Image: from article Closing the Loops on Southern Ocean Dynamics: From the Circumpolar Current to Ice Shelves and From Bottom Mixing to Surface Waves https://doi.org/10.1029/2022RG000781
Notes:
Carbon sink – the ocean’s dense water absorbs carbon then sinks it.
Ice – the Antarctic ice sheet is the largest ice mass on earth. It flows onto the Southern Ocean in the form of gigantic ice shelves, which are being eaten away by a warming ocean. This has huge implications for sea level rise.
Water – the Southern Ocean’s dense water mass, produced in only a relatively few small regions of Antarctica accounts for an extraordinary 40% of the global ocean volume.
Currents – the Antarctic Circumpolar Current is the largest ocean current on Earth, and helps keep Antarctica frozen.
The Southern Ocean is wild and dynamic. It experiences Earth’s strongest winds and largest waves. It is home to city-sized icebergs and the biggest ocean current on the globe, as well as tiny turbulent flows that fit inside a teacup.
The Southern Ocean is also crucial to Earth’s natural systems. It forms the dense water that fills the world’s deep oceans. It stores heat and carbon resulting from human-caused global warming, and controls the flux of heat to the huge ice sheet of Antarctica – the greatest threat to runaway global sea-level rise.
The scale and complexity of the Southern Ocean can be hard to comprehend. But our new paper may help. It summarises the present state of understanding of the Southern Ocean, how it is changing, and where the knowledge gaps lie.
Scientists and others regularly voyage to the Southern Ocean’s furthermost icy reaches – but more research is needed. The scientific and broader community must join together to advance Southern Ocean science and protect this vital natural asset.
Masses of ice at risk
The Antarctic Ice Sheet is the largest ice mass on Earth, equivalent to 58 metres of the global sea level.
The ice sheet flows onto the Southern Ocean surface in the form of giant ice shelves. Many of these ice shelves are being eaten away from below by a warmer ocean, or crumbling and becoming icebergs at a faster rate than before.
Beyond the ice shelves, millions of square kilometres of the Southern Ocean surface is frozen into a layer of sea ice. This acts as a giant solar reflector and shields ice shelves from powerful Southern Ocean waves.
After decades of seemingly defying warming temperatures, the Southern Ocean’s sea ice has dramatically declined in recent years. This puts ice shelves and the ice sheet under even greater stress.
Filling Earth’s oceans
Much of the sea ice is produced in small regions of open water, called “polynyas”, formed by strong and cold winds blowing off Antarctica. These winds cool the ocean surface below the freezing point, causing ice to form.
As the ice forms it ejects salt into the ocean surface. This extra salt, in addition to cooling effects from the atmosphere, makes the surface seawater heavier, or more “dense”.
The dense water sinks in turbulent plumes (imagine an upside-down volcano) and cascades through underwater canyons into the deep ocean, while mixing with overlying waters.
The resulting dense water mass, produced in only a few relatively small regions of Antarctica, accounts for an extraordinary 40% of the global ocean volume. It is ultimately lifted back to the ocean surface by centimetre-scale turbulent eddies – of the type you see when mixing milk into your tea.
In the deep ocean, this mixing is largely driven by ocean tides that slosh over the rough seafloor and produce internal waves.
Climate system at risk
It takes many hundreds of years for the ocean water to cycle from the surface Southern Ocean to the deep and back. Water returning to the surface today is like a time capsule, reflecting the cooler, pre-industrial climate when it first sank to the ocean depths.
The water that sinks today has absorbed more carbon to store in the deep ocean, helping to limit global warming.
However, models and observations suggest reductions in sea ice and ice shelves are weakening this crucial climate system. They are making the water warmer, less salty and more buoyant, so less prone to sinking. This means less carbon storage and a warmer atmosphere in the years ahead.
So much we don’t know
Making measurements in the Southern Ocean is immensely challenging due to its remote location and hostile conditions. This means in many cases data is sparse, and so scientists don’t know exactly how quickly changes are occurring.
Our review identified several areas as a key priority for future Southern Ocean research. They include observations of ocean temperatures and melting beneath ice shelves, as well as long term measurements of dense water formation.
