The Southern Ocean, the fourth largest of the world’s five major ocean basins, is the vast body of water encircling Antarctica, generally defined by the Antarctic Circumpolar Current (ACC). Unlike the Atlantic, Pacific, and Indian Oceans, its northern boundary does not abut a landmass but is defined by a circulation zone that connects all three. This unique, unimpeded flow has created a marine environment that acts as a primary control system for the entire planet. The Southern Ocean governs global ocean currents, regulates atmospheric composition, and sustains marine life far beyond its own boundaries.
The Engine of Global Ocean Circulation
The physical movement of water in the Southern Ocean is the primary driver of the global thermohaline circulation, often called the “Ocean Conveyor Belt.” This massive system distributes heat, salt, and oxygen throughout the world’s oceans. The Antarctic Circumpolar Current (ACC), the world’s largest and strongest ocean current, flows eastward around the continent.
The ACC transports more water than any other current, acting as a barrier that thermally isolates Antarctica from warmer northern waters. This isolation is a major factor in maintaining the continent’s cold climate and ice sheet stability.
The Southern Ocean is also the main source region for the densest water mass on Earth, known as Antarctic Bottom Water (AABW). AABW forms primarily in polynyas—areas of open water surrounded by sea ice—along the Antarctic continental shelf during winter. As sea ice forms, salt is rejected into the remaining water, making it extremely cold, saline, and dense. This heavy water sinks to the ocean floor, where it spreads northward into the abyssal depths of the Atlantic, Indian, and Pacific Oceans, ventilating the deep ocean with dissolved oxygen and driving the lower limb of the global circulation system.
Earth’s Primary Carbon and Heat Sink
The Southern Ocean moderates global climate by absorbing vast amounts of anthropogenic heat and carbon dioxide (CO2). It takes up an estimated 70% of the excess heat absorbed by the global ocean each year, and approximately 40% of the human-generated CO2 absorbed by all oceans.
This uptake is facilitated by the solubility pump, where cold ocean water is more capable of dissolving atmospheric CO2 than warmer water. The unique circulation brings deep, carbon-rich water to the surface, where it exchanges with the atmosphere, and then locks away the absorbed heat and carbon when surface waters sink to form AABW. This process sequesters the heat and carbon in the deep ocean, isolating it from the atmosphere for centuries.
The efficiency of this carbon and heat uptake helps slow the rate of atmospheric warming. However, continued warming and changes to the circulation, such as freshwater input from melting ice, could reduce the ocean’s capacity to absorb these substances. A decrease in AABW formation, for example, is projected to decrease the deep ocean carbon storage rate, which would accelerate the rise of atmospheric CO2 concentrations.
Foundation of Global Marine Ecosystems
The Southern Ocean acts as a global nutrient hub. The strong westerly winds that drive the ACC cause a process called upwelling, which pulls deep, nutrient-rich water toward the surface. This deep water contains high concentrations of macronutrients like nitrate and phosphate, which are remnants of decomposed organic matter.
When this nutrient-rich water reaches the sunlit surface layer, it fuels phytoplankton blooms, forming the base of the food web. While the Southern Ocean itself is often limited by iron, the volume of nutrients brought to the surface supports high primary productivity. Much of this upwelled water then flows northward, carrying these nutrients into the Atlantic, Indian, and Pacific Oceans.
It is estimated that upwelling in the Southern Ocean maintains three-quarters of the biological production outside of the Southern Ocean itself. Within the Southern Ocean, the abundance of phytoplankton supports enormous populations of Antarctic krill, a species foundational to the ecosystem. Krill serve as the primary food source for a wide array of marine predators, including seals, whales, and penguins.
Influence on Global Sea Levels and Weather Systems
The physical interaction between the Southern Ocean and the Antarctic Ice Sheet directly impacts global sea level rise. Warm ocean water moving beneath the floating ice shelves can accelerate melting from below. This melt process does not directly raise sea level, but it destabilizes the ice shelves, which act as buttresses holding back the grounded ice sheet on the continent.
The melting of these ice shelves allows the grounded ice to flow more quickly into the ocean, contributing significantly to global sea level rise. The influx of freshwater from melting ice also affects ocean circulation by making the surface layer less dense. This can slow the exchange between surface and deep waters, further impacting heat and carbon absorption.
The Southern Ocean also has a strong influence on weather systems across the Southern Hemisphere through its coupling with the atmosphere. The Southern Annular Mode (SAM), a major pattern of atmospheric variability, is closely linked to Southern Ocean dynamics. Changes in the SAM influence precipitation and storm tracks across southern latitudes, affecting climates as far away as Australia and South America.