Antarctic Circumpolar Current Transport Research Articles

Interbasin Differences in Interannual Variations of the Antarctic Circumpolar Current Transport

AbstractDespite the critical role of the Antarctic Circumpolar Current (ACC) in global convey belt, its temporal variability and spatial difference are poorly investigated due to the lack of full‐depth observations. This study quantitatively investigates the interannual variability of the ACC transport in different basins using Gravity Recovery and Climate Experiment (GRACE) ocean bottom pressure (OBP) data and In Situ Analysis System 17 (ISAS17) temperature and salinity fields. The results reveal that the interannual variation differs significantly in the Indian basin, the Pacific and Atlantic basins, and is likely caused by the meridional distribution of the wind stress curl, rather than the westerly winds. The interannual variation is correlated with the Southern Annular Mode (SAM) in the Indian basin, but also modulated by El Niño–Southern Oscillation (ENSO) in the Pacific and Atlantic basins. The results are further verified by a mass conservation ocean model with elaborating the important influence of both local and global atmospheric variability on the spatial‐temporal variability of the ACC. The difference of the ACC transports in the major basins might be ascribed to the respective meridional transport in each basin.

Lee Waves Break Eddy Saturation of the Antarctic Circumpolar Current

AbstractEddy‐resolving ocean models suggest that the transport of the Antarctic Circumpolar Current (ACC) may be insensitive to increasing wind. This insensitivity is due to eddies that flatten the isopycnals and compensate for their wind‐driven steepening. However, the eddy‐resolving models do not accurately represent the eddy dissipation processes that occur at scales smaller than the model resolution, including lee wave generation at rough topography. Using a lee wave parameterization in an idealized model of the Southern Ocean, we show that the ACC transport becomes more sensitive to wind when the lee wave drag is included. The sensitivity arises from the dependence of the lee wave drag on the bottom stratification. When the bottom stratification increases in response to wind, it increases the lee wave generation, and hence the eddy dissipation, at rough topography. As a result, the ACC shear (baroclinic transport) increases to drive stronger eddy generation to compensate.

“Eddy” Saturation of the Antarctic Circumpolar Current by Standing Waves

Abstract It is now well established that changes in the zonal wind stress over the Antarctic Circumpolar Current (ACC) do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation.” Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes in the efficiency with which transient eddies transfer momentum to the sea floor; on the other extreme, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates for changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that eddy saturation primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasigeostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance.

Optimally growing initial error for predicting the sudden shift in the Antarctic Circumpolar Current transport and its application to targeted observation

Optimally growing initial error for predicting the sudden shift in the Antarctic Circumpolar Current transport and its application to targeted observation

The Impact of Southern Ocean Topographic Barriers on the Ocean Circulation and the Overlying Atmosphere

Abstract Southern Ocean bathymetry constrains the path of the Antarctic Circumpolar Current (ACC), but the bathymetric influence on the coupled ocean–atmosphere system is poorly understood. Here, we investigate this impact by respectively flattening large topographic barriers around the Kerguelen Plateau, Campbell Plateau, Mid-Atlantic Ridge, and Drake Passage in four simulations in a coupled climate model. The barriers impact both the wind and buoyancy forcing of the ACC transport, which increases by between 4% and 14% when barriers are removed individually and by 56% when all barriers are removed simultaneously. The removal of Kerguelen Plateau bathymetry increases convection south of the plateau and the removal of Drake Passage bathymetry reduces convection upstream in the Ross Sea. When the barriers are removed, zonal flattening of the currents leads to sea surface temperature (SST) anomalies that strongly correlate to precipitation anomalies, with correlation coefficients ranging between r = 0.92 and r = 0.97 in the four experiments. The SST anomalies correlate to the surface winds too in some locations. However, they also generate circumpolar waves of sea level pressure (SLP) anomalies, which induce remote wind speed changes that are unconnected to the underlying SST field. The meridional variability in the wind stress curl contours over the Mid-Atlantic Ridge, the Kerguelen Plateau, and the Campbell Plateau disappears when these barriers are removed, confirming the impact of bathymetry on surface winds. However, bathymetry-induced wind changes are too small to affect the overall wave-3 asymmetry in the Southern Hemisphere westerlies. Removal of Southern Hemisphere orography is also inconsequential to the wave-3 pattern. Significance Statement Several studies in the past have pointed to the controlling effect of bathymetry on currents in the Southern Ocean circulation, but a clear idea of the importance of the major topographic barriers in the Southern Ocean is lacking. By removing these barriers systematically in a coupled climate model, we can evaluate their impact on several important components of the climate system, such as the Antarctic Circumpolar Current (ACC) pathways and strength, Antarctic Bottom Water formation, sea surface temperature, overlying winds, air–sea fluxes, and even precipitation. This helps in our understanding of what controls the pathways of the ACC and how much it matters for climate.

