Cold, densecontinually sinks from the surface into the depths of the North Atlantic Ocean and the high- latitude Southern Ocean—the circumpolarsurrounding Antarctica. Although the processes and pathways by which thatenters the deep ocean are not completely understood, it’s clear that densermust descend into the abyss to replace lighterbelow. That’s because oceantend to be stratified into progressively denser layers. A puzzling challenge has been to determine how that densereturns from the deep. Without an exit pathway back to the surface, the entire ocean would fill up with cold, dense

Oceanographers used to think that drainage of the deep ocean occurs primarily by the vertical mixing of the density layers throughout the global ocean.According to this theory, downward turbulent fluxes of heat warm the deepwhich allows it to be displaced upward and replaced with newly cooled, denseBut observations do not support that as the sole mechanism for transporting deepback to the surface. Indeed, measurements using dye tracers suggest that vertical mixing in the upper 2 km of the ocean is an order of magnitude too small to account for the requisite warming.

An alternative theory, which has gained wide acceptance over the past two decades, is that the primary return pathway for deepis in the(see the article by J. Robert Toggweiler, November 1994, page 45 ). The westerly winds in the Southern Hemisphere drive a strongly divergent surface flow that draws upfrom below in a wide ring circling the Antarctic continent. Observations indicate that as much as 80% of deepresurfaces in theThe majority of the denseupwells from a depth of roughly 2–3 km along sloping density layers with little heat input or mixing required.(Boxdescribes the circulation through the global overturning loop.)

The upwelling exerts a huge influence on Earth’s atmosphere and climate. Because the newly exposed water is cold, it absorbs a vast amount of excess heat from the atmosphere. Thanks to the decomposition of organic matter that rains into the oceans, the upwelled water is a rich source of the nutrients that supply most biological production in the global ocean. Because the upwelled water continually replaces surface water, it absorbs a significant amount of excess carbon from the atmosphere.

Theis up to 1000 years old.The dissolvedcontent and temperature of deepin thethus reflect a preindustrial world that no longer exists—one with an atmospheric COconcentration some 120 ppm lower than its current value near 400 ppm, and a 0.8 °C cooler global surface atmospheric temperature.

In isolation, the wind-driven flow would eventually expel all lightfrom theand thereby increasingly steepen the tilt of its density layers. However, two key processes limit that steepening. First, rainfall and warming by the atmosphere reduce the density of the upwelledas it flows northward along the surface. Second, mesoscalevortices about 10 km across, move lightsouthward—box 2 explains how—and partially flatten the density layers. The net effect of the competition between the Ekman transport, theand the surface buoyancy flux is an equilibrium in which the deep, dense layers of the world ocean slope upwards and intersect the surface in theThe primary reason that thein thedominates on a global scale is due to the steep slope of its density layers, which provide a route for denseto resurface with minimal heat input.

Simple volume conservation explains why theoccurs. The strength of the westerly winds, and therefore the Ekman transport, varies with latitude—the maximum northward surface transport occurs at about 50° S and decreases south of that.must be drawn up from below in order to balance the difference between the larger northward transport at 50° S, say, compared with the smaller northward transport at 60° S. The broad ring ofshown in figurea starts close to the Antarctic continent and extends all the way to roughly 50° S.

A key driver of thecirculation is the westerly winds—the strongest mean sea-surface winds on Earth. Because the planet rotates, momentum transferred from the atmosphere to the upper hundred meters of the ocean produces a flow not in the direction of the winds but to the left of them (in the Southern Hemisphere). That flow, shown in figureand known as the Ekman transport, moves lighternorthward and draws large quantities of deep, denseto the surface in the south.

In addition to those changes in the upper ocean, Antarctic bottom water—the deepestmass in the Southern Ocean—has also warmed rapidly in recent decades.It’s not clear what’s causing the abyssal warming, which is strongest in the south and concentrated along the northward pathways. But possible causes include a reduced rate at which denseforms, due to a freshening of theas the Antarctic ice sheet melts, or an enhanced mixing between Antarctic bottomand the overlying warmermasses.

It is theof cold, deepin theand their exposure to the now warmer atmosphere that has led to the rapid heat uptake there (see figureb).The strongest warming is concentrated in the upper 1 km and penetrates much deeper than elsewhere in the global ocean.Greater wind speeds have also likely contributed to the anomalously large heat uptake. The westerly winds have increased by some 10–20% in recent decades due to the combined effects of the Antarctic ozone hole—see the article by Anne Douglass, Paul Newman, and Susan Solomon, July 2014, page 42 —and greenhouse gas warming. And although it is impossible to directly measure the strength of thechanges in the distribution of chlorofluorocarbons in the ocean suggest thatmay have increased in response to the wind increase.

