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The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen and nutrients throughout the world ocean1,2. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes2–4, yet it is unclear what processes have driven this warming, and whether this warming is linked to a slowdown in the ocean’s overturning circulation. Furthermore, attributing change to specific drivers is difficult owing to limited measurements, and because coupled climate models exhibit biases in the region5–7. In addition, future change remains uncertain, with the latest coordinated climate model projections not accounting for dynamic ice-sheet melt. Here we use a transient forced high-resolution coupled ocean–sea-ice model to show that under a high-emissions scenario, abyssal warming is set to accelerate over the next 30 years. We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements. In contrast, projected wind and thermal forcing has little impact on the properties, age and volume of AABW. These results highlight the critical importance of Antarctic meltwater in setting the abyssal ocean overturning, with implications for global ocean biogeochemistry and climate that could last for centuries.

Antarctic Bottom Water (AABW) formation and transport constitute a key component of the global ocean circulation. Direct observations suggest that AABW volumes and transport rates may be decreasing, but these observations are too temporally or spatially sparse to determine the cause. To address this problem, we develop a new method to reconstruct AABW transport variability using data from the GRACE (Gravity Recovery and Climate Experiment) satellite mission. We use an ocean general circulation model to investigate the relationship between ocean bottom pressure and AABW: we calculate both of these quantities in the model, and link them using a regularized linear regression. Our reconstruction from modeled ocean bottom pressure can capture 65%–90% of modeled AABW transport variability, depending on the ocean basin. When realistic observational uncertainty values are added to the modeled ocean bottom pressure, the reconstruction can still capture 30%–80% of AABW transport variability. Using the same regression values, the reconstruction skill is within the same range in a second, independent, general circulation model. We conclude that our reconstruction method is not unique to the model in which it was developed and can be applied to GRACE satellite observations of ocean bottom pressure. These advances allow us to create the first global reconstruction of AABW transport variability over the satellite era. Our reconstruction provides information on the interannual variability of AABW transport, but more accurate observations are needed to discern AABW transport trends. Ocean circulation moves heat and carbon around the globe. Changes in the way this circulation moves heat and carbon influence future climate. One part of this ocean circulation is Antarctic Bottom Water, which forms around Antarctica and flows north along the ocean floor into the Pacific, Atlantic and Indian Oceans. Observations of Antarctic Bottom Water are sparse. Those which exist suggest that the volume of Antarctic Bottom Water is declining, but are insufficient to explain why this is happening. We design a new method to try and measure Antarctic Bottom Water transport. The physical equations describing fluid flows suggest gravity signals measured by satellites might be useful. To establish how useful this data is, we simulate the observations of these satellites in an ocean model. We also calculate the transport of Antarctic Bottom Water in the model. This means we can investigate how effective the modeled satellite data is at measuring modeled Antarctic Bottom Water. Our method of using the satellite data skilfully measures Antarctic Bottom Water transport, so we use this method to calculate Antarctic Bottom Water from the real‐world satellite observations. We use estimates of ocean bottom pressure from the Gravity Recovery and Climate Experiment (GRACE) satellites as a proxy for Antarctic Bottom Water transportThe largest source of uncertainty in our reconstruction is satellite measurement uncertaintyWe reconstruct Antarctic Bottom Water transport anomalies, capturing an estimated 30%–80% of variance We use estimates of ocean bottom pressure from the Gravity Recovery and Climate Experiment (GRACE) satellites as a proxy for Antarctic Bottom Water transport The largest source of uncertainty in our reconstruction is satellite measurement uncertainty We reconstruct Antarctic Bottom Water transport anomalies, capturing an estimated 30%–80% of variance

