Polynyas are regions of open water or reduced ice concentration bordered by water that is ice covered. They are typically rectangular or elliptical in shape with sizes ranging from a few hundred meters to hundreds of kilometers (Smith et al., 1990). Polynyas appear in winter when air temperatures are well below the freezing point for sea water. Therefore, some mechanism(s) must exist by which ice is removed as it forms or ice is prevented from forming in these regions.
First, ice can be continually removed from a region by the actions of winds or ocean currents. The heat required to balance the loss to the atmosphere (which again is much colder than the air) and hence maintain the open water area (i.e., the polynya) is provided by the release of latent heat when ice is formed (which again is continually removed from the region). Polynyas maintained by this mechanism are called Latent Heat Polynyas. If the offshore transport of ice caused by winds and currents is greater than the rate of frazil ice formation, the polynya becomes larger. Conversely, if the rate of ice formation required to balance the sensible heat loss to the atmosphere exceeds the transport of ice by the winds, the polynya becomes smaller. So, the transportation, or advection, of the sea ice by the winds must be balanced by ice formation for the polynya to exist in steady state.
In the second mechanism, sea ice is prevented from forming in a region by warmer waters entering the region by upwelling or mixing in sufficient quantities to balance the sensible heat loss to the atmosphere. Since these polynyas have an additional source of heat (sensible heat from the warmer waters), ice formation and the associated release of latent heat is not required to balance the heat loss to the atmosphere. This type of polynya is called a Sensible Heat Polynya. If the sensible heat entering a region is greater than the sensible heat loss to the atmosphere the polynya becomes larger. Conversely, if the sensible heat entering a region is less than the sensible heat loss to the atmosphere ice forms (to release latent heat and balance heat lost to the atmosphere) and the polynya becomes smaller. So the atmospheric sensible heat loss must be balanced by the sensible heat entering a region via upwelling, mixing, or advection for the polynya to exist in steady state.
Latent heat polynyas are capable of forming far more sea ice than sensible heat polynyas.