As a result of the increased radiative trapping of greenhouse gases in recent decades, Earth is no longer in radiative equilibrium; more energy enters the top of the atmosphere than is emitted back to space. (See the article by Raymond Pierrehumbert, January 2011, page 33 .) Of the excess energy that has been absorbed by the climate system over the past 50 years, more than 90% has gone into warming the world’s oceans.Historical observations oftemperature are scarce, which makes it difficult to estimate the distribution of anthropogenic heat the ocean has absorbed. Results from climate-model simulations, though, suggest that thedominates the global oceanic heat uptake,with up to three quarters of the additional heat flux occurring south of 30° S. Although the heat uptake has been crucial in limiting atmospheric warming, it has also been, through thermal expansion, the major contributor to sea-level rise.

The first reason is that theas we’ve discussed, is the part of the global ocean where deepenriched inby the biological pump, upwell to the surface. The second, more subtle reason is that surfaceare not as efficiently stripped from thesurface by the biological pump. Iron and light, both necessary for phytoplankton growth, are simply too scarce in that part of the world. A decade ago one of us (Sarmiento) and colleagues estimated from model simulations that three-quarters of the global ocean’s biological production outside theis maintained byoriginating from the Southern Ocean’ssupply.

The surface of the global ocean between 40° S and 40° N is nearly devoid ofThe scarcity is a consequence of the biological pump, discussed in box 3 , that exports organic material from the ocean’s surface, where it forms by photosynthesis, to the deep interior, where it decomposes. Without a return pathway, the continual loss offrom the upper layers would pose a serious threat to marine life. Fortunately,do find their way back into the ocean’s upper reaches for two reasons, both involving the

Another feedback could arise from the projected wind-driven increase indue to a complex interplay of processes.First, the delivery of more carbon-richto the surface increases the degassing of CO. Second, the greaterdelivery and changed surface-water properties alter the efficiency of the biological pump. And third, a greater volume of deepexposed to the atmosphere increases the uptake of anthropogenic

The ocean is able to act like a sponge for anthropogenic CO 2 because of the high concentration of carbonate ions, which react with excess CO 2 to form bicarbonate. That process keeps the oceanic CO 2 concentration low, and thus allows for more uptake. But as the ocean takes up more carbon, the carbonate ions are reduced, which reduces the ocean’s ability to absorb CO 2 . As ocean surface temperature increases in the future, the solubility of CO 2 will decrease, which will, in turn, increase the oceanic partial pressure of CO 2 and decrease the rate of ocean carbon uptake. In short, those two processes lead to a positive feedback on global warming: As the ocean warms, it removes less CO 2 from the atmosphere, which leads to increased warming.

Although the Southern Ocean has taken up a major fraction of anthropogenic carbon in recent decades, whether it will continue to absorb CO 2 so rapidly is unclear. The change in the air–sea CO 2 flux has so far been dominated by the substantial increases in atmospheric CO 2 . However, changing ocean circulation and chemistry will likely impact the future rate of ocean carbon uptake.

Since 1750 the global ocean has absorbed nearly 30% of anthropogenic COemissions.Observational and model-based estimates suggest that themay be responsible for up to half of that uptake.South of 45° S, the elevated atmospheric COconcentration has reduced the difference between atmospheric and oceanic COpartial pressures and thereby decreased the amount of degassing from the carbon-rich upwelledA comparison between figuresc andd illustrates the point. North of 45° S, the flux ofinto the ocean has increased, because the partial pressure of COin the atmosphere is increasing faster than in the ocean.

In preindustrial times, thereleased large quantities ofinto the atmosphere, and that amount was balanced by uptake elsewhere in the global ocean, as shown in figurec. Boxexplains how the variation in COsolubility with temperature, the decomposition of organic matter, and physicalcombine to determine the net air–sea COflux. South of about 45° S, COdegassing from theof carbon-rich deepused to exceed the uptake ofby phytoplankton.

The global ocean is the largest reservoir ofin the climate system. Prior to any anthropogenic influence, it contained around 60 times the amount ofstored in the atmosphere. The ocean therefore is a central player in Earth’scycle and affects climate by either absorbing COfrom the atmosphere or releasing it. Whereas absorption cools the climate, the release of COwarms it, a process oceanographers believe happened during Earth’s many glacial to interglacial transitions.