More data is needed to monitor changes and provide early warning of significant climate events, such as ice sheet collapse. Crucially, more data is also needed to inform and assess the computer models on which government, industry and society rely to predict future climate.
Unfortunately, ocean observations are expensive. For example, Australia’s premier research vessel, the RV Investigator, costs more than A$100,000 a day to run. And the new SWOT satellite– a joint project of the European Union and United States to measure the ocean surface at unprecedented resolution – cost more than US$1 billion.
These costs also highlight the need for enhanced national and international collaboration. This would make the best use of available resources, and promote technological innovation to develop more cost-effective observing systems such as drones and drifting robotic instruments.
The federal government’s scientific priorities and funding decisions should reflect the crucial importance of Southern Ocean science.
We are currently in the UN Decade of Ocean Science, which aims to improve predictions of ocean and climate change. Improved understanding of the Southern Ocean is vital to this effort.
Closing the Loops on Southern Ocean Dynamics: From the Circumpolar Current to Ice Shelves and From Bottom Mixing to Surface Waves
Abstract
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.
Key Points
- 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
Plain Language Summary
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.
1 Introduction
The Southern Ocean is a harsh, dynamic, and remote environment, which profoundly influences Earth’s present and future climates. It is home to the global ocean’s strongest winds, coldest ocean surface temperatures, largest ice shelves, most voluminous ocean currents, most extreme surface waves, and more. The Southern Ocean acts as a central hub of the global ocean where waters from the Atlantic, Pacific and Indian basins converge and mix. As such, it regulates the uptake of heat and carbon at a global scale. To the south, the unique dynamics of the Southern Ocean control the flux of heat to Antarctica’s fringes, thus controlling the stability of the Antarctic Ice Sheet (which holds the volumetric equivalent of about 60 m in global mean sea level; Fretwell et al., 2013; Morlighem et al., 2020). However, the Southern Ocean is experiencing profound, large-scale changes, many at unprecedented and accelerating rates. These include the lowest ever recorded sea ice minima in the past two Austral summers (Fetterer et al., 2017), rapid melting of the West Antarctic ice shelves (Paolo et al., 2015), and the warming and freshening of the abyssal waters formed in the Southern Ocean (Purkey & Johnson, 2013).
The observed large-scale changes in the Southern Ocean climate encompass a rich spectrum of dynamics, spanning thousand-kilometer scale ocean currents, tens- to hundred-kilometer scale polynyas, ten-kilometer wide eddies on the continental shelf, kilometer-scale convection, hundred-meter scale surface waves, meter-scale pancake sea ice formation and millimeter-scale turbulent mixing. The network of linkages and feedbacks between the different components of the Southern Ocean dynamic system creates challenges in understanding and predicting this vitally important region and its role in the global climate and ecosystems. The objective of this review is to close loops in understanding of the Southern Ocean dynamic system by drawing together contemporary perspectives on the different components of the system from different research communities within the broader field of Southern Ocean physical science. It aims to help the broad range of Southern Ocean researchers understand the context of their own work within the overall field, thereby facilitating better informed collaborations. As such, the focus is on a holistic physical understanding of the Southern Ocean, rather than associated aspects of atmospheric dynamics, land-based ice, and dynamical interactions with biogeochemistry. Instead, the reader is directed to reviews by Noble et al. (2020) for Antarctic Ice Sheet dynamics and Henley et al. (2020) for Southern Ocean biogeochemistry. In addition, there exist a number of reviews into different aspects of atmospheric dynamics and air–sea coupling, including the Southern Annular Mode (Fogt & Marshall, 2020), Southern Ocean precipitation (Siems et al., 2022) and air-sea-ice exchanges (S. Swart et al., 2019).