A holistic review is given of the Southern Ocean dynamic system, in the context of the crucial role it plays in the global climate and the profound changes it is experiencing. The review focuses on connections between different components of the Southern Ocean dynamic system, drawing together contemporary perspectives from different research communities, with the objective of closing loops in our understanding of the complex network of feedbacks in the overall system. The review is targeted at researchers in Southern Ocean physical science with the ambition of broadening their knowledge beyond their specific field, and aims at facilitating better‐informed interdisciplinary collaborations. For the purposes of this review, the Southern Ocean dynamic system is divided into four main components: large‐scale circulation; cryosphere; turbulence; and gravity waves. Overviews are given of the key dynamical phenomena for each component, before describing the linkages between the components. The reviews are complemented by an overview of observed Southern Ocean trends and future climate projections. Priority research areas are identified to close remaining loops in our understanding of the Southern Ocean system. The United Nations has identified 2021–2030 as the Decade of Ocean Science, with a goal to improve predictions of ocean and climate change. Improved understanding of the Southern Ocean is crucial to this effort, as it is the central hub of the global ocean. The Southern Ocean is the formation site for much of the dense water that fills the deep ocean, sequesters the majority of anthropogenic heat and carbon, and controls the flux of heat to Antarctica. The large‐scale circulation of the Southern Ocean is strongly influenced by interactions with sea ice and ice shelves, and is mediated by smaller scale processes, including eddies, waves, and mixing. The complex interplay between these dynamic processes remains poorly understood, limiting our ability to understand, model and predict changes to the Southern Ocean, global climate and sea level. This article provides a holistic review of Southern Ocean processes, connecting the smallest scales of ocean mixing to the global circulation and climate. It seeks to develop a common language and knowledge‐base across the Southern Ocean physical science community to facilitate knowledge‐sharing and collaboration, with the aim of closing loops on our understanding of one of the world's most dynamic regions. Contemporary perspectives are reviewed on the different components of the Southern Ocean dynamic system from distinct research communitiesKey connections between different components of Southern Ocean dynamics are highlightedCross‐cutting priorities for future Southern Ocean physical science are identified Contemporary perspectives are reviewed on the different components of the Southern Ocean dynamic system from distinct research communities Key connections between different components of Southern Ocean dynamics are highlighted Cross‐cutting priorities for future Southern Ocean physical science are identified

Ocean ventilation is the transfer of tracers and young water from the surface down into the ocean interior. The tracers that can be transported to depth include anthropogenic heat and carbon, both of which are critical to understanding future climate trajectories. Ventilation occurs in both high- and midlatitude regions, but it is the southern midlatitudes that are responsible for the largest fraction of anthropogenic heat and carbon uptake; such Southern Ocean ventilation is the focus of this review. Southern Ocean ventilation occurs through a chain of interconnected mechanisms, including the zonally averaged meridional overturning circulation, localized subduction, eddy-driven mixing along isopycnals, and lateral transport by subtropical gyres. To unravel the complex pathways of ventilation and reconcile conflicting results, here we assess the relative contribution of each of thesemechanisms, emphasizing the three-dimensional and temporally varying nature of the ventilation of the Southern Ocean pycnocline. We conclude that Southern Ocean ventilation depends on multiple processes and that simplified frameworks that explain ventilation changes through a single process are insufficient.

Oceanic mesoscale eddies play a profound role in mixing tracers such as heat, carbon and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how the eddy field has varied over the past few decades. Furthermore, climate model predictions generally do not resolve mesoscale eddies, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of ocean eddy activity using two independent observational datasets of surface mesoscale eddy variability (one estimates surface currents, and the other is derived from sea surface temperature). Maps of mesoscale variability trends show heterogeneous patterns, with eddy-rich regions showing a significant increase in mesoscale variability of 2–5% per decade, while the tropical oceans show a decrease in mesoscale variability. This readjustment of the surface mesoscale ocean circulation has important implications for the exchange of heat and carbon between the ocean and atmosphere.