New tools Section: Choose Top of page ABSTRACT What comes around Rapid heat uptake Nutrient supply Carbon sink New tools << REFERENCES CITING ARTICLES

The Southern Ocean’s remoteness and hostile environment make observations difficult. Few measurements of carbon, nutrients, surface buoyancy fluxes, and in situ current speeds exist, and those that do were mainly taken during summer months. Furthermore, model simulations of the region are complicated by the extremely high resolution required to resolve small-scale eddies, which are crucial for understanding how the Southern Ocean will respond to climate change.

Physics Today ). As a result of a highly successful international collaboration, there are currently more than 3500 Argo floats throughout the upper 2 km of the global ocean. Although the floats provide invaluable insights into the ocean’s heat storage and circulation, measurements of the Southern Ocean carbon uptake and nutrient resupply are still restricted to sparse, summer-biased, ship-based observations. In one of the most impressive oceanographic achievements of the past decade, physical oceanographers have developed autonomous, free-drifting Argo floats equipped with sensors for temperature, salinity, and pressure (see July 2000, page 50 ). As a result of a highly successful international collaboration, there are currently more than 3500 Argo floats throughout the upper 2 km of the global ocean. Although the floats provide invaluable insights into the ocean’s heat storage and circulation, measurements of theuptake andresupply are still restricted to sparse, summer-biased, ship-based observations.

nutrients and chlorophyll have been released into the Southern Ocean. Another 200 or so are planned to be deployed in the region over the next six years under the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project 16 et al. , Ocean Carbon and Biogeochemistry News , Fall 2014, p. 1. 16. See J. Russell, Fall 2014, p. 1. http://www.us-ocb.org/publications/OCB_NEWS_FALL14.pdf carbon storage and changes in nutrient supply. Earlier this year, however, the first 10 of a newly developed set of Argo floats equipped with sensors to measure pH and concentrations ofand chlorophyll have been released into theAnother 200 or so are planned to be deployed in the region over the next six years under theand Climate Observations and Modeling (SOCCOM) projectwith support from NSF’s Division of Polar Programs, the National Oceanic and Atmospheric Administration, and NASA. The new SOCCOM floats are designed to provide near real-time monitoring of oceanstorage and changes insupply.

Southern Ocean research is the ability of computational models to resolve the small-scale eddies, which, as outlined in box upwelling strength and its sensitivity to climate change. A decade ago the best climate models were limited to a horizontal resolution on the scale of 100 km in the Southern Ocean, an order of magnitude greater than the roughly 10-km length scale of an eddy. But thanks to faster, more powerful computers, climate simulations, such as the ones shown in figure 2 15 43, 140 (2013); 41, 2261 (2011). 15. A. K. Morrison, A. M. Hogg, J. Phys. Oceanogr., 140 (2013); https://doi.org/10.1175/JPO-D-12-057.1 R. Abernathey, J. Marshall, D. Ferreira, J. Phys. Oceanogr., 2261 (2011). https://doi.org/10.1175/JPO-D-11-023.1 , 17 et al. , J. Clim. (in press), 17. S. M. Griffies, J. Clim. (in press), https://doi.org/10.1175/JCLI-D-14-00353.1 upwelling is likely to respond to climate change and changes in wind speeds are crucial for quantifying heat uptake, carbon uptake, and changes in nutrient supply. The next generation of climate models is likely to be greatly improved in that regard. The second recent advance inresearch is the ability of computational models to resolve the small-scalewhich, as outlined in box 2 , directly affect thestrength and its sensitivity to climate change. A decade ago the best climate models were limited to a horizontal resolution on the scale of 100 km in thean order of magnitude greater than the roughly 10-km length scale of anBut thanks to faster, more powerful computers, climate simulations, such as the ones shown in figure, can now resolve features smaller than 10 km in that part of the world.Accurate predictions for how theis likely to respond to climate change and changes in wind speeds are crucial for quantifying heat uptake,uptake, and changes insupply. The next generation of climate models is likely to be greatly improved in that regard.

nutrient and carbon cycles stems from the fact that it is the primary gateway through which Earth’s deep and bottom waters interact with the atmosphere. But although oceanographers are beginning to understand the basic processes leading to heat and carbon uptake in the Southern Ocean, quantifying the changes precisely remains difficult. Observational uncertainties 6 et al. , Geophys. Res. Lett. 39, L10603 (2012), 6. S. Levitus, Geophys. Res. Lett. 39, L10603 (2012), https://doi.org/10.1029/2012GL051106 , 13 et al. , Biogeosciences 10, 2169 (2013). 13. S. Khatiwala, Biogeosciences, 2169 (2013). https://doi.org/10.5194/bg-10-2169-2013 The Southern Ocean’s prominent influence on the global heat balance andandcycles stems from the fact that it is the primary gateway through which Earth’s deep and bottominteract with the atmosphere. But although oceanographers are beginning to understand the basic processes leading to heat anduptake in thequantifying the changes precisely remains difficult. Observational uncertaintiesare roughly 20–30%. The hope is that they will sharply decrease during the coming decades as records from the Argo floats accumulate and the resolution of simulations sharpens.