There are several definitions of the Southern Ocean extent; we take a dynamical perspective and consider the Southern Ocean system to be bounded by the northern extent of the Antarctic Circumpolar Current, and that its southern boundary includes the sub-ice-shelf cavities fringing the Antarctic continent, which terminate at the glacial ice shelf grounding zone. In the vertical direction, we consider dynamics stretching from the ocean surface, which is occupied by surface gravity waves and/or sea ice, to the ocean bottom, which is a key region for the generation of internal waves and subsequent mixing. We divide the Southern Ocean dynamic system into four main components: large-scale circulation; cryosphere; turbulence; and gravity waves. Large-scale circulation incorporates the Antarctic Circumpolar Current, Antarctic Slope Current, sub-polar gyres, and the meridional overturning circulation. The cryosphere includes sea ice and glacial ice shelves, as well as dynamic phenomena in the sub-ice-shelf cavities. We define turbulence as chaotic dynamics spanning from mesoscale eddies and polynya convection at the largest end, down to millimeter-scale diapycnal mixing. Gravity waves includes surface waves, internal waves and tides. Figure 1 gives a spatio-temporal perspective of the phenomena reviewed, which shows the broad range of scales covered. We focus the review on the connected nature of interactions between the different phenomena.
Figure 1
Spatio-temporal perspective of key dynamic phenomena reviewed, where colors indicate association to the four key dynamical components. The scales represented in this diagram indicate the time (and space) scales of the phenomena themselves, rather the much broader range of time scales over which these phenomena vary (e.g., internal waves exist at timescales of hours, but internal wave amplitudes vary on daily, seasonal and interannual timescales due to changes in stratification and atmospheric forcing). The ACC refers to the Antarctic Circumpolar Current.
We structure the review around the four main dynamical components identified above, commencing with large-scale circulation (Section 2) to provide a global perspective on the Southern Ocean dynamical environment, followed by cryosphere (Section 3), turbulence (Section 4) and gravity waves (Section 5). In each section, we give an overview of the fundamental physics of the dynamical component being considered, before describing the linkages between these components. In prioritizing these linkages, we focus on the most impactful, those in areas of growing research activity, and those where significant outstanding questions remain. We typically describe the linkages in the section corresponding to the component that is being impacted, thereby minimizing repetition. Each section ends with a short overview of the impacts of the component in the other sections to “close the loops”. The sections dedicated to the four dynamical components are followed by overviews of relevant Southern Ocean climate trends and future climate projections (Section 6) and cross-cutting priorities for future Southern Ocean physical science (Section 7), and brief closing remarks (Section 8).
2 Large-Scale Circulation
Figure 2
Schematic of the Southern Ocean’s large-scale circulation, where the ocean colors indicate the density, ranging from lighter (dark orange) to denser (dark blue) waters, and isopycnal contours are the interfaces between the layers. The horizontal gradients in density are correlated with largely geostrophic currents, including the Antarctic Circumpolar Current (ACC) and Antarctic Slope Current, above the shelf slope/break. Antarctic Bottom Water is generated by convection and brine rejection on the continental shelf, and flows down into the abyssal ocean. Warmer Circumpolar Deep Water is upwelled in the mid-depths and plays a key role in the melt rate of glacial ice shelves. These processes collectively form the Southern Ocean component of the upper and abyssal overturning cells, as indicated by the dashed lines. Farther to the north, at the density fronts of the ACC, are the formation sites of northward flowing mode and intermediate waters. The topography, isopycnals, and glacial ice shelf profile on the southern side of the schematic are from observations in the Ross Sea, although they are artificially extended to the north to represent a more typical condition for the ACC. Note that the depth scale is not linear.
Figure 3
A schematic plan view of the large-scale circulation of the Southern Ocean. The key features are the braided network of eddies and jets, circumnavigating the continent, that comprise the eastward-flowing Antarctic Circumpolar Current (ACC) (for which the northern and southern limits, or fronts, are represented by black contours), the Weddell and Ross gyres (blue dotted lines), and the westward-flowing Antarctic Slope Current (ASC) nearer the continent (brown line). The ASC exists everywhere except along the western side of the Antarctic Peninsula, where the ACC flows very close to the shelf slope. The current/gyre lines represent contours of streamfunction, sketched based on typical time-mean flows in a global ocean model. The background image shows a typical snapshot of daily mean surface flow speed from the ACCESS-OM2-01 global ocean model (Kiss et al., 2020).