The storage of anomalous heat and carbon in the Southern Ocean in response to increasing greenhouse gases greatly mitigates atmospheric warming and exerts a large impact on the marine ecosystem. However, the mechanisms driving the ocean storage patterns are uncertain. Here using recent hydrographic observations, we compare for the first time the spatial patterns of heat and carbon storage, which show substantial differences in the Southern Ocean, in contrast with the conventional view of simple passive subduction into the thermocline. Using an eddy‐rich global climate model, we demonstrate that redistribution of the preindustrial temperature field is the dominant control on the heat storage pattern, whereas carbon storage largely results from passive transport of anthropogenic carbon uptake at the surface. Lastly, this study highlights the importance of realistic representation of wind and surface buoyancy flux in climate models to improve future projection of circulation change and thus heat and carbon storage. Observed excess heat (Q′) and anthropogenic carbon storage in the Southern Ocean shows contrasting spatial patternsEddy‐resolving model reveals that circulation change controls Q′storage; carbon storage anomaly (C′) behaves more like a passive tracerA responsive surface buoyancy flux results in a greater sensitivity of net circulation change in response to wind stress change

Antarctica has long been considered biologically isolated1. Global warming will make parts of Antarctica more habitable for invasive taxa, yet presumed barriers to dispersal—especially the Southern Ocean’s strong, circumpolar winds, ocean currents and fronts—have been thought to protect the region from non-anthropogenic colonizations from the north1,2. We combine molecular and oceanographic tools to directly test for biological dispersal across the Southern Ocean. Genomic analyses reveal that rafting keystone kelps recently travelled >20,000 km and crossed several ocean-front ‘barriers’ to reach Antarctica from mid-latitude source populations. High-resolution ocean circulation models, incorporating both mesoscale eddies and wave-driven Stokes drift, indicate that such Antarctic incursions are remarkably frequent and rapid. Our results demonstrate that storm-forced surface waves and ocean eddies can dramatically enhance oceanographic connectivity for drift particles in surface layers, and show that Antarctica is not biologically isolated. We infer that Antarctica’s long-standing ecological differences have been the result of environmental extremes that have precluded the establishment of temperate-adapted taxa, but that such taxa nonetheless frequently disperse to the region. Global warming thus has the potential to allow the establishment of diverse new species—including keystone kelps that would drastically alter ecosystem dynamics—even without anthropogenic introductions.

The Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current. The Antarctic Slope Current is a narrow ocean current that travels around Antarctica following the continental slope. It separates the shallow and cold continental shelf from much warmer waters in the open ocean. Intrusions of the relatively warm water across the continental slope impacts melting of Antarctic ice shelves and global sea level rise. Understanding what controls the strength and variability of the Antarctic Slope Current is therefore important. The Antarctic Slope Current usually has the largest velocities near the surface, but there are regions where there is also a strong bottom flow. We use a coupled ocean‐sea ice model and show that the bottom flow is controlled by the export of very dense shelf water that flows down the continental slope in a few locations around the continent. However, the bottom and surface flow does not vary together on interannual time scales which tells us that the two components of the Antarctic Slope Current are largely independent. The result is important as the formation of dense shelf water is expected to reduce in the future which will impact the deep flow, but not the surface component. Simulations reveal a close spatial relationship between the bottom‐intensified Antarctic Slope Current and Dense Shelf Water exportInterannual variability of Dense Shelf Water is reflected in the bottom speed of the bottom‐intensified Antarctic Slope CurrentThe surface component varies independently from the bottom‐intensified flow implying two distinct, co‐located currents Simulations reveal a close spatial relationship between the bottom‐intensified Antarctic Slope Current and Dense Shelf Water export Interannual variability of Dense Shelf Water is reflected in the bottom speed of the bottom‐intensified Antarctic Slope Current The surface component varies independently from the bottom‐intensified flow implying two distinct, co‐located currents