Changes in Global Ocean Circulation due to Isopycnal Diffusion

Abstract We investigate changes in the ocean circulation due to the variation of isopycnal diffusivity (κiso) in a global non-eddy-resolving model. Although isopycnal diffusion is thought to have minor effects on interior density gradients, the model circulation shows a surprisingly large sensitivity to the changes: with increasing κiso, the strength of the Atlantic residual overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) transport weaken. At high latitudes, the isopycnal diffusion diffuses temperature and salinity upward and poleward, and at low latitudes downward close to the surface. Increasing isopycnal diffusivity increases the meridional isopycnal fluxes whose meridional gradient is equatorward, hence leading to a negative contribution to the flux divergence in the tracer equations and predominant cooling and freshening equatorward of 40°. The effect on temperature overcompensates the countering effect of salinity diffusion, such that the meridional density differences decrease, along with which ACC and AMOC decrease. We diagnose the adjustment process to the new equilibrium with increased isopycnal diffusion to assess how the other terms in the tracer equations react to the increased κiso. It reveals that around ±40° latitude, the cooling induced by the increased isopycnal flux is only partly compensated by warming by advection, explaining the net cooling. Overall, the results emphasize the importance of isopycnal diffusion on ocean circulation and dynamics, and hence the necessity of its careful representation in models. Significance Statement The effect of mixing by mesoscale eddies, represented as diffusion along surfaces of constant density in models, on the ocean circulation is not well understood. Here, we show that an increase in the eddy diffusivity in different setups of a global ocean model leads to a surprisingly large change of the ocean circulation. The strength of the Atlantic overturning circulation and the Antarctic Circumpolar Current decrease. We find that the interior ocean becomes cooler and fresher and that the temperature effect on density dominates over salinity, resulting in a decrease in the density gradients. Our results point out the importance of eddy diffusion on ocean circulation, and hence the necessity of its correct representation in ocean and climate models.

The Coupled Atmosphere–Ocean Response to Antarctic Sea Ice Loss

Abstract Antarctic sea ice is projected to decrease in response to increasing greenhouse gas concentrations. Limited studies so far have examined the coupled atmosphere–ocean response to Antarctic sea ice loss. Here, we isolate the response to Antarctic sea ice loss in the atmosphere and ocean using bespoke sea ice albedo perturbation experiments with HadGEM3-GC3.1-LL, provide the first detailed examination of the global ocean response, and quantify the importance of atmosphere–ocean coupling, through comparison to uncoupled experiments with prescribed Antarctic sea ice loss. Lower-tropospheric warming and moistening over regions of sea ice loss and the nearby Southern Ocean are simulated in both coupled and uncoupled configurations but are of greater magnitude in the coupled model. A weakening and equatorward shift of the tropospheric westerly jet are simulated in both configurations, but are also larger in the coupled model. Ocean coupling allows the warming response to spread northward, and by poleward atmospheric energy transport, back to the Antarctic interior. Warmer tropical sea surface temperatures enhance atmospheric convection, driving upper-tropospheric warming and triggering atmospheric teleconnections to the extratropics, including a weakened Aleutian low. A 20% reduction in Antarctic Circumpolar Current transport and a weakening of the shallow tropical convergence cell are simulated. Surface waters warm and freshen globally, becoming more stratified and stable in the Southern Ocean, with similar changes, but of lesser magnitude, in the Arctic Ocean, where sea ice declines. Our results suggest that the climate effects of Antarctic sea ice loss stretch from pole to pole and from the heights of the tropical troposphere to the depths of the Southern Ocean.

