadal surface cooling trend over the subpolar Southern Ocean (SO). In this study we use large ensembles of simulations with multiple climate models to study the role of the SO meridional overturning circulation (MOC) in these sea surface temperature (SST) trends. We find that multiple competing processes play prominent roles, consistent with multiple mechanisms proposed in the literature for the observed cooling. Early in the simulations (twentieth century and early twenty-first century) internal variability of the MOC can have a large impact, in part due to substantial simulated multidecadal variability of the MOC. Ensemble members with initially strong convection (and related surface warming due to convective mixing of subsurface warmth to the surface) tend to subsequently cool at the surface as convection associated with internal variability weakens. A second process occurs in the late-twentieth and twenty-first centuries, as weakening of oceanic convection associated with global warming and high-latitude freshening can contribute to the surface cooling trend by suppressing convection and associated vertical mixing of subsurface heat. As the simulations progress, the multidecadal SO variability is suppressed due to forced changes in the mean state and increased oceanic stratification. As a third process, the shallower mixed layers can then rapidly warm due to increasing forcing from greenhouse gas warming. Also, during this period the ensemble spread of SO SST trend partly arises from the spread of the wind-driven Deacon cell strength. Thus, different processes could conceivably have led to the observed cooling trend, consistent with the range of possibilities presented in the literature. To better understand the causes of the observed trend, it is important to better understand the characteristics of internal low-frequency variability in the SO and the response of that variability to global warming.

The effects of historical ozone changes on Southern Ocean heat uptake and storage

Atmospheric ozone concentrations have dramatically changed in the last five decades of past century. Herein we explore the effects of historical ozone changes that include stratospheric ozone depletion on Southern Ocean heat uptake and storage, by comparing CESM1 large ensemble simulations with fixed-ozone experiment. During 1958–2005, the ozone changes contribute to about 50% of poleward intensification of the Southern Hemisphere westerly winds in historical simulations, which intensifies the Deacon Cell and residual meridional overturning circulation, thus contributing to heat redistribution in the Southern Ocean. Heat budget analysis shows that, in response to historical ozone changes, heat is taken up between 50 and 58 °S mainly through changes in sensible heat flux, shortwave radiation flux, and the flux due to seasonal sea ice formation and melt. A major part of the absorbed heat, however, is redistributed equatorward primarily through Eulerian mean ocean heat transport such that ocean heat storage peaks at lower latitudes, around 44 °S. The ozone-induced interior warming contributes to about 22% of the historical Southern Ocean warming over 1958–2005. Poleward of 62 °S where a subsurface temperature inversion occurs, shoaling isopycnals lead to warming and salinification in the upper ocean. To the north of 50 °S, the deep-reaching warming and freshening that correspond to the ocean heat storage maximum are primarily set by the deepening of isopycnals. The large-scale patterns of isopycnal shoaling (deepening) at high (middle) latitudes are consistent with the overlying negative (positive) wind stress curl anomalies related to the poleward intensification of westerly winds, suggesting that the wind changes play an important role in the Southern Ocean heat redistribution under the ozone forcing.

Simulated changes of the Southern Ocean air-sea heat flux feedback in a warmer climate

In this paper, we have evaluated the Southern Ocean (SO) heat flux feedback in a fully coupled model and for the first time examined how this feedback evolves in response to global warming. The model broadly captures the observed characteristics of heat flux feedback over the SO. The heat flux tends to damp SST anomalies over the SO and thus the feedback is negative. In a warmer climate, the negative heat flux feedback in the SO, contributed mainly from turbulent component, becomes stronger. The turbulent feedback in the present day is primarily balanced by the upper boundary that strongly depends on background SST and wind and the thermal adjustment of boundary layer to SST anomalies. It is found that this balance shifts a little bit under global warming scenario. The upper limit increases in a warmer climate due to warm SST responses. The thermal adjustment of boundary layer becomes weaker in a warmer climate because of decreased atmospheric background heat convergence. The mean Deacon Cell transports anomalous heat caused by the greenhouse gas effect northward, leading to a heat convergence along the northern flank of the Antarctic Circumpolar Current. Constrained by the energy, the atmospheric northward heat transport has a corresponding divergence north of 55°S. This anomalous heat transport divergence favors air heat leaving away from 55°S–35°S regions to the polar region, leads to smaller air temperature tendencies in the local compared to the present day and therefore leads to a weakened thermal adjustment of boundary layer. Therefore, both changes in the upper limit and thermal adjustment of boundary layer contribute positively to the enhanced turbulent feedback in a warmer climate. The dynamic component due to changes in wind tends to compensate these two positive contributors, but its magnitude is too small to become a dominant factor.

Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

Eddy Cancellation of the Ekman Cell in Subtropical Gyres

AbstractThe presence of large-scale Ekman pumping associated with the climatological wind stress curl is the textbook explanation for low biological activity in the subtropical gyres. Using an idealized, eddy-resolving model, it is shown that Eulerian-mean Ekman pumping may be opposed by an eddy-driven circulation, analogous to the way in which the atmospheric Ferrel cell and the Southern Ocean Deacon cell are opposed by eddy-driven circulations. Lagrangian particle tracking, potential vorticity fluxes, and depth–density streamfunctions are used to show that, in the model, the rectified effect of eddies acts to largely cancel the Eulerian-mean Ekman downwelling. To distinguish this effect from eddy compensation, it is proposed that the suppression of Eulerian-mean downwelling by eddies be called “eddy cancellation.”

Improvements in LICOM2. Part I: Vertical Mixing

Abstract Better computational stability is achieved in an improved version of the National Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)/Institute of Atmospheric Physics (IAP) Climate Ocean Model, version 2 (LICOM2, the standard version), after improvements to the implementations of the vertical mixing, mesoscale eddy parameterization, and bottom drag schemes. The large warm biases of LICOM2 in the western Pacific Ocean and eastern Indian Ocean warm pool and on the east coast of the Pacific Ocean are significantly improved. The salinity bias in the tropical Pacific Ocean related to the warm bias of the warm pool is also alleviated. The simulation of the Atlantic meridional overturning circulation is improved because of enhanced vertical mixing in the high latitudes of the North Atlantic Ocean. The new version also presents a stronger Deacon cell, and thus a more powerful Antarctic Circumpolar Current that is closer to the observation, due to weaker southward mesoscale eddy transport in the Southern Ocean.

Variability in Southern Hemisphere Ocean Circulation from the 1980s to the 2000s

Abstract Interannual-to-decadal variability of ocean circulation in the Southern Hemisphere was examined using data from the 1980s to the 2000s in a box inverse model to estimate transport across hydrographic sections and three ocean general circulation models (OGCMs). The westerly wind stress over the OGCM Southern Ocean showed a steady increase of 5%–8% decade−1. The meridional overturning circulation was quantified by the transport across 30°S. The OGCMs suggested a slight strengthening [from 0.2 ± 1.0 to 0.8 ± 1.3 Sv decade−1 (1 Sv ≡ 106 m3 s−1)] of the upper meridional cell (Deacon cell) and two OGCMs showed a weakening (−0.8 ± 0.6 and −1.0 ± 0.3 Sv decade−1) of the lower meridional [Antarctic Bottom Water (AABW)] cell, partly explained by contraction of the AABW volume. The box inverse estimates did not contradict these two findings. For Antarctic Circumpolar Current transport, quantified by zonal transport across four key sections, the box inverse model estimated a decrease of 5–21 Sv. Decomposition of the decrease into baroclinic transport by the Subantarctic and Polar Fronts, barotropic transport, and others shows that the decrease is mostly due to barotropic transport and transport carried by the flow north of the Subantarctic Front and south of the Polar Front. In the OGCMs, the variability of transport across key sections is often correlated with transport carried by a flow south of the Polar Front and with the southern annular mode index. In all models, then, the transport of the Antarctic Circumpolar Current, defined as the transport carried by the fronts, has not decreased significantly over the study period.