Recent ice loss on the western Antarctic Peninsula has been driven by warming ocean waters on the continental shelf. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. High‐resolution ocean model simulations show that the temperature variability along the western Antarctic Peninsula is controlled by the rate of dense water formation in the Weddell Sea. Passive tracer advection reveals connectivity between the Weddell Sea and the coastline of the western Antarctic Peninsula and Bellingshausen Sea. During multi‐year periods of weak Weddell dense water formation, dense overflow transport in the Weddell Sea decreases, while the transport of cold water around the tip of the Antarctic Peninsula strengthens, driving a temperature decrease of 0.4°C along the western Antarctic Peninsula. This mechanism implies that western Antarctic Peninsula coastal ocean temperature may cool in the future if Weddell Dense Shelf Water production slows down. Melting of the ice sheet along the western Antarctic Peninsula has been driven by warming ocean waters that are in contact with the underside of the ice. It is therefore important that we understand what processes drive variation in ocean temperature. However, due to the short observational record, our understanding of the dynamics and variability in this region remains poor. Using a high‐resolution ocean model, we identify a new mechanism that controls the ocean temperature variability along the western Antarctic Peninsula that is linked to the formation of dense water to the east in the Weddell Sea. During years when dense water formation is weak in the Weddell Sea, there is an increased transport of cold waters westward around the tip of the Antarctic Peninsula that flood the coast of the western Antarctic Peninsula with cold waters. Conversely, when dense water formation is strong in the Weddell Sea, there is decreased inflow of cold waters and the ocean along the coast of the western Antarctic Peninsula warms. This mechanism implies that the ocean along the western Antarctic Peninsula may temporarily cool in the future if dense water formation in the Weddell Sea slows down. A high resolution ocean model reveals connectivity from the Weddell Sea to the western Antarctic Peninsula and Bellingshausen SeaWhen Weddell Sea dense water formation is weak, transport of cold water along the coastline of the western Antarctic Peninsula increasesThis mechanism controls the simulated decadal variability along the western Antarctic Peninsula, and cools coastal waters by 0.4°C A high resolution ocean model reveals connectivity from the Weddell Sea to the western Antarctic Peninsula and Bellingshausen Sea When Weddell Sea dense water formation is weak, transport of cold water along the coastline of the western Antarctic Peninsula increases This mechanism controls the simulated decadal variability along the western Antarctic Peninsula, and cools coastal waters by 0.4°C

In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high‐resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse‐resolution models, potentially affecting the skill of centennial scale climate change projections. In this paper we use a variety of ocean models to investigate how long it takes for deep ocean waters to upwell to the surface of the Southern Ocean around Antarctica. We track virtual particles in our simulated currents and show how they spiral southward and upward toward the surface. We find that this journey takes 87 years in a standard coarse‐resolution climate model but only 17 years in a state of the art high‐resolution climate model. We argue that the difference between the models is due to vortices which vigorously upwell the virtual particles but are too small to be represented in standard climate models. Particles also only loop around Antarctica 0.9 times in the high‐resolution model compared to 3.2 times in the coarse‐resolution model, suggesting that different kinds of upwelling waters have less time to mix with each other in coarse‐resolution models. These differences in timescale and the number of loops suggest that there exist biases in long‐term climate change projections using standard coarse‐resolution climate models. The timescales and 3‐D structure of Circumpolar Deep Water (CDW) upwelling are missing from the zonally integrated overturning frameworkLagrangian transit times of upwelling CDW are longer with coarser grid spacing or longer temporal averaging of the sampled velocity fieldAs horizontal model resolution increases, particles complete fewer circumpolar loops thereby limiting interbasin merging of CDW