Acute Sensitivity of Global Ocean Circulation and Heat Content to Eddy Energy Dissipation Timescale

AbstractThe global ocean overturning circulation, critically dependent on the global density stratification, plays a central role in regulating climate evolution. While it is well known that the global stratification profile exhibits a strong dependence to Southern Ocean dynamics and in particular to wind and buoyancy forcing, we demonstrate here that the stratification is also acutely sensitive to the mesoscale eddy energy dissipation timescale. Within the context of a global ocean circulation model with an energy constrained mesoscale eddy parameterization, it is shown that modest variations in the eddy energy dissipation timescale lead to significant variations in key metrics relating to ocean circulation, namely the Antarctic Circumpolar Current transport, Atlantic Meridional Overturning Circulation strength, and global ocean heat content, over long timescales. The results highlight a need to constrain uncertainties associated with eddy energy dissipation for climate model projections over centennial timescales and also for paleoclimate simulations over millennial timescales.

Optimal Precursors Triggering Sudden Shifts in the Antarctic Circumpolar Current Transport Through Drake Passage

AbstractDue to the high nonlinearity, the accurate prediction of the Antarctic circumpolar current (ACC) transport is still challenging. Using the eddy‐permitting regional ocean modeling system and the conditional nonlinear optimal perturbation approach, the sudden shift in the ACC transport through Drake Passage (DP) is investigated by exploring its optimal precursor (OPR). Here, the sudden shift in the ACC transport is defined as a fluctuation exceeding double STD (∼16 Sv; 1 Sv = 106 m3 s−1) within 30 days. The results indicate that the OPRs for all three cases exhibit specific structures in the middle DP (58°–62°S, 72°–64°W) at the depth of 1,000–3,000 m, implying that the ACC transport is most sensitive to the initial perturbations there. The OPRs' evolutions show similar features: the OPR for each case triggers an eddy‐like dipole perturbation with a northern cyclone and a southern anticyclone, leading to a sudden reduction in the ACC transport of ∼40 Sv. It is verified that the density components in the OPRs determine such evolution processes. Furthermore, baroclinic instability is diagnosed to play a dominant role in the development of OPRs by analyzing the perturbed kinetic energy budget. This study suggests that the deep‐layer density perturbations in the Southern Ocean should be carefully monitored when considering the short‐range prediction of the ACC transport.

Testing Methods for Reconstructing Glacial Antarctic Circumpolar Current Transport in an Isotope‐Enabled Climate Model

AbstractThe Antarctic Circumpolar Current (ACC) plays a vital role in the interbasin exchange of ocean properties. A robust method to reconstruct the ACC baroclinic transport is helpful to assess the ACC's sensitivity to a changed climate. Here we test the reconstruction methods at the Last Glacial Maximum (LGM; ∼20 ka) using end‐member water masses in a fully coupled, isotope‐enabled Community Earth System Model. Model results suggest that the density profile at the northern side of ocean margins across the ACC can be reconstructed well from end‐member water masses of Subtropical Surface Water (STSW), Antarctic Intermediate Water (AAIW), and Lower Circumpolar Deep Water. One additional pore fluid observation at 1,000 m can substantially improve transport reconstruction and is essential to constrain the sign of change in ACC transport during the LGM. Moreover, the uncertainty in transport calculation is large when salinities for STSW and AAIW are reconstructed independently based on the δ18Osw‐Salinity relationship of surface and intermediate waters in the South Indian Ocean. More direct measurements of LGM temperature and salinity may allow better transport reconstruction.

