Polynyas are persistent areas of open water found within the vast, ice-covered polar regions. They exist where one would expect solid ice, making them significant features in the frozen landscapes of the Arctic and Antarctic. These regions of unfrozen seawater within continuous pack ice or fast ice often range in size from hundreds of meters to hundreds of square miles.
What Are Polynyas?
Polynyas are areas of open water surrounded by sea ice. Found in both the Arctic and Southern Oceans, they appear as “ice oases” or “open water holes” in otherwise frozen seas. These areas can be semi-permanent, lasting for days, weeks, or even entire seasons.
Polynyas are categorized into two main types based on their formation. Sensible heat polynyas, also known as open-ocean polynyas, are thermodynamically driven, occurring when warmer deep water rises to the surface. Latent heat polynyas are driven by strong winds that push ice away from a fixed boundary, such as a coastline.
How Polynyas Form
Latent heat polynyas form when strong, persistent winds, often katabatic winds blowing off ice sheets, continuously push newly formed ice away from a coastline or ice edge. This constant removal of ice exposes open water, which then rapidly freezes, only for the new ice to be swept away again, creating a continuous “ice factory” effect. This process maintains an area of open water even in freezing temperatures.
Sensible heat polynyas, conversely, are formed by the upwelling of warmer, deeper ocean water to the surface. This warmer water melts existing ice or prevents new ice from forming, keeping the surface free of ice. These polynyas often occur over specific oceanographic features like seamounts, where topographic interactions can induce the necessary upwelling or vertical mixing of subsurface water. The Weddell Polynya in the Antarctic Weddell Sea is a well-known example of a sensible heat polynya.
Ecological Significance of Polynyas
Polynyas function as biological hotspots in the polar environment, supporting a rich diversity of marine life. The open water and increased light penetration in these areas allow for significant primary productivity. This leads to large blooms of phytoplankton, which form the base of the marine food web.
This abundance of primary producers sustains a thriving ecosystem. Krill, fish like Arctic cod, and various seabirds, including murres and kittiwakes, gather in these areas for feeding and breeding. Marine mammals such as seals, walruses, beluga whales, and narwhals also rely on polynyas for food and as overwintering grounds. Many penguin colonies in Antarctica are located near recurrent coastal polynyas, relying on these ice-free areas for feeding.
Polynyas and Ocean-Atmosphere Interaction
Polynyas play a role in large-scale oceanographic and atmospheric processes. They act as “ice factories,” where amounts of new ice are continuously formed. As seawater freezes, salt is expelled into the remaining water, increasing its density in a process called brine rejection.
This cold, dense, salty water then sinks, contributing to the formation of deep and bottom waters, such as Antarctic Bottom Water (AABW), which drives global ocean circulation. Polynyas also influence heat exchange between the ocean and the atmosphere, releasing amounts of heat from the relatively warm ocean to the colder air. This heat release, which can be seven to nine times higher than in adjacent ice-covered areas, influences regional weather patterns and broader climate systems.
Monitoring Polynyas
Scientists monitor polynyas using a combination of satellite imagery, remote sensing, and in-situ observations. Satellite passive microwave radiometers are used due to their independence from cloud cover and daylight, providing daily information on polynya extent. However, their coarse resolution can make it difficult to distinguish between sea ice and open water.
Thermal infrared (TIR) sensors, such as those on the Moderate Resolution Imaging Spectroradiometer (MODIS), offer higher resolution imagery, typically 1 kilometer, multiple times a day, which is useful for determining polynya size. While sensitive to clouds, these observations are supplemented by synthetic aperture radar (SAR) and optical sensors for even finer spatial detail, although these have more limited coverage. In-situ observations provide localized data to complement satellite information. This comprehensive monitoring helps researchers understand polynya dynamics, predict their behavior, and assess their response to climate change.