The polar oceans: Arctic Ocean, Southern Ocean and krill

Updated November 2026

The Arctic Ocean is a small, nearly enclosed sea capped by ice; the Southern Ocean is a wide-open ring of water around Antarctica. Between them they help drive the deep currents of the whole planet, soak up heat and carbon dioxide, and feed some of Earth’s richest food webs.

The Arctic Ocean

The Arctic Ocean is the smallest of the world’s five ocean basins, with an area of about 14 million square kilometers (5.4 million square miles), according to NOAA Ocean Exploration. Its deepest point, in the Canada Basin, is about 5,441 meters down. Parts of it stay covered by sea ice all year, though that cover is shrinking (see Polar ice).

The Arctic Ocean is changing fast. NOAA’s Arctic Report Card 2025 describes Atlantification: warmer, saltier water from the Atlantic now reaches the central Arctic Ocean, hundreds of miles beyond its former edge, and weakens the layering that used to keep warm water away from the ice. In August 2025 the Atlantic-side marginal seas were about 7 °C (13 °F) warmer than the 1991–2020 August average. With more open water and light, plant plankton have grown: from 2003 to 2025 their productivity rose 80% in the Eurasian Arctic, 34% in the Barents Sea and 27% in Hudson Bay.

The Southern Ocean

Most countries, including the United States, now count the Southern Ocean as the fifth ocean. The US Board on Geographic Names defines it as the water from the coast of Antarctica north to 60 degrees South. Those limits were proposed to the International Hydrographic Organization in 2000 but have not been formally approved, because some countries disagree.

The Southern Ocean takes up a large share of the heat that greenhouse gases trap. The IPCC found that it accounted for 35–43% of the heat gained by the upper 2,000 meters of the global ocean between 1970 and 2017, and 45–62% between 2005 and 2017.

The Antarctic Circumpolar Current

Nothing blocks the Southern Ocean as it circles the globe, so strong westerly winds push a current all the way around Antarctica. The Antarctic Circumpolar Current is the only current that flows completely around the planet and the largest wind-driven current on Earth. It reaches from the surface down to about 4,000 meters, is more than 120 miles (about 190 km) wide, and links the Atlantic, Pacific and Indian oceans (Woods Hole Oceanographic Institution).

Deep water formation

The polar oceans are where the deep ocean is “fed.” Cold, salty water is dense and sinks; warm water stays at the surface. NOAA describes a global “conveyor belt” of deep currents that starts in the Norwegian Sea, where the ocean loses heat to the air, becomes denser and sinks. That cold bottom water flows south, past the equator, all the way to Antarctica, and eventually returns to the surface through mixing and upwelling.

Around Antarctica, especially in the Weddell and Ross seas, freezing sea ice leaves salt behind in the water below. That cold, salty water becomes Antarctic Bottom Water, the densest water in the world, which sinks below about 4,000 meters and spreads through the deep ocean.

Why it matters. Surface warming and extra fresh water from melting ice make surface water lighter, which slows mixing with deeper water. The IPCC found that layering (stratification) in the upper 200 meters of the ocean grew by about 2.3% between 1971–1990 and 1998–2017.

Ocean acidification

When carbon dioxide dissolves in seawater it forms an acid. The IPCC found that the ocean has absorbed 20–30% of human carbon dioxide emissions since the 1980s, and that open-ocean surface pH has fallen by 0.017–0.027 units per decade since the late 1980s. Since the industrial revolution, surface pH has dropped by about 0.1 units. Because the pH scale is logarithmic, that small number means about a 30% increase in acidity (NOAA).

More acidic water makes it harder for animals to build shells from calcium carbonate. Tiny swimming snails called pteropods, or “sea butterflies,” are a warning sign: in lab water matching conditions expected by 2100, their shells slowly dissolved within 45 days, and shell damage has already been found in the Southern Ocean. The IPCC expects warming, acidification and sea ice loss together to affect polar marine life through changes to habitats and populations.

Krill: the base of the Antarctic food web

Antarctic krill are shrimp-like animals about 6 cm long that weigh just over a gram. They may live 5 to 10 years, eat mainly phytoplankton, and gather in swarms that can reach 30,000 animals per cubic meter. The Australian Antarctic Division estimates there are about 400 million tonnes of them in the Southern Ocean. Seals, whales, seabirds, fish and squid depend on them directly or indirectly.

A quick calculation shows the scale: if each krill weighs about 1 gram, 400 million tonnes (400 million × 1,000,000 grams = 400 trillion grams) is roughly 400 trillion krill. The IPCC links a southward shift of krill in the South Atlantic to climate change and projects their habitat will contract further south this century. See Polar wildlife for the animals that rely on them.

Check yourself

Why can a current flow all the way around Antarctica but not around the Arctic?

No continent blocks the Southern Ocean’s path around the globe, so westerly winds can drive the Antarctic Circumpolar Current in a complete loop. The Arctic Ocean is almost surrounded by land.

How does freezing sea ice help make the world’s densest water?

When seawater freezes, much of the salt stays behind in the water below. That water becomes colder and saltier, so it is very dense and sinks to the ocean floor as Antarctic Bottom Water.

Why does a pH drop of only 0.1 matter?

The pH scale is logarithmic, so a 0.1 drop means about a 30% increase in acidity. That makes it harder for shell-building animals like pteropods to make and keep their shells.

Sources