Southern Ocean polynyas in CMIP6 models

Abstract. Polynyas facilitate air–sea fluxes, impacting climate-relevant properties such as sea ice formation and deep water production. Despite their importance, polynyas have been poorly represented in past generations of climate models. Here we present a method to track the presence, frequency and spatial distribution of polynyas in the Southern Ocean in 27 models participating in the Climate Model Intercomparison Project Phase 6 (CMIP6) and two satellite-based sea ice products. Only half of the 27 models form open-water polynyas (OWPs), and most underestimate their area. As in satellite observations, three models show episodes of high OWP activity separated by decades of no OWP, while other models unrealistically create OWPs nearly every year. In contrast, the coastal polynya area is overestimated in most models, with the least accurate representations occurring in the models with the coarsest horizontal resolution. We show that the presence or absence of OWPs is linked to changes in the regional hydrography, specifically the linkages between polynya activity with deep water convection and/or the shoaling of the upper water column thermocline. Models with an accurate Antarctic Circumpolar Current transport and wind stress curl have too frequent OWPs. Biases in polynya representation continue to exist in climate models, which has an impact on the regional ocean circulation and ventilation that should be addressed. However, emerging iceberg discharge schemes, more adequate vertical grid type or overflow parameterisation are anticipated to improve polynya representations and associated climate prediction in the future.

Untangling local and remote influences in two major petrel habitats in the oligotrophic Southern Ocean.

Ocean circulation connects geographically distinct ecosystems across a wide range of spatial and temporal scales via exchanges of physical and biogeochemical properties. Remote oceanographic processes can be especially important for ecosystems in the Southern Ocean, where the Antarctic Circumpolar Current transports properties across ocean basins through both advection and mixing. Recent tracking studies have indicated the existence of two large-scale, open ocean habitats in the Southern Ocean used by grey petrels (Procellaria cinerea) from two populations (i.e., Kerguelen and Antipodes islands) during their nonbreeding season for extended periods during austral summer (i.e., October to February). In this work, we use a novel combination of large-scale oceanographic observations, surface drifter data, satellite-derived primary productivity, numerical adjoint sensitivity experiments, and output from a biogeochemical state estimate to examine local and remote influences on these grey petrel habitats. Our aim is to understand the oceanographic features that control these isolated foraging areas and to evaluate their ecological value as oligotrophic open ocean habitats. We estimate the minimum local primary productivity required to support these populations to be much <1% of the estimated local primary productivity. The region in the southeast Indian Ocean used by the birds from Kerguelen is connected by circulation to the productive Kerguelen shelf. In contrast, the region in the south-central Pacific Ocean used by seabirds from the Antipodes is relatively isolated suggesting it is more influenced by local factors or the cumulative effects of many seasonal cycles. This work exemplifies the potential use of predator distributions and oceanographic data to highlight areas of the open ocean that may be more dynamic and productive than previously thought. Our results highlight the need to consider advective connections between ecosystems in the Southern Ocean and to re-evaluate the ecological relevance of oligotrophic Southern Ocean regions from a conservation perspective.

Effects of Buoyancy and Wind Forcing on Southern Ocean Climate Change

AbstractObservations show that since the 1950s, the Southern Ocean has stored a large amount of anthropogenic heat and has freshened at the surface. These patterns can be attributed to two components of surface forcing: poleward-intensified westerly winds and increased buoyancy flux from freshwater and heat. Here we separate the effects of these two forcing components by using a novel partial-coupling technique. We show that buoyancy forcing dominates the overall response in the temperature and salinity structure of the Southern Ocean. Wind stress change results in changes in subsurface temperature and salinity that are closely related to intensified residual meridional overturning circulation. As an important result, we show that buoyancy and wind forcing result in opposing changes in salinity: the wind-induced surface salinity increase due to upwelling of saltier subsurface water offsets surface freshening due to amplification of the global hydrological cycle. Buoyancy and wind forcing further lead to different vertical structures of Antarctic Circumpolar Current (ACC) transport change; buoyancy forcing causes an ACC transport increase (3.1 ± 1.6 Sv; 1 Sv ≡ 106m3s−1) by increasing the meridional density gradient across the ACC in the upper 2000 m, while the wind-induced response is more barotropic, with the whole column transport increased by 8.7 ± 2.3 Sv. While previous research focused on the wind effect on ACC intensity, we show that surface horizontal current acceleration within the ACC is dominated by buoyancy forcing. These results shed light on how the Southern Ocean might change under global warming, contributing to more reliable future projections.