2.1 Antarctic Circumpolar Current
2.2 Antarctic Slope Current
2.3 Weddell and Ross Gyres
2.4 Upper Overturning Circulation
Figure 4
Simulated eddy compensation of the upper overturning circulation in a numerical model with no eddy parameterization. With no eddy compensation (by resolved or parameterized eddies), the overturning would linearly increase with the magnitude of the westerly wind stress following the surface Ekman transport (black dashed line). As model resolution is increased such that mesoscale eddies become fully resolved (red line), the sensitivity of the overturning circulation to wind stress decreases, but remains non-zero. Figure reproduced from Morrison and Hogg (2013).
Figure 5
Impact of the nonlinear equation of state (i.e., the equation describing the dependence of the density of seawater on temperature, salinity and pressure) on simulated Antarctic Intermediate Water formation. (a, b) Latitudinal transects along 23.5°W of salinity (color, with contours labeled in black) and potential density (white labeled contours) in the South Atlantic. The simulation shown in (a) uses a full non-linear equation of state, while (b) uses a linear equation of state. Antarctic Intermediate Water (blue to green freshwater pathway shown in a) forms through isopycnal mixing leading to cabbeling and is only able to form in the model configuration using a nonlinear equation of state. Figure reproduced from Nycander et al. (2015).
2.5 Abyssal Overturning Circulation
2.6 Closing the Loops
3 Cryosphere
Figure 6
Schematic of the oceanic margin of the southern cryosphere, including key dynamic connections with the Southern Ocean. The ice shelf is the floating extension of the Antarctic Ice Sheet formed from glaciers flowing onto the ocean surface. Adjacent glaciers may fuse together as suture zones. The ice shelf contains features, such as melt ponds at its surface (that can result in hydrofracture), crevasses and meltwater channels at its base, and rifts that extend throughout the shelf depth and propagate to the shelf front to calve tabular icebergs, from which bergy bits break off. Here, the giant ice shelf partially encloses a cold-water cavity that experiences Mode One circulation (Section 3.1.1), involving bottom inflow of cold water fed by dense shelf water created in a polynya, and outflow of basal meltwater that exits the cavity as a plume of Ice Shelf Water that can create platelets of marine ice that attach to the underside of local sea ice. At the ice shelf grounding zone, subglacial discharge of ice sheet meltwater can accelerate melting and create subglacial meltwater channels. The shelf front is occupied by a polynya (created by katabatic winds) and immobile landfast sea ice (attached to the ice shelf). Pack ice bounds the polynya and landfast sea ice toward the ocean. The pack ice consists predominantly of semi-consolidated sea ice with features such as pressure ridges, leads, and fractures, but with a shear zone at its boundary with the landfast sea ice and a marginal ice zone at its boundary with the open ocean, where floe sizes are relatively small due to the presence of surface waves. Large-scale sea ice drift is dictated by winds, such as those associated with polar cyclones, as well as ocean tides, currents and gyres.
3.1 Ice Shelves and Sub-Ice Shelf Cavities
3.1.1 Ice Shelf Cavity Exchange With the Southern Ocean
Figure 7
Idealized modes of cavity circulation using the Ross sector as an exemplar (Jacobs et al., 1992; Tinto et al., 2019) and showing the effect of polynyas. The ice sheet domain comprises the East and West Antarctic Ice Sheets (EAIS and WAIS, respectively), and the Discovery Deep is the lowest known point in the Ross cavity. The light-brown band denotes the extension of the ice shelf and sub-ice-shelf water cavity onto the continental shelf. Modes One, Two, and Three are shown together for convenience but do not necessarily co-exist nor is there a substantial amount of direct observation of these modes. Mode 2* indicates the uncertainty regarding penetration of modified Circumpolar Deep Water (mCDW) into the cavity. Additional features include (a) melt water from the east (Nakayama, Timmermann, et al., 2014), (b) Antarctic Slope Current (A. L. Stewart et al., 2019), (c) continental shelf troughs and possible penetration of mCDW, and (d) high salinity shelf water draining off the continental shelf. On the continental shelf itself there are (e) sea ice driving a polynya and convection, and (f) the shelf front wedge, which is a buoyant front associated with summer warming that interacts with the Mode Three circulation (Malyarenko et al., 2019). Within the cavity, there are (g) cavity interleaving (Stevens et al., 2020) affecting the cavity circulation, and (h) subglacial discharge flows at the grounding line.
