Southern Ocean overturning circulation

• 1. Introduction

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  The Southern Ocean overturning circulation lifts water that has been isolated from the atmosphere for decades, sometimes for more than a century. Half the North Atlantic Deep Water it brings upward takes between 60 and 90 years just to reach the surface. As the southern half of a global planetary loop, it connects every ocean basin on Earth. Scientists call its northern twin the Atlantic meridional overturning circulation, or AMOC. Together they transport vast quantities of warm and cold water, along with dissolved oxygen, dissolved carbon, and nutrients including iron. What makes the southern half distinct from AMOC? How is a warming climate reshaping it? And what does the science say about whether it could collapse entirely? At any given moment, roughly 27 Sverdrup of deep water wells up in the Southern Ocean. One Sverdrup represents a flow of one million cubic meters per second.

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• 2. Two Cells, Two Drivers

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  The wind-driven portion of the Southern Ocean overturning circulation is called the Deacon cell. Powerful westerly winds blow over the Antarctic Circumpolar Current, generating a process called Ekman transport. This draws deep water, specifically Circumpolar Deep Water, to the surface near the pole. At the surface, precipitation and summer sea-ice melt freshen this water and reduce its density. It is then transformed into lighter water types, including Subantarctic Mode Water and Antarctic Intermediate Water. The potential energy stored by this wind-driven circulation is later released through energetic, time-dependent eddying motions.

  Dense Shelf Water, the foundation of the lower cell, forms when sea ice crystallizes along the Antarctic coast. As ice forms, brine is expelled into the surrounding water, making it saltier, denser, and heavier. This cold, salty water sinks to the ocean floor, where it becomes Antarctic Bottom Water. Antarctic Bottom Water originates from the Ross Sea, the Weddell Sea, and the eastern coast of Antarctica. The lower cell accounts for around a third of total Antarctic Bottom Water formation globally.

  Harald Sverdrup had already identified the resolution to a puzzle hiding within these mechanics. Classical theory required vigorous vertical mixing in the thermocline to lift dense water upward. But observations show no such mixing. Instead, dense water ascends along sloping density surfaces in nearly adiabatic pathways, exchanging very little heat with surrounding water.

  The Southern Ocean is where approximately 80 percent of the world's deep water returns to the surface, making it the dominant place where the ocean interior reconnects with the atmosphere.

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• 3. The Carbon Cycle

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  The Southern Ocean absorbs around 40 percent of all oceanic carbon uptake from human-caused emissions. Oceans as a whole pull in roughly 25 percent of the carbon dioxide that human activity releases into the atmosphere. Before the Industrial Revolution, the ocean and atmosphere held carbon dioxide in rough balance. Rising emissions have since shifted that balance, turning the oceans into a net carbon sink.

  Water reaching the surface in the Southern Ocean has typically spent many decades in the deep ocean, cut off from the atmosphere. Its dissolved carbon concentrations are far lower than those of modern surface water, so it can absorb a large amount of carbon after surfacing. Downwelling then carries that carbon-rich water back to the depths. In regions where warm circumpolar water rises to the surface, some carbon escapes back into the air, partially offsetting the sink effect.

  Iron, along with other minerals, is carried upward from the deep ocean by upwelling. These nutrients fuel phytoplankton growth and boost ocean primary production. When phytoplankton die, downwelling carries the dead organic matter to the depths before it can decompose and return carbon to the air. Scientists call this the biological pump. A completely abiotic Southern Ocean would be a net carbon source rather than a sink.

  Through the 2000s, some scientists proposed that shifting winds in the Southern Hemisphere were curtailing the ocean's carbon absorption. Later work found the opposite: the sink had been stronger than those earlier estimates by 14 to 18 percent. Since 1971, the ocean has taken in over 90 percent of the extra heat that Earth's climate system has accumulated. The extratropical Southern Hemisphere ocean south of 30 degrees South has absorbed a disproportionate share of that warming.

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• 4. A System Under Pressure

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  The upper cell has grown by 3 to 4 Sverdrup since the early 1970s, an increase of 50 to 60 percent in its flow. The lower cell has weakened by a similar absolute number of Sverdrup. Because it is the larger of the two, those changes amount to only a 10 to 20 percent reduction. Climate change drove much of both trends, but the natural cycle of the Interdecadal Pacific Oscillation also played an important role.

  West Antarctica's upper ocean has warmed by 1 degree Celsius since 1955. The Antarctic Circumpolar Current is warming faster than the global average. Greater warming accelerates ice loss from Antarctica, generating between 1,100 and 1,500 billion tons of fresh meltwater per year. As this meltwater mixes into the Southern Ocean, it dilutes the salty Antarctic Bottom Water that powers the lower cell. The result is increased stratification, with surface layers becoming more stable and less prone to mixing downward.

  The Southern Annular Mode, the dominant climate pattern over the extratropical Southern Hemisphere, has been spending more time in its positive phase. In that phase, stronger westerly winds bring more warming and more precipitation over the Southern Ocean, freshening it further. Climate change and the lingering aftermath of ozone depletion have both contributed to this shift. Models currently disagree on whether the circulation will continue responding to SAM shifts as it does today, or will eventually adjust.

  As of the early 2020s, models suggest the lower cell will continue to weaken. The upper cell could grow around 20 percent stronger over the 21st century. Both projections carry limited confidence. Even the CMIP6 models, the most advanced generation available, represent ocean stratification poorly and inconsistently. Antarctic ice loss has long been the least certain element of sea level rise projections. It is also the largest long-term factor shaping the Southern Ocean overturning circulation's future.

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• 5. Collapse and Consequences

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  One study projects that under the worst projected climate trajectory, the circulation could drop to half its current strength by 2050. Greater losses would follow in the centuries afterward. Even if collapse begins in the near future, full collapse would not arrive until close to 2300.

  Paleoclimate records show the circulation has been substantially weaker, and has collapsed outright, during past periods both warmer and colder than today. Some early research places the threshold for likely collapse at between 1.7 and 3 degrees Celsius of global warming. Scientists acknowledge this is among the most uncertain of climate tipping point estimates.

  Australia and other Southern Hemisphere countries would face a long-term decline in precipitation. The Northern Hemisphere would experience a corresponding increase. Both effects are expected to unfold over multiple centuries.

  As the lower cell weakens, it delivers fewer nutrients to the Southern Ocean's sunlit surface. Marine productivity would decline, and certain fish species could face extinction. Some marine ecosystems could collapse entirely. Reduced marine productivity would also mean less carbon absorption over geological timescales, potentially raising long-term warming and the overall climate sensitivity to human emissions.

  A similar process is unfolding with AMOC in the Northern Hemisphere. Greenland's ice sheet is also in decline, pouring fresh meltwater into the North Atlantic. This affects AMOC much as Antarctic meltwater affects the Southern Ocean circulation. Scientists suggest both circulations could eventually collapse to much weaker states, in ways that would be difficult to reverse. Such transitions would constitute tipping points in the climate system. Were both to collapse, their opposite effects on precipitation in the two hemispheres would partially counteract each other.

  The Southern Hemisphere is home to only about 10 percent of all people on Earth. Partly as a result, the Southern Ocean overturning circulation has received far less scientific study than AMOC. In all scenarios where greenhouse gas emissions have not been substantially reduced, global warming will reach 2 degrees Celsius. Only total greenhouse gas emissions matter more than the state of this circulation for determining when that threshold arrives.

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