Sensitivity of transient climate change to tidal mixing: Southern Ocean heat uptake in climate change experiments performed with ECHAM5/MPIOM

We investigate the sensitivity of the transient climate change to a tidal mixing scheme. The scheme parameterizes diapycnal diffusivity depending on the location of energy dissipation over rough topography, whereas the standard configuration uses horizontally con- stant diffusivity. We perform ensemble climate change experiments with two setups of MPIOM/ECHAM5, one setup with the tidal mixing scheme and the second setup with the standard configuration. Analysis of the responses of the transient climate change to CO2 increase reveals that the implementation of tidal mixing leads to a significant reduction of the transient surface warming by 9 %. The weaker surface warming in the tidal run is localized par- ticularly over the Weddell Sea, likely caused by a stronger ocean heat uptake in the Southern Ocean. The analysis of the ocean heat budget reveals that the ocean heat uptake in both experiments is caused by changes in convection and advection. In the upper ocean, heat uptake is caused by reduced convection and enhancement of the Deacon Cell, which appears also in isopycnal coordinates. In the deeper ocean, heat uptake is caused by reduction of convective cooling associated with the circulation polewards of 65S. Tidal mixing leads to stronger heat uptake in the Southern Ocean by causing stronger changes in advection, namely a stronger increase in the Deacon Cell and a stronger reduction in advective cooling by the circulation polewards of 65S. Counter-intuitively, the relation between tidal mixing and greater heat storage in the deep ocean is an indirect one, through the influence of tidal mixing on the circulation.

The Ventilated Ocean

Abstract Adiabatic theories of ocean circulation and density structure have a long tradition, from the concept of the ventilated thermocline to the notion that deep ocean ventilation is controlled by westerly winds over the Southern Ocean. This study explores these ideas using a recently developed Lagrangian ocean model (LOM), which simulates ocean motions by computing trajectories of water parcels. A unique feature of the LOM is its capacity to model ocean circulations in the adiabatic limit, in which water parcels exactly conserve their densities when they are not in contact with the ocean surface. The authors take advantage of this property of the LOM and consider the circulation and stratification that develop in an ocean with a fully adiabatic interior (with both isopycnal and diapycnal diffusivities set to zero). The ocean basin in the study mimics that of the Atlantic Ocean and includes a circumpolar channel. The model is forced by zonal wind stress and a density restoring at the surface. Despite the idealized geometry, the relatively coarse model resolution, and the lack of atmospheric coupling, the nondiffusive ocean maintains a density structure and meridional overturning that are broadly in line with those observed in the Atlantic Ocean. These are generated by just a handful of key water pathways, including shallow tropical cells described by ventilated thermocline theory; a deep overturning cell in which sinking North Atlantic Deep Water eventually upwells in the Southern Ocean before returning northward as Antarctic Intermediate Water; a Deacon cell that results from a topographically steered and corkscrewing circumpolar current; and weakly overturning Antarctic Bottom Water, which is effectively ventilated only in the Southern Hemisphere. The main conclusion of this study is that the adiabatic limit for the ocean interior provides the leading-order solution for ocean overturning and density structure, with tracer diffusion contributing first-order perturbations. Comparing nondiffusive and diffusive experiments helps to quantify the changes in stratification and circulation that result from adding a moderate amount of tracer diffusion in the ocean model, and these include an increase in the amplitude of the deep meridional overturning cell of several Sverdrups, a 10%–20% increase in Northern Hemispheric northward heat transport, a stronger stratification just below the main thermocline, and a more realistic bottom overturning cell.

The Adiabatic Pole-to-Pole Overturning Circulation

Abstract The adiabatic pole-to-pole cell of the residual overturning circulation (ROC) is studied in a two-hemisphere, semienclosed basin, with a zonally reentrant channel occupying the southernmost eighth of the domain. Three different models of increasing complexity are used: a simple, analytically tractable zonally averaged model; a coarse-resolution numerical model with parameterized eddies; and an eddy-resolving general circulation model. Two elements are found to be necessary for the existence of an adiabatic pole-to-pole cell: 1) a thermally indirect, wind-driven overturning circulation in the zonally reentrant channel, analogous to the Deacon cell in the Antarctic Circumpolar Current (ACC) region, and 2) a set of outcropping isopycnals shared between the channel and the semienclosed region of the Northern Hemisphere. These points are supported by several computations varying the domain geometry, the surface buoyancy distribution, and the wind forcing. All three models give results that are qualitatively very similar, indicating that the two requirements above are general and robust. The zonally averaged model parameterizes the streamfunction associated with adiabatic buoyancy fluxes as downgradient diffusion of buoyancy thickness, with a diffusivity in the semienclosed region of the Northern Hemisphere much larger than that in the ACC region. In the simple model, the disparity in diffusivities is necessary to obtain a substantial pole-to-pole ROC. The simple model also illustrates how the geometry of the isopycnals is shaped by the interhemispheric ROC, leading to three major thermostads, which the authors identify with the major water masses of the Atlantic: that is, North Atlantic Deep Water, Antarctic Intermediate Water, and Antarctic Bottom Water.

