Antarctica is the coldest continent on Earth, encircled by the vast Southern Ocean. Unlike freshwater, which freezes at 0° Celsius, the ocean water remains consistently liquid at temperatures below this common freezing point. This immense body of water acts as a massive thermal reservoir, maintaining temperatures near the extreme lower limit for liquid water. This pervasive cold dictates the physics of the ocean and the biology of the life within it.
The Specific Temperature Range
The surface waters of the Southern Ocean maintain a narrow temperature band due to the presence of ice. Seawater immediately surrounding the continent typically ranges between approximately 0°C and its freezing point, about -1.8°C. This frigid zone contrasts sharply with temperate oceans, where surface temperatures can exceed 25°C. In localized coastal areas, where the water is particularly dense with salt, temperatures have been recorded as low as -2.6°C while remaining liquid.
The temperature stability extends deep into the ocean basin, influencing global circulation patterns. A massive, cold water mass known as Antarctic Bottom Water (AABW) forms here, exhibiting temperatures between -1.9°C and 2°C. This dense, cold layer occupies the abyssal zones below 4,000 meters across much of the world’s oceans. The persistent cold across both surface and deep layers characterizes the Antarctic marine habitat as thermally stable, yet extreme.
The Science of Sub-Zero Water
Antarctic seawater remains liquid below the freezing point of pure water due to a physical phenomenon called Freezing Point Depression. This process is governed by the concentration of dissolved salts, which interfere with the formation of ice crystals. Seawater with a standard salinity of around 35 parts per thousand has a freezing point lowered to approximately -1.8°C.
As the water cools, salt ions prevent water molecules from locking into a solid lattice until the temperature drops sufficiently low. If the water’s salt content increases, such as through sea ice formation, the freezing point is depressed further. The coldest seawater is often the saltiest. High salinity is the dominant factor allowing the Antarctic Ocean to maintain its liquid state at sub-zero temperatures, outweighing the slight effect of pressure.
Driving Factors of Antarctic Cold
The persistent cold of the Southern Ocean is maintained by large-scale oceanographic and atmospheric dynamics. The primary mechanism is sea ice formation, which initiates brine rejection. As surface water freezes, pure water molecules form ice crystals, expelling salt into the remaining liquid water underneath. This rejection increases the salinity and density of the water just beneath the ice.
This super-cooled, hyper-saline water sinks rapidly down the continental slope, forming Antarctic Bottom Water (AABW). The continuous sinking of AABW drives the lower limb of the global thermohaline circulation, locking the extreme cold into deep ocean basins worldwide. Furthermore, the Antarctic Circumpolar Current (ACC) acts as a powerful boundary that isolates this cold water mass, preventing warmer currents from the north from penetrating the area. This isolation helps the Southern Ocean maintain its unique thermal characteristics.
Adapting to the Extreme Cold
Life in the cold Antarctic waters requires biological adaptations to avoid freezing solid at temperatures below the freezing point of their body fluids. Antarctic notothenioid fish, which dominate the region’s fish fauna, possess specialized molecules called antifreeze glycoproteins (AFGPs). These proteins bind to incipient ice crystals in the fish’s blood and tissues, physically preventing their growth and stopping the freezing process.
The evolution of AFGPs arose in notothenioids from a gene that codes for the digestive enzyme trypsinogen. Beyond these chemical defenses, many Antarctic marine species exhibit physiological changes to cope with the cold. Some species display polar gigantism, growing to unusually large sizes, while nearly all cold-adapted marine life shows significantly slower metabolic rates. These adaptations allow them to survive in an environment where their body fluids’ freezing point, naturally around -0.7°C, is significantly higher than the surrounding seawater.