3.1.2 Cavities, Gyres, and Eddies
3.1.3 Tidal Influence on Cavities
3.1.4 Meltwater Plumes and Marine Ice
3.1.5 Subglacial Discharge
Figure 8
Small-scale views of an ice shelf and sub-ice-shelf water cavity showing some of the under-observed but critical processes likely to be present. (a) Grounding zone region including subglacial discharge of meltwater from beneath the ice sheet, basal crevasses, stratification/baroclinic waves (dashed lines), in/outflow. (b) The basal boundary layer (bbl), temperature and salt stratification and roughness variations. The basal boundary layer shows the 1–10 m thick region close to the ice-shelf base, where the fluid velocity and turbulence is affected by the presence of buoyant meltwater. Temperature and salinity increase rapidly through the boundary layer from diffusion-controlled melting conditions at the ice-ocean interface through to the boundary layer itself and then to ocean-cavity conditions at the edge of the boundary (the circulation of which is not well known).
3.1.6 Cavity Basal Boundary Layers
3.1.7 Iceberg Calving
3.2 Sea Ice
3.2.1 Sea Ice Drift
3.2.2 Surface Wave–Floe Interactions in the Marginal Ice Zone
Figure 9
Schematic of surface wave–ice floe interaction processes in the marginal ice zone, including (from left to right): wave-induced breakup of a large floe; subsequent northward drift of small broken floes due to off-ice winds, and enhanced melt in summer (indicates by white tear drops below ice floes); wave overwash of a floe; herding and rafting of small floes; floe-floe collisions; production of frazil in the open water created between floes during winter. The spirals indicate turbulent mixing.
3.2.3 Brine Inclusions to Convective Channels
Figure 10
The main panel (left-hand side) is a schematic of sea ice at finescale. Above the dashed line, air bubbles (white circles) and brine inclusions (elongated blue shapes) are trapped within the impermeable, solid ice (sky blue). Below the line the ice is permeable, allowing brine drainage and fresh water inflow, that is, a mushy layer. A zoom in on the microscale for a brine inclusion is given (right). The liquid brine region is surrounded by ice, and the arrows point in the direction of the salt flux during the freezing process.
4 Turbulence
Figure 11
Schematic to illustrate surface, interior, and bottom boundary layers in the Southern Ocean, with a summary of turbulence processes acting in each layer. The ocean colors indicate the density, from lighter (dark orange) to denser (dark blue) waters, and isopycnal contours are the interfaces between the layers. Note that the three layers are offset in latitude and disconnected in the vertical, where the surface layer is 0–300 m in depth, the interior layer is 1,000–4,000 m and the bottom layer is 4,500–5,000 m. The water masses shown are Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), Circumpolar Deep Water (CDW), and Antarctic Bottom Water (AABW). Also shown on the top panel are the Antarctic Circumpolar Current (ACC), the Polar Front (PF) and the Subantarctic Front (SAF).
4.1 Eddies, Jets, and Fronts
Figure 12
Mesoscale (and submesoscale) turbulent structures are ubiquitous in the Southern Ocean. The Rossby number, defined as the vertical component of relative vorticity (∂v/∂x − ∂u/∂y) divided by the planetary vorticity (f), highlights dynamical features. The four zoomed-in panels show: the Antarctic Slope Current (top right), a large-scale meander near the Macquarie Ridge and the associated energetic mesoscale eddy field (bottom right), the spatial variation in the dominant dynamical scale in the Southern Ocean (bottom left), and the highly energetic turbulence in Drake Passage (top left). The velocity fields are snapshots from a regional simulation around Antarctica at a 1/20° lateral resolution, performed with the Modular Ocean Model, version 6 (Adcroft et al., 2019) by the Consortium for Ocean and Sea Ice Modeling in Australia (Kiss et al., 2020).