Lagrangian decomposition of the Deacon Cell

The meridional overturning cells in the Southern Ocean are decomposed by Lagrangian tracing using velocity and density fields simulated with an ocean general circulation model. Particular emphasis is given to the Deacon Cell. The flow is divided into four major components: (1) water circling around Antarctica in the Antarctic Circumpolar Current (ACC), (2) water leaving the ACC toward the north into the three world oceans, (3) water coming from the north and joining the ACC, mainly consisting of North Atlantic Deep Water (NADW), and (4) interocean exchange between the three world oceans without circling around Antarctica. The Deacon Cell has an amplitude of 20 Sv, of which 6 Sv can be explained by the the east‐west tilt of the ACC, 5 Sv by the east‐west tilt of the subtropical gyre, and the remaining 9 Sv by the differences of the slope and depth of the southward transport of NADW and its return flow as less dense water. The diabatic or cross‐isopycnal Deacon Cell is only 2 Sv.

On the fast response of the Southern Ocean to changes in the zonal wind

Abstract. Model studies of the Southern Ocean, reported here, show that the Antarctic Circumpolar Current responds within two days to changes in the zonal wind stress at the latitudes of Drake Passage. Further investigation shows that the response is primarily barotropic and that, as one might expect, it is controlled by topography. Analysis of the results show that the changes in the barotropic flow are sufficient to transfer the changed surface wind stress to the underlying topography and that during this initial phase baroclinic processes are not involved. The model results also show that the Deacon Cell responds to changes in the wind stress on the same rapid time scale. It is shown that the changes in the Deacon Cell can also be explained by the change in the barotropic velocity field, an increase in the zonal wind stress producing an increased northward flow in shallow regions and southward flow where the ocean is deep. This new explanation is unexpected as previously the Deacon Cell has been thought of as a baroclinic feature of the ocean. The results imply that where baroclinic processes do appear to be involved in either the zonal momentum balance of the Southern Ocean or the formation of the Deacon Cell, they are part of the long term baroclinic response of the ocean's density field to the changes in the barotropic flow.

Connection between the decadal variability in the Southern Ocean circulation and the Southern Annular Mode

Previous studies demonstrated the remarkable upward trend of the Southern Annular Mode (SAM) and Southern Ocean wind stress in association with anthropogenic forcing. An oceanic reanalysis data set is used to investigate the response of the circulation in the Southern Ocean to the decadal variability of SAM. Our results indicate the strengthening and the poleward shift of the northward Ekman velocity as well as the Ekman pumping rate, which led to a corresponding strengthening trend in the Deacon Cell. This strengthening, in turn, intensified the meridional density gradient and the tilting of the isopycnal surfaces. On the interannual time scale, the Antarctic Circumpolar Currents (ACC) transport exhibits a positive correlation with SAM index as seen separately in observations. However, there is no significant trend in the total transport of ACC. Possible reasons are discussed.