Box 1. Draining the deep ocean waters that fill Earth’s deep oceans are formed primarily in the Southern Ocean, with the other 40% formed in the North Atlantic Ocean. The figure 2 37, 2550 (2007); 26(1), 80 (2013). 2. R. Lumpkin, K. Speer, J. Phys. Oceanogr., 2550 (2007); https://doi.org/10.1175/JPO3130.1 L. D. Talley, Oceanography(1), 80 (2013). https://doi.org/10.5670/oceanog.2013.07 waters follow over the roughly 1000 years it takes them to complete the circuit. The pathways are intertwined between three great ocean basins and over the full depth of the global ocean. About 60% of the cold, densethat fill Earth’s deep oceans are formed primarily in thewith the other 40% formed in the North Atlantic Ocean. Thehere, adapted from Lynne Talley’s masterful summary of global tracer observations and ocean circulation estimates,depicts the average pathways that thosefollow over the roughly 1000 years it takes them to complete the circuit. The pathways are intertwined between three great ocean basins and over the full depth of the global ocean. water that cools and sinks in the Southern Ocean is the densest water mass in the world, and it spreads northward (shown here as blue ribbons) to occupy the abyss of the three ocean basins. The deep water formed in the North Atlantic (green) is less dense and therefore lies above the Antarctic-sourced bottom water in the Atlantic Ocean. Vertical mixing in the upper 2 km, roughly half the ocean’s depth, is insufficient for the bottom water to upwell all the way to the surface as it spreads into the Northern Hemisphere. However, the interaction of the abyssal flow with seamounts and mid-ocean ridges, which are more plentiful below 2 km, can vigorously mix the bottom water and higher layers enough to transform them into less dense deep water (orange and green). The wind-driven upwelling in the Southern Ocean is sufficiently powerful to drain around 80% of those deep waters. 2 37, 2550 (2007); 26(1), 80 (2013). 2. R. Lumpkin, K. Speer, J. Phys. Oceanogr., 2550 (2007); https://doi.org/10.1175/JPO3130.1 L. D. Talley, Oceanography(1), 80 (2013). https://doi.org/10.5670/oceanog.2013.07 Thethat cools and sinks in theis the densestmass in the world, and it spreads northward (shown here as blue ribbons) to occupy the abyss of the three ocean basins. The deepformed in the North Atlantic (green) is less dense and therefore lies above the Antarctic-sourced bottomin the Atlantic Ocean. Vertical mixing in the upper 2 km, roughly half the ocean’s depth, is insufficient for the bottomto upwell all the way to the surface as it spreads into the Northern Hemisphere. However, the interaction of the abyssal flow with seamounts and mid-ocean ridges, which are more plentiful below 2 km, can vigorously mix the bottomand higher layers enough to transform them into less dense deep(orange and green). The wind-drivenin theis sufficiently powerful to drain around 80% of those deep water upwell in the Southern Ocean: One originates from the North Atlantic (green), the other from the Pacific and Indian Oceans (orange). The densest of these can be traced back to the North Atlantic by the high salinity content that is a signature of the larger evaporation compared with rainfall in the Atlantic. Overlying the North Atlantic–sourced deep water are the deep waters that upwell from the Pacific and Indian Oceans. They are about 1000 years old 5 41, 2831 (2011), 5. T. DeVries, F. Primeau, J. Phys. Oceanogr., 2831 (2011), https://doi.org/10.1175/JPO-D-10-05011.1 Two distinct flavors of deepupwell in theOne originates from the North Atlantic (green), the other from the Pacific and Indian Oceans (orange). The densest of these can be traced back to the North Atlantic by the high salinity content that is a signature of the larger evaporation compared with rainfall in the Atlantic. Overlying the North Atlantic–sourced deepare the deepthat upwell from the Pacific and Indian Oceans. They are about 1000 years oldand can thus be distinguished by their low oxygen concentration, which is depleted over time by deep-sea bacteria as they decompose organic matter. Because the deep waters from the Pacific and Indian Oceans lie on top of the North Atlantic–sourced deep water, they upwell in the northern part of the Southern Ocean. Approximately half of that flavor of deep water is freshened and warmed at the surface, which turns it into two other major water masses—the lighter so-called mode and intermediate waters (red)—that eventually make their way to the North Atlantic via surface and upper-ocean pathways (purple), only to sink again. The remainder, including nearly all the North Atlantic–sourced deep water, flows to the south under the influence of the easterly winds near Antarctica and is again converted to very dense Antarctic bottom water through cooling. Hence the global overturning may be crudely represented by a figure-eight loop in which surface water (purple) sinks in the North Atlantic (green), upwells in the Southern Ocean, sinks again around Antarctica (blue), and upwells again in the Southern Ocean via the Pacific and Indian Oceans (orange) before finally returning northward (red) along the surface (purple) to the North Atlantic (green).