Ice sheet melting into the Southern Ocean can change the formation and properties of the Antarctic Bottom Water (AABW). Ocean models often mimic ice sheet melting by adding freshwater fluxes in the Southern Ocean under diverse spatial distributions. We use a global ocean and sea‐ice model to explore whether the spatial distribution and magnitude of meltwater fluxes can alter AABW properties and formation. We find that a realistic spatially varying meltwater flux sustains AABW with higher salinities compared to simulations with uniform meltwater fluxes. Finally, we show that increases in ice sheet melting above 12% since 1958 can trigger AABW freshening rates similar to those observed in the Southern Ocean since 1990, suggesting that the increasing Antarctic meltwater discharge can drive the observed AABW freshening. Previous research suggests that increased Antarctic ice sheet (AIS) melting may be driving freshening in the abyssal Southern Ocean, and Antarctic sea ice expansion since 1978. However, the main tools we have to assess abyssal ocean changes are ocean models, and they often misrepresent Antarctic ice melting by assuming it occurs uniformly along the Antarctic coast. In this study, we use a global ocean model to assess if correcting the spatial distribution of Antarctic ice melting (from uniform to spatially varying), and increasing its magnitude, can change the salinity in the abyssal Southern Ocean. We show that correcting the spatial representation of the AIS melting results in abyssal waters with higher salinities, correcting 30% the ocean model fresh bias in the abyssal cells. We also show that a 12% increase in Antarctic ice melting can trigger freshening of the abyssal Southern Ocean at rates similar to the trends observed since the 1990s, thus suggesting that enhanced melting of Antarctic land ice may be the main driver of the recently observed AABW freshening. An ocean and sea‐ice model is used to explore how spatiotemporal variations in meltwater fluxes affect the Antarctic Bottom Water (AABW)Spatially varying meltwater fluxes reduces the low‐salinity bias of AABW in the ocean and sea‐ice model used by 30%Increasing meltwater fluxes by 12% over 60 years induces AABW freshening at rates consistent to observed AABW trends since 1990 An ocean and sea‐ice model is used to explore how spatiotemporal variations in meltwater fluxes affect the Antarctic Bottom Water (AABW) Spatially varying meltwater fluxes reduces the low‐salinity bias of AABW in the ocean and sea‐ice model used by 30% Increasing meltwater fluxes by 12% over 60 years induces AABW freshening at rates consistent to observed AABW trends since 1990

Antarctic Bottom Water (AABW) is a major component of the global overturning circulation, originating around the Antarctic continental margin. In recent decades AABW has both warmed and freshened, but there is also evidence of large interannual variability. The causes of this underlying variability are not yet fully understood, in part due to a lack of ocean and air‐sea‐ice flux measurements in the region. Here, we simulate the formation and export of AABW from 1958 to 2018 using a global, eddying ocean–sea‐ice model in which the four AABW formation regions and transports agree reasonably well with observations. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. Reservoirs of very dense waters at depth in the Weddell and Ross Seas following 1–2 years of strong surface water mass transformation can lead to higher AABW export for up to a decade. In Prydz Bay and at the Adélie Coast in contrast, dense water reservoirs do not persist beyond 1 year which we attribute to the narrower shelf extent in the East Antarctic AABW formation regions. The main factor controlling years of high AABW formation are weaker easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air‐sea buoyancy loss and convective overturning. Our study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage. Antarctic Bottom Water (AABW) originates over the Antarctic shelf when sea ice formation and mixing leave behind very cold, salty and dense waters. One third of the total ocean volume is filled by AABW, the densest water mass in the global ocean. AABW is important for climate as it regulates the global ocean overturning of heat, freshwater, oxygen, carbon and nutrients. Although we know that AABW has been warming in recent decades, there are only very limited observations of the year‐to‐year variability in AABW formation rates. Here, we use a numerical simulation of the global ocean to quantify how much AABW is formed and exported to depth over the period 1958 to 2018. We found that the AABW formation and export changes strongly from year to year. We attribute this variability to changes in wind and sea ice around Antarctica. In general, more AABW is exported during years when the polar easterly winds are weaker, as this influences the sea ice distribution and sea ice formation in the regions of AABW formation. Our findings help to interpret trends and variations in observational data of AABW where only limited duration and coverage is available. Strong interannual variability of modeled Antarctic Bottom Water (AABW) formation and export is identified at the four AABW formation sitesReservoirs of dense waters in the Weddell/Ross Sea due to strong water mass transformation can lead to higher AABW export for up to a decadeIncreased AABW formation and export is linked to weaker easterlies, less sea ice import, and more areas of open water and sea ice formation Strong interannual variability of modeled Antarctic Bottom Water (AABW) formation and export is identified at the four AABW formation sites Reservoirs of dense waters in the Weddell/Ross Sea due to strong water mass transformation can lead to higher AABW export for up to a decade Increased AABW formation and export is linked to weaker easterlies, less sea ice import, and more areas of open water and sea ice formation