Adjoint Sensitivity of an Ocean General Circulation Model to Bottom Topography

Abstract Bottom topography, or more generally the geometry of the ocean basins, is an important ingredient in numerical ocean modeling. With the help of an adjoint model, it is shown that scalar diagnostics or objective functions in a coarse-resolution model, such as the transport through Drake Passage, the strength of the Atlantic Ocean meridional overturning circulation, the Deacon cell, and the meridional heat transport across 32°S, are sensitive to bottom topography as much as they are to surface boundary conditions. For example, adjoint topography sensitivities of the transport through Drake Passage are large in choke-point areas such as the Crozet–Kerguélen Plateau and south of New Zealand; the Atlantic meridional overturning circulation is sensitive to topography in the western boundary region of the North Atlantic Ocean and along the Scotland–Iceland Ridge. Many sensitivities are connected to steep topography and can be interpreted in terms of bottom form stress, that is, the product of bottom pressure and topography gradient. The adjoint sensitivities are found to agree with direct perturbation methods with deviations smaller than 30% for significant perturbations on time scales of 100 yr, so that the assumption of quasi linearity that is implicit in the adjoint method holds. The horizontal resolution of the numerical model affects the sensitivities to bottom topography, but large-scale patterns and the overall impact of changes in topography appear to be robust. The relative impact of changes in topography and surface boundary conditions on the model circulation is estimated by multiplying the adjoint sensitivities with assumed uncertainties. If the uncertainties are correlated in space, changing the surface boundary conditions has a larger impact on the scalar diagnostics than topography does, but the effects can locally be on the same order of magnitude if uncorrelated uncertainties are assumed. In either case, bottom topography variations within their prior uncertainties affect the solution of an ocean circulation model. To this extent, including topography in the control vector can be expected to compensate for identifiable model errors and, thus, to improve the solutions of estimation problems.

What Sets the Surface Eddy Mass Flux in the Southern Ocean?

Abstract The Ocean Circulation and Climate Advanced Modelling (OCCAM) global, eddy-permitting ocean general circulation model has been used to investigate the surface eddy mass flux in the Southern Ocean. The isopycnal eddy mass flux in the surface layer is almost uniformly poleward and scales well with the local Ekman transport. This seems at odds with other models and observations suggesting topographic localization of the eddy fluxes with locally, large rotational components. Integrated over the thermocline depth the eddy fluxes do show such topographic localization. The surface eddy mass flux is mainly a consequence of the intermittent deepening of the mixed layer with the seasonal cycle, which redistributes the Ekman transport over the stack of layers that eventually become ventilated. Baroclinic instability gives rise to much smaller eddy-induced transports. Independent of the framework in which the residual mean flow is analyzed (isopycnal or geometric), the eddy-induced transport that opposes the wind-driven Ekman flow only partially compensates the Deacon cell. The associated overturning cell is about 5 Sv (where 1 Sv ≡ 106 m3 s−1), responsible for a cancellation of the Deacon cell of 30%. In geometric coordinates, a strong signature (14 Sv) of the Deacon cell remains for the residual mean flow. Only after transformation to density coordinates is a further reduction with 10 Sv obtained. Zonal tilting of isopycnals makes along-isopycnal recirculations appear as vertical overturning cells in geometric coordinates. These cells disappear in the isopycnal framework without any eddy-induced transport being involved.

Isopycnal Averaging at Constant Height. Part I: The Formulation and a Case Study

Abstract Simple Eulerian averaging of velocities, density, and tracers at constant position is the most natural way of averaging. However, Eulerian averaging gives incorrect watermass distributions and properties as well as spurious diabatic circulations such as the Deacon cell. Instead of averaging at constant height, averaging along isopycnals removes such fictitious mixing and diabatic circulations. Such isopycnal averaging is normally performed at constant latitude, that is, averaging along isopynals as they heave up and down. As a result, height information is lost and the sea surface becomes much warmer (or lighter) than with simple Eulerian averaging. In fact, averaging can be performed along arbitrarily aligned surfaces. This study considers a particular case in which isopycnal averaging is performed at constant height. Thus, this new isopycnal averaging follows isopycnals as they meander horizontally at constant z. Height information is now retained at the cost of losing latitudinal information. The advantage of this averaging is that it avoids the problem of giving a surface that is too warm. Associated with this new isopycnal averaging, a “vertical” transport streamfunction in (ρ, z) space can be defined, in analogy to the conventional meridional overturning streamfunction in (y, ρ) space. Here in Part I, this constant-height isopycnal averaging is explained and illustrated in an idealized zonal channel model. In Part II the relationship between the two different isopycnal averagings and the Eulerian mean eddy flux divergence is explored.