4.1.1 Wind Forcing
4.1.2 Buoyancy Forcing
4.1.3 Internal Wave Interactions
4.1.4 Mesoscale Turbulence Self-Interactions
4.1.5 Topographic Effects
Figure 13
Eddies, fronts and jets in the Kerguelen Plateau region. (a) Snapshot of sea surface temperature (colors) and sea surface height (cyan contours) from the ACCESS-OM2-01 model (Kiss et al., 2020). (b) Eddy kinetic energy (colors), sea surface height and southward eddy thickness fluxes (from results by Yung et al., 2022) averaged over a 10-year simulation. Gray contours in both panels show bathymetry.
4.1.6 Dissipation of Eddy Kinetic Energy
4.2 Convection
4.2.1 Upper Mixed Layer Convection
Figure 14
Surface fluxes and mixed layer depth (MLD) in the Southern Ocean for Austral (a–c) summer and (d–f) winter. (a, d) The buoyancy flux due to the net surface heat flux, Bq. (b, e) The buoyancy flux due to the net surface salt flux, Bp. (c, f) MLD. Fine black lines represent (a, b, d, e) sea ice extent, and (c, f) main fronts of the Antarctic Circumpolar Current, with the thick black line corresponding to the maximum seasonal sea ice extent. Fluxes are calculated based on the SOSE reanalysis product (Mazloff et al., 2010). (c, f) Reproduced from Pellichero et al. (2017).
4.2.2 Coastal Polynya Convection
4.2.3 Open Ocean Polynya Convection
Figure 15
4.3 Mixing
4.3.1 Upper Ocean Mixing
4.3.2 Interior Diapycnal Mixing
4.4 Closing the Loops
5 Gravity Waves
Figure 16
Schematic of gravity wave processes in the Southern Ocean including surface waves, internal waves and tides. At the surface, strong storm systems generate surface waves and (near-inertial) internal waves. The gravitational force of the moon and sun generate bulk motions of the water column (tides) that, in combination with other ocean flows, generate internal waves at the seafloor. The waves interact with other components of the Southern Ocean system. For example, surface waves are dissipated in the marginal ice zone, while internal waves may be trapped in eddies and currents, and/or drive diapycnal mixing in the ocean interior. Color contours show a typical density field, ranging from lighter (dark orange) to denser (dark blue) waters.
5.1 Surface Waves
5.1.1 Surface Wave Breaking
Figure 17
Schematic of a breaking surface gravity wave (not to scale). The wave propagates in the direction of the wind and grows with time until it becomes too steep and breaks. Wave breaking induces near-surface turbulence, which generates air bubbles and entrains them into the sub-surface ocean, mediating air-sea fluxes of momentum, energy, moisture, and biological constituents with the ambient atmosphere. Turbulent oscillatory motion (from both breaking and non-breaking waves) drives vertical mixing (blue spirals) through the water column to a depth comparable to the wavelength, contributing to the mixed ocean surface layer.
5.1.2 Influence of Mesoscale Turbulence in the Antarctic Circumpolar Current
5.1.3 Attenuation, Dissipation, and Scattering by Sea Ice
5.2 Tides
Figure 18
Schematic of the primary roles of tides in the Southern Ocean system. Under the ice shelf, tidal currents generate friction that modifies hydrographic properties of the water column, influencing the basal melting of the ice shelf. At the ice front and shelf break, rectified tidal currents modify water mass transport along and across these topographic barriers. Over the continental shelf and slope, tidal currents modify sea ice production and concentration. Stress at the base of landfast sea ice affects melting, and mixing controls on surface mixed-layer depth. Mixing and rectification of tidal flows alters the production of Antarctic Bottom Water. Farther north, tidal flows over steep and rough topography of mid-ocean ridges generates internal (baroclinic) tides that can drive mixing in the ocean interior. Baroclinic tides may also be generated over the continental slope. The notation λcrit(M2) denotes the critical latitude at which the M2 tidal frequency equals the Coriolis frequency.