The Antarctic Slope Current (ASC) and Antarctic Coastal Current advect heat, freshwater, nutrients, and biological organisms westward around the Antarctic margin, providing a connective link between different sectors of the continental shelf. Yet the strength and pathways of connectivity around the continent, and the timescales of advection, remain poorly understood. We use daily velocity fields from a global high‐resolution ocean‐sea ice model, combined with Lagrangian particle tracking, to shed light on these timescales and improve our understanding of circumpolar connectivity around Antarctica. Virtual particles were released along vertical transects over the continental shelf every 5 days for a year and were tracked forward in time for 21 years. Analysis of the resulting particle trajectories highlights that the West Antarctic sector has widespread connectivity with all regions of the Antarctic shelf. Advection around the continent is typically rapid with peak transit times of 1–5 years for particles to travel 90° of longitude downstream. The ASC plays a key role in driving connectivity in East Antarctica and the Weddell Sea, while the Coastal Current controls connectivity in West Antarctica, the eastern Antarctic Peninsula, and along the continental shelf east of Prydz Bay. Connectivity around the shelf is impeded in two main locations, namely, the tip of the Antarctic Peninsula and Cape Adare in the Ross Sea, where significant export of water from the continental shelf is found. These findings help to understand the locations and timescales over which anomalies, such as meltwater from the Antarctic Ice Sheet, can be redistributed downstream. There are two main currents that flow westward around the Antarctic continent. Both of these currents transport heat, salt, marine organisms, and nutrients around the continent, acting as a link between different regions of the Antarctic coastline. Yet little is known about the strength of this link, and the timescales over which it occurs. We investigate this connective strength, and transport timescales, using computer software and an ocean model to track water parcels as they travel around the continent. We find substantial lateral connections between many sectors of the Antarctic coastline. Water masses originating in West Antarctica, for example, can reach every region of the continental shelf. Typically, water masses on the shelf can travel 90° of longitude downstream within one to 5 years. The faster timescales tend to occur in East Antarctica, with the slower timescales found in West Antarctica and around the Antarctic Peninsula. Water movement along the shelf is disrupted in two main locations. One in the western Ross Sea, and the other at the Antarctic Peninsula. Most of the water found along the Antarctic continental shelf is exported from the shelf region to the open ocean at these two locations. Particle tracking in an ocean model shows water masses on the Antarctic shelf are typically advected 90° downstream within 1–5 yearsThe Antarctic Peninsula acts as both a receiving point, and export point, for upstream waters on the continental shelfWater masses originating in West Antarctica can reach all regions of the Antarctic continental shelf within 15 years Particle tracking in an ocean model shows water masses on the Antarctic shelf are typically advected 90° downstream within 1–5 years The Antarctic Peninsula acts as both a receiving point, and export point, for upstream waters on the continental shelf Water masses originating in West Antarctica can reach all regions of the Antarctic continental shelf within 15 years

Understanding the causes of recent climatic trends and variability in the high-latitude Southern Hemisphere is hampered by a short instrumental record. Here, we analyse recent atmosphere, surface ocean and sea-ice observations in this region and assess their trends in the context of palaeoclimate records and climate model simulations. Over the 36-year satellite era, significant linear trends in annual mean sea-ice extent, surface temperature and sea-level pressure are superimposed on large interannual to decadal variability. Most observed trends, however, are not unusual when compared with Antarctic palaeoclimate records of the past two centuries. With the exception of the positive trend in the Southern Annular Mode, climate model simulations that include anthropogenic forcing are not compatible with the observed trends. This suggests that natural variability overwhelms the forced response in the observations, but the models may not fully represent this natural variability or may overestimate the magnitude of the forced response.