The Baltic Haline Conveyor Belt or The Overturning Circulation and Mixing in the Baltic

A study of the water-mass circulation of the Baltic has been undertaken by making use of a three dimensional Baltic Sea model simulation. The saline water from the North Atlantic is traced through the Danish Sounds into the Baltic where it upwells and mixes with the fresh water inflow from the rivers forming a Baltic haline conveyor belt. The mixing of the saline water from the Great Belt and Oresund with the fresh water is investigated making use of overturning stream functions and Lagrangian trajectories. The overturning stream function was calculated as a function of four different vertical coordinates (depth, salinity, temperature and density) in order to understand the path of the water and where it upwells and mixes. Evidence of a fictive depth overturning cell similar to the Deacon Cell in the Southern Ocean was found in the Baltic proper corresponding to the gyre circulation around Gotland, which vanishes when the overturning stream function is projected on density layers. A Lagrangian trajectory study was performed to obtain a better view of the circulation and mixing of the saline and fresh waters. The residence time of the water masses in the Baltic is calculated to be 26-29 years and the Lagrangian dispersion reaches basin saturation after 5 years.

Eddy mass transport for the Southern Ocean in an eddy-permitting global ocean model

The eddy-induced mass transport is diagnosed for the Southern Ocean in an eddy-permitting global ocean model (OCCAM). The focus is on the transport by transient eddies in the deep ocean. The transport streamfunction is calculated in four different combinations of coordinate system. Depending on the coordinate system employed, the strength of transient eddy transport varies from 6 Sv meridional transport in latitude-density coordinates to 20 Sv across-streamline transport in streamline-depth coordinates. It is shown that transient eddies as well as standing eddies are necessary for cancelling the Deacon cell. In the Antarctic bottom water density layer, the major contribution of the transient eddies towards net equatorward transport occurs (a) as a strong transport over the narrow Drake Passage and (b) as a weaker but systematic transport over a broader region in the southeast Pacific where the Antarctic circumpolar current breaks up into multiple jets. In contrast, in the North Atlantic deep water density layer the net poleward eddy transport is spread out almost everywhere. This suggests that attention to eddies should not be restricted to places where the eddy transport has large magnitude.

Thermohaline and Wind Forcing of a Circumpolar Channel withBlocked Geostrophic Contours

Abstract The Antarctic Circumpolar Current is governed by unique dynamics. Because the latitude belt of Drake Passage is not zonally bounded by continents, the Sverdrup theory does not apply to the Antarctic Circumpolar Current. However, most of the geostrophic contours are blocked at Drake Passage, which provides an important dynamic constraint for the vorticity equation of the depth averaged flow. This study addresses the effects of thermohaline and wind forcing on the large-scale transport of a circumpolar current with blocked geostrophic contours. Various numerical experiments with three different idealized model geometries were conducted. Based on the results and theoretical arguments, the authors promote an indirect wind effect on the circumpolar current: while the direct effects of the wind in driving the circumpolar current through a vertical transfer of the applied wind stress are of minor importance, the wind does substantially influence the circumpolar current transport through its effects on the density field. This indirect wind effect is discussed in two steps. First, at the latitudes of the circumpolar current and longitudes where the geostrophic contours are blocked, the meridional gradient of the mass transport streamfunction is to leading order balanced by the meridional gradient of the baroclinic potential energy. This balance implies that the total transport is to leading order baroclinic and that the deep transport is small. For this statement, some theoretical arguments are offered. Second, a simplified analytical model is used to obtain the distribution of the baroclinic potential energy. Assuming an advective–diffusive balance for the densities in the deep downwelling northern branch of the Deacon cell, this model reproduces the qualitative dependence of the circumpolar current transport on the imposed wind and thermohaline forcing as well as on the turbulent diffusivities.

Chaotic and regular trajectories in the Antarctic Circumpolar Current

Methods from chaos theory are applied to the analysis of the circulation in the Southern Ocean, using velocity fields produced by a realistic global ocean model. We plot the intersections ofindividual trajectories encircling Antarctica with a vertical plane in the Drake passage. Thisso-called Poincare section shows a drastic difference between regular trajectories in a coreregion of the Antarctic Circumpolar Current (ACC), and chaotic, mixing trajectories in thesurrounding region. It also shows that there is a region with overturning circulation of approximately3.5 Sv in the ACC, with downwelling on the northern side and upwelling on the southernside, which may be related to the Deacon cell.