What Is the Antarctic Ocean?

The Antarctic Ocean, more commonly called the Southern Ocean, is the body of water encircling Antarctica, bounded to the south by the continent and to the north by the waters of the Atlantic, Pacific, and Indian Oceans. Unlike the other ocean basins it has no continental margin on its northern side, so its limit is defined by circulation rather than by coastline: most oceanographers place the boundary near 60 degrees south latitude or, on physical grounds, at the Antarctic Convergence, where cold polar surface water sinks beneath warmer subantarctic water. That convergence is a sharp gradient in temperature, salinity, and biological community, which is why it works as a boundary at all.

The region matters out of proportion to its area because the water that fills the deep basins of every other ocean is largely made there. Cooling, sea ice formation, and brine rejection along the Antarctic margin produce the densest water in the world ocean, and wind-driven upwelling brings old, carbon-rich deep water back to the surface. The result is a set of processes that regulate global heat and carbon distribution on timescales from seasons to millennia.

The Antarctic Circumpolar Current

The dominant circulation feature is the Antarctic Circumpolar Current, the only current that flows unbroken around the globe, driven by persistent westerly winds and constrained by the Drake Passage gap. It transports on the order of 150 million cubic meters of water per second, more than any other current, and it links the three major ocean basins into a single system. The current is not a smooth ribbon but a set of narrow jets separated by fronts, each associated with distinct water mass properties and each subject to intense mesoscale eddy activity. Analysis of the response of the Antarctic Circumpolar Current to recent climate change shows that stronger winds have intensified eddies rather than accelerating the mean flow, a saturation behavior that constrains how the system responds to changing forcing. The dynamics, the transport estimates, and the frontal structure are summarized in the United Nations World Ocean Assessment chapter on the current.

Sea Ice and Water Mass Formation

Antarctic sea ice is highly seasonal, expanding from roughly 3 million square kilometers in February to about 18 million in September, a swing far larger than the Arctic's. Freezing expels salt into the water below, and in coastal polynyas, areas of open water held clear by offshore winds, that dense brine sinks down the continental slope to form Antarctic Bottom Water, the deepest limb of the global overturning circulation. Ice extent also controls how much heat and gas the ocean exchanges with the atmosphere and how much sunlight the surface reflects. After decades of modest growth, Antarctic sea ice extent dropped sharply from 2016 onward, and the question of whether that shift represents a regime change is examined by researchers at the National Snow and Ice Data Center.

Carbon, Heat, and Observation

The Southern Ocean accounts for a disproportionate share of the ocean's uptake of anthropogenic heat and carbon dioxide, while simultaneously venting natural carbon brought up from depth by upwelling, so the net flux is a small difference between two large opposing terms. Quantifying it demands measurements in one of the least accessible places on Earth. Observation therefore leans on autonomous platforms and satellites: profiling floats with biogeochemical sensors, instrumented seals, moorings, and satellite altimetry, scatterometry, and passive microwave imaging that has tracked ice extent continuously since 1979. Gravity missions add ice sheet mass change, and ship-based hydrographic sections still supply the deep, high-accuracy reference data against which autonomous sensors are calibrated.

Applications

Study of the Antarctic Ocean has applications in fields including:

• Climate modeling and projection of sea level rise
• Satellite remote sensing of sea ice, ocean color, and surface height
• Autonomous underwater vehicle and profiling float engineering
• Fisheries management, particularly for krill and toothfish stocks
• Polar ship design, navigation, and ice forecasting
• Paleoclimate reconstruction from sediment and ice cores