Response of the Pacific Sector of the Southern Ocean to Wind Stress Variability From 1995 to 2017

AbstractDrifter, satellite, expendable bathythermograph (XBT), and Argo float data are used to study the response of the Pacific Sector of the Southern Ocean (PSSO) to the wind stress field in the period 1995–2017, in terms of eddy field, water mass transport, and heat fluxes at large and regional scales. Increasing wind stress over the PSSO in those two decades led to a growth of the Eddy Kinetic Energy (EKE) in the region of the Antarctic Circumpolar Current (ACC). Increases of the EKE occur with delays of 1–4 years with respect to peaks in the zonal component of the wind stress. The persistent ACC meander located south of New Zealand (between 150°E and 180°W and 50°S to 66°S) responds to the interannual wind variations earlier than the entire ACC branch in the PSSO. In the same area, an estimate of the ACC transport based on in situ data shows interannual variability but no significant decadal trend over the study period. The effects of the EKE variability on the meridional eddy heat fluxes are significant on interannual scales. The strengthening of the EKE field leads to a local increase in the poleward meridional eddy heat fluxes in the PSSO, especially in the ACC band. The weakening of the EKE field defines an area of equatorward meridional eddy heat fluxes in the middle of the PSSO (south of 40°S, between 130°W and 160°W) and prevalent poleward fluxes in the further western and eastern regions.

Climate Response to Increasing Antarctic Iceberg and Ice Shelf Melt

AbstractMass loss from the Antarctic continent is increasing; however, climate models either assume a constant mass loss rate or return snowfall over land to the ocean to maintain equilibrium. Numerous studies have investigated sea ice and ocean sensitivity to this assumption and reached different conclusions, possibly due to different representations of melt fluxes. The coupled atmosphere–land–ocean–sea ice model, HadGEM3-GC3.1, includes a realistic spatial distribution of coastal melt fluxes, a new ice shelf cavity parameterization, and explicit representation of icebergs. This configuration makes it appropriate to revisit how increasing melt fluxes influence ocean and sea ice and to assess whether responses to melt from ice shelves and icebergs are distinguishable. We present results from simulated scenarios of increasing meltwater fluxes and show that these drive sea ice increases and, for increasing ice shelf melt, a decline in Antarctic Bottom Water formation. In our experiments, the mixed layer around the Antarctic coast deepens in response to rising ice shelf meltwater and shallows in response to stratification driven by iceberg melt. We find similar surface temperature and salinity responses to increasing meltwater fluxes from ice shelves and icebergs, but midlayer waters warm to greater depths and farther north when ice shelf melt is present. We show that as meltwater fluxes increase, snowfall becomes more likely at lower latitudes and Antarctic Circumpolar Current transport declines. These insights are helpful for interpretation of climate simulations that assume constant mass loss rates and demonstrate the importance of representing increasing melt rates for both ice shelves and icebergs.

Representation of Southern Ocean Properties across Coupled Model Intercomparison Project Generations: CMIP3 to CMIP6

AbstractThe air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.

Antarctic Circumpolar Current Transport Through Drake Passage: What Can We Learn From Comparing High‐Resolution Model Results to Observations?

AbstractUncertainty exists in the time‐mean total transport of the Antarctic Circumpolar Current (ACC), the world's strongest ocean current. The two most recent observational programs in Drake Passage, DRAKE and cDrake, yielded transports of 141 and 173.3 Sv, respectively. In this paper, we use a realistic 1/12° global ocean simulation to interpret these observational estimates and reconcile their differences. We first show that the modeled ACC transport in the upper 1,000 m is in excellent agreement with repeat shipboard acoustic Doppler current profiler (SADCP) transects and that the exponentially decaying transport profile in the model is consistent with the profile derived from repeat hydrographic data. By further comparing the model results to the cDrake and DRAKE observations, we argue that the modeled 157.3 Sv transport, that is, approximately the average of the cDrake and DRAKE estimates, is actually representative of the time‐mean ACC transport through the Drake Passage. The cDrake experiment overestimated the barotropic contribution in part because the array undersampled the deep recirculation southwest of the Shackleton Fracture Zone, whereas the surface geostrophic currents used in the DRAKE estimate yielded a weaker near‐surface transport than implied by the SADCP data. We also find that the modeled baroclinic and barotropic transports are not correlated; thus, monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport.