Figure 19
Schematic of near-inertial wave generation, propagation and dissipation. Storms generate inertial oscillations in the ocean mixed layer which drive horizontal convergences and divergences that lead to vertical velocities. These pump the base of the mixed layer generating internal waves near the local inertial frequency (i.e., wave frequencies 1–1.2f) that have counterclockwise polarization in the Southern Ocean. High mode near-inertial waves propagate downward and equatorward and tend to break locally due to high shear. Low mode internal waves propagate further equatorward. The interactions of near-inertial internal waves with other internal waves and with the background mesoscale flow are not represented here. Figure adapted from Alford et al. (2016).
Figure 20
Schematic of internal lee wave generation and the associated vertical transfer of horizontal momentum flux via lee wave induced form stress across isopycnal layers. The pressure is increased on the upstream side of the hill (+ΔP) and decreased on the downstream side (−ΔP), resulting in a force from the fluid on the hill. The breaking of the wave at a critical level drives turbulent mixing and deposition of the wave momentum, resulting in a net force on the background mesoscale flow. For lee waves, this force always acts to decelerate the flow. For lee waves, critical levels only occur when the velocity reduces with height along the waves propagation path, as shown here; this usually occurs when the wave reaches the horizontal boundary of an eddy/jet, but for simplicity, the flow in this schematic is represented as horizontally uniform.
6 Climate Trends and Future Projections
The Southern Ocean dynamic system is changing in response to global warming (Section 1) and changes in its atmospheric drivers. In recent decades, surface wind speeds have increased over the Southern Ocean (Young & Ribal, 2019; Young et al., 2011) and the maximum in the wind speed shifted southward; these changes are often described in terms of a strengthening and poleward contraction of the Southern Annular Mode (the dominant mode of atmospheric variability over the Southern Ocean; Arblaster & Meehl, 2006; D. W. Thompson et al., 2011; Toggweiler, 2009). Precipitation has decreased at lower latitudes and increased at higher latitudes (Manton et al., 2020), and evaporation has decreased over the Southern Ocean (Boisvert et al., 2020). This section reviews the key trends in the different components of the Southern Ocean dynamical system and projections for future trends where available. These trends in dynamical components are strongly influenced by ongoing changes in the thermohaline structure of the Southern Ocean. For a more detailed review of recent trends in the physical climate, the reader is referred to J. M. Jones et al. (2016).
7 Research Priorities to Close the Loops on Southern Ocean Dynamics
Many key research questions remain regarding interactions between the different components of the Southern Ocean dynamic system, and how their recent trends (Section 6) will affect the interactions. The knowledge gaps compromise our ability to represent the Southern Ocean in global models accurately and, hence, make well informed projections of future climate change and sea level rise. Indeed, over half the uncertainty in projections of global mean sea level is due to Antarctic Ice Sheet melting (Kopp et al., 2014). Reducing this uncertainty requires advances on multiple fronts due to the range of processes that influence the melt rate (Cook et al., 2023). Necessary advances include predicting trends in the large-scale circulation and temperature of the Southern Ocean beyond the continental shelf, understanding the transport mechanisms that flux heat onto the shelf and into the ice shelf cavities, and developing accurate parameterizations of the finescale convection and turbulence that melts the ice shelves. Similarly, a key contributor to uncertainty in global mean air temperatures on long timescales is the rate of heat storage in the abyssal ocean (Abraham et al., 2013). Most anthropogenic heat is currently stored in the upper ocean (Levitus et al., 2012), which overturns faster; however, the abyssal ocean is playing an increasing role and will be crucial to the long-timescale evolution of climate change and it remains an open question whether this abyssal overturning will increase or decrease under climate change (Section 6.1). The sign and magnitude of the trend is influenced by a host of processes including the poorly understood dynamics of the Ross and Weddell gyres, changes in sea ice cover and brine rejection, the small-scale convection that leads to Dense Shelf Water, and the unknown distribution and magnitude of mixing in the abyssal Southern Ocean.
The overarching research priority is improving our ability to model the Southern Ocean system and its response to anthropogenic forcing. This requires a multi-disciplinary community effort, involving researchers across the different components of Southern Ocean dynamics, and spanning advances in theoretical understanding of individual processes, technological developments to improve observations, novel data analysis techniques, innovative numerical methods, and, finally, putting these components together to develop the global ocean–sea ice models that are used in climate and sea level projections. We now describe the specific priorities that feed into addressing the uncertainties identified above, which we broadly divide in process-based models (Section 7.1), observations (Section 7.2), and regional and global models (Section 7.3).