Box 2. Ocean eddies and upwelling Southern Ocean tends to tilt the otherwise flat density layers in the ocean and increases the ocean’s potential energy. That potential energy is released as kinetic energy through a process known as baroclinic instability, which causes eddies about 10 km in size to form. The eddies move light, upper-ocean water southward and dense, lower-ocean water northward, which acts to flatten the density levels. In the surface layer the southward eddy advection directly opposes the northward, wind-driven Ekman transport, as shown in figure 1 eddies reduce the magnitude of the upwelling. 3 41, 1795 (2011); et al. , Nature 501, 408 (2013). 3. C. L. Wolfe, P. Cessi, J. Phys. Oceanogr., 1795 (2011); https://doi.org/10.1175/2011JPO4570.1 A. J. Watson, Nature, 408 (2013). https://doi.org/10.1038/nature12432 The steady blowing of winds over thetends to tilt the otherwise flat density layers in the ocean and increases the ocean’s potential energy. That potential energy is released as kinetic energy through a process known as baroclinic instability, which causesabout 10 km in size to form. Themove light, upper-oceansouthward and dense, lower-oceannorthward, which acts to flatten the density levels. In the surface layer the southwardadvection directly opposes the northward, wind-driven Ekman transport, as shown in figure. Hence, thereduce the magnitude of the For steady-state flow in the ocean’s interior, the Coriolis force in the east–west direction is balanced by east–west pressure gradients: where ρ is water density, ƒ is the Coriolis parameter (a function of Earth’s rotation rate and latitude), v is the water’s velocity in the north–south direction, and p is pressure. The flow of the strong eastward Antarctic circumpolar current over topographic ridges sets up east–west pressure gradients that drive the southward upwelling flow. eddy advection is crucial. Specifically, it allows the deep southward flow to upwell past the reach of topography to the surface. 3 41, 1795 (2011); et al. , Nature 501, 408 (2013). 3. C. L. Wolfe, P. Cessi, J. Phys. Oceanogr., 1795 (2011); https://doi.org/10.1175/2011JPO4570.1 A. J. Watson, Nature, 408 (2013). https://doi.org/10.1038/nature12432 However, in the latitudes between Antarctica and South America, the highest topography reaches only to about 2 km below the surface. In the absence of topographic pressure gradients above that depth to drive the transport along density layers, the southwardadvection is crucial. Specifically, it allows the deep southward flow to upwell past the reach of topography to the surface. In short, eddies contribute to steady-state upwelling dynamics in the Southern Ocean in two significant and seemingly conflicting ways. At the surface, their southward flow opposes the northward Ekman transport and thus reduces the upwelling. But below the surface, down to some 2000 m, the eddies enhance the upwelling. And in the latitude range unblocked by topography, it would not be possible for the southward flow to reach the surface in their absence. In addition to being a controlling factor for steady-state upwelling transport, eddies also influence the response of upwelling to changing winds. Enhanced westerly winds increase the northward Ekman transport and the divergence of surface currents. In isolation, that would lead to a matching increase in the upwelling of deep water. The eddies’ kinetic energy, however, also scales linearly with wind stress, which results in an increase in the opposing southward eddy flow at the surface. upwelling transport may increase by about 60% for a doubling of wind stress. 15 43, 140 (2013); 41, 2261 (2011). 15. A. K. Morrison, A. M. Hogg, J. Phys. Oceanogr., 140 (2013); https://doi.org/10.1175/JPO-D-12-057.1 R. Abernathey, J. Marshall, D. Ferreira, J. Phys. Oceanogr., 2261 (2011). https://doi.org/10.1175/JPO-D-11-023.1 New eddy-resolving numerical models suggest that nettransport may increase by about 60% for a doubling of wind stress.