Twenty-five years of Mercator ocean reanalysis GLORYS12 at Drake Passage: Velocity assessment and total volume transport

Velocities in Drake Passage from the 25-year GLORYS12 Mercator Ocean reanalysis were compared with satellite altimetry-derived surface velocities and independent in-situ velocity measurements from the DRAKE (2006–2009) and cDrake (2007–2011) experiments. GLORYS12 assimilates satellite along-track sea level anomalies and, as expected, model velocities are in rather good agreement with gridded altimetry derived velocities (r ~ 0.5, above 99% confidence level). GLORYS12 velocities also compared well with the current-meter data from the DRAKE (in the water column) and cDrake (50 m above seafloor) experiments in terms of means and standard deviations; most correlations between the reanalysis and directly measured velocity time series were significant (r ~ 0.5 for DRAKE and r ~ 0.4 for cDrake, above 99% confidence level). We used the GLORYS12 reanalysis to examine the Antarctic Circumpolar Current (ACC) total volume transport across three sections in Drake Passage. The 25 years of the ACC total volume transport has a mean of 155 ± 3 Sv and a standard deviation of 6.7 Sv. Annual mean values span over a range of 12 Sv. The spectrum of the GLORYS12 total transport time series shows energy peaks (above 95% confidence level) at the intraseasonal, semi-annual and annual periods and no significant low-frequency variations (beyond 2 years) or trend, in agreement with previous studies. This first assessment of GLORYS12 velocities in Drake Passage is very encouraging for monitoring the regional variability of the ACC.

Driving mechanisms of the variability and long-term trend of the Brazil–Malvinas confluence during the 21st century

Over the last decade, several studies have identified a southward drift of the mean meridional position of the Brazil–Malvinas confluence (BMC). Although this trend has been ascribed to different mechanisms, the most recent study found a reduction of the Malvinas current (MC) as the main reason behind it. It is unclear, however, how this mechanism would persist in the face of global warming and under projected increased winds over the Southern Ocean, as the response of the Antarctic Circumpolar Current (ACC) varies within different Earth System Models (ESM), ultimately impacting the MC. We ran a high-resolution ( $$1/12^{\circ }$$ ) ocean model driven with results from the Max-Planck-Institute–ESM to verify how the confluence will respond to anthropogenic climate change, by downscaling results from the pre-industrial control and RCP8.5 scenarios. Our results show that the southward confluence shift is only persistent under anthropogenic forcing and is led by a reduction of the MC volume transport. This reduction of the Malvinas transport is induced by a shift of the main ACC flow closer to Antarctica’s shelf, in response to a southward movement of the westerlies band, even if no long-term changes on the total ACC transport can be found. Our results corroborate previous evidence regarding the MC as the main responsible behind the observed BMC southward shift in the recent past but points toward anthropogenic climate change as the triggering mechanism, with various effects cascading from its impact on the Southern Ocean. This also has consequences for the BMC variability, whose amplitude reduces as we approach the end of the 21st century.

Assessing the Quality of Southern Ocean Circulation in CMIP5 AOGCM and Earth System Model Simulations

Abstract The Southern Ocean (SO) is vital to Earth’s climate system due to its dominant role in exchanging carbon and heat between the ocean and atmosphere and transforming water masses. Evaluating the ability of fully coupled climate models to accurately simulate SO circulation and properties is crucial for building confidence in model projections and advancing model fidelity. By analyzing multiple biases collectively across large model ensembles, physical mechanisms governing the diverse mean-state SO circulation found across models can be identified. This analysis 1) assesses the ability of a large ensemble of models contributed to phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate observationally based metrics associated with an accurate representation of the Antarctic Circumpolar Current (ACC), and 2) presents a framework by which the quality of the simulation can be categorized and mechanisms governing the resulting circulation can be deduced. Different combinations of biases in critical metrics including the magnitude and position of the zonally averaged westerly wind stress maximum, wind-driven surface divergence, surface buoyancy fluxes, and properties and transport of North Atlantic Deep Water entering the SO produce distinct mean-state ACC transports. Relative to CMIP3, the quality of the CMIP5 SO simulations has improved. Eight of the thirty-one models simulate an ACC within observational uncertainty (2σ) for approximately the right reasons; that is, the models achieve accuracy in the surface wind stress forcing and the representation of the difference in the meridional density across the current. Improved observations allow for a better assessment of the SO circulation and its properties.