Antarctic coastal polynyas are important areas of sea ice formation during the winter. For example, the Mertz Glacier Polynya, located in East Antarctica, covers only 0.001% of the overall Antarctic sea ice zone at its maximum winter extent, but is responsible for 1% of the total sea ice production in the Southern Ocean.
A polynya is a large body of open water or an area covered by very thin ice that persists in the middle of winter sea ice in the polar regions. The name comes from the Russian word for open or hollow. Due to the physical processes responsible for their formation, many polynyas recur in the same region every year. Polynyas may range in size from a few square kilometres to hundreds of thousands of square kilometres. The 350,000 square kilometre Weddell Sea Polynya near the Greenwich Meridian, which occurred from 1974 to 1976, was the largest polynya ever observed.
If a polynya occurs adjacent to a coastal promontory or blocking feature, it is called a coastal polynya. Some authorities also classify polynyas as deep water (those that form at or beyond the continental shelf break) or shelf water (occurring over the shelves).
There are two main mechanisms by which Antarctic polynyas are formed:
Sensible heat polynyas are formed by the upwelling of warm water, which makes the surface water warm enough to melt existing ice and/or prevent new ice from forming. The size is determined by surface area of warm water. These are areas of low ice production.
Latent heat polynyas form in areas in which sea ice is removed from the region of origin by winds and/or ocean currents as quickly as it forms. Given their high rates of sea ice formation, these polynyas are often called the ‘ice factories’ of the Antarctic sea ice zone.
Many polynyas are formed by a combination of the two mechanisms, with one predominating. Most coastal or shelf polynyas are predominantly latent heat polynyas. They tend to form where there are strong winds off land and downwind or down-current of landfast ice, glacier tongues or grounded icebergs.
Polynyas of this type have two distinct regions – a central open-water area where ice is constantly being formed (the ‘active polynya’) and an outer ring where the ice piles up as it is moved from the centre (the ‘young ice’ area).
Although relatively small in area, coastal polynyas play a disproportionately important role in many important physical and biological processes in the high-latitude Southern Ocean, as well as having an impact on global ocean circulation.
Coastal polynyas in some areas around Antarctica are important contributors to global ocean circulation. Vertical mixing of ocean water, known as ‘overturning circulation’ or ‘thermohaline circulation’, is an important aspect of the global current system that is driven primarily by rising and sinking of water masses at high latitudes in both the northern and southern hemispheres.
Salt is expelled as the ocean water freezes to form sea ice. This creates dense brine that sinks and flows down the continental shelf of Antarctica to form Antarctic Bottom Water – the densest water in the open ocean. This water flows outward from the Southern Ocean and through other ocean basins as part of the global ocean circulation ‘conveyor belt’ that distributes heat, nutrients and gases around the world. There are only a few areas in the world where this dense water is formed, an example being the Mertz Glacier Polynya to the south of Tasmania.
The sinking brine also helps lower atmospheric concentrations of carbon dioxide by carrying it from the surface to the deep ocean where it may be kept from contact with the atmosphere for thousands of years.
The open water areas within polynyas are important for marine mammals such as killer whales (giving them places where they can surface and breathe) and sea birds, especially in winter.
Antarctic coastal polynyas are biologically very productive, and offer ideal conditions for seasonally early and intense phytoplankton blooms. Due in part to the lack of a thick ice cover, polynyas form ‘windows’ through which the ocean can receive relatively high levels of sunlight from early spring onwards. The open waters then retain more heat that continues to thin the ice cover.
Penguin colonies also tend to be closely associated with polynyas. In Eastern Antarctica, for example, more than 90% of Adélie penguin colonies are situated next to recurrent coastal polynyas. Recent research shows that changes in polynya productivity from year to year affects the numbers of penguins in nearby colonies.
Surface heat and moisture loss from polynyas also helps warm the surrounding atmosphere and affects regional atmospheric circulation.