8 Closing Remarks
In many respects, the Southern Ocean is the final frontier of ocean science. It is a vast, poorly observed, inhospitable and almost untouched region that has fascinated humankind since the discovery of Antarctica in the 1820s. Scientific interest in the Southern Ocean has grown rapidly in recent times, along with understanding of the control that Southern Ocean dynamics exert on global climate and climate change. However, progress has sometimes been stymied by a lack of effective communication between scientists in different disciplines and using different methodologies. This holistic review of Southern Ocean dynamics has sought to provide a common language and knowledge-base across the Southern Ocean physical science community to facilitate future knowledge-sharing and collaboration. Such collaboration is critical to address the key scientific priorities identified above that span the disciplines of mathematics, fluid mechanics, software engineering, glaciology and oceanography, and methodologies as diverse as laboratory experiments of individual processes through to numerical modeling of the entire Southern Ocean system. All of these disciplines and methodologies—and many more—have a crucial role to play in accelerating our understanding of Southern Ocean dynamics in the years ahead, and thereby improving our ability to predict ocean and climate change. This outcome is critical for the global community, and indeed forms one of the goals of the United Nations Decade of Ocean Science 2021–2030. Facilitated by this review, we encourage the entire Southern Ocean community to come together to support this objective.
Acknowledgments
We thank the three anonymous referees and the handling editor (Claudine Stirling) for their insightful comments on the article. We acknowledge the receipt of the Elizabeth and Frederick White Research Conference Award from the Australian Academy of Science, which funded the initial meeting that led to this review. The meeting received additional funding from the Australian Research Council Centre of Excellence for Climate Extremes (CE170100023). The Australian Research Council supported the authors via its various grant schemes (Grants FL150100090; CE170100023; DE170100184; DP190100494; DP190101173; FT180100037; FT190100404; FT190100413; DE200100414; DP200102828; LP200100406; SR200100008; DE210100749; DE21010004; DE220101027; DP230102994). LGB acknowledges support from the Australian Antarctic Science Program (AAS4528). PH acknowledges support from the Australian Antarctic Science Program (AAS4496, AAS4506, AAS4625) and grant funding from the International Space Science Institute (Switzerland; Project 405). GJS acknowledges support from the Banting Postdoctoral Fellowship through funding (180031). FM acknowledges support by the Marsden Fund (Grants 18-UOO-216 and 20-UOO-173) administered by Royal Society Te Apārangi. CS, AM, and FM acknowledge the Ministry of Business, Innovation and Employment Antarctic Science platform (ANTA1801). LP acknowledges support from the U.S. National Science Foundation, Grant 1744789, and NASA Grants 80NSSC19K1201 and 80NSSC21K0911. Figures 2, 6–11, 16–20 were produced by Stacey McCormack. Figure 3 background image uses data from Kiss et al. (2020). Figure 4 is reproduced from Morrison and Hogg (2013). Figure 5 is reproduced from Nycander et al. (2015). Figure 12 uses data from Adcroft et al. (2019) and Kiss et al. (2020). Figure 13a uses data from Kiss et al. (2020). Figure 13b uses results from Yung et al. (2022), for which Claire Yung provided the code used. Figure 14 uses data from Mazloff et al. (2010). Figures 14c–14f reproduced from Pellichero et al. (2017). Figure 15 reproduced from Vreugdenhil and Gayen (2021). Many figures were produced using computational resources and data hosted by the National Computational Infrastructure, Canberra, Australia, a facility of the Australian Commonwealth Government. In keeping with the spirit of this review, we also acknowledge the many collaborative research consortia that supported this work including the: Consortium for Ocean Sea Ice Modeling in Australia, ARC Centre of Excellence in Climate Extremes, ARC Centre for Excellence in Antarctic Science, Australian Antarctic Partnership Program, and the Australian Government’s National Environmental Science Program Climate Systems Hub.
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