Summary
Polynyas are persistent and recurrent regions of open water and/or thin ice or reduced ice concentration, tens to tens of thousands of square kilometers in areal extent, that occur within the sea ice zones of both hemispheres at locations where a more consolidated and thicker ice cover would be climatologically expected. Rather than simply constituting recurrent "windows" in the sea ice, polynyas are profoundly affected by, and intimately linked to, local and even regional ice conditions (i.e., the "icescape"). They respond sensitively to thermodynamic and dynamic forcing by the ocean and atmosphere and entail ecologically important "oases" that enable birds and mammals to overwinter at high latitudes and encourage enhanced primary production in the spring. In this review, we introduce the concept of polynyas from the perspective of the sea ice conditions/processes that define them. We discuss the unique characteristics of polynyas in both polar regions, and assess their possible response/contribution to climate variability and change. An inventory of Northern Hemisphere polynyas is presented, based primarily of satellite data analysis but also on information from the literature and aboriginal peoples. Summary statistics on polynya opening and closing dates are also provided, along with information on the availability of light relative to the seasonal cycles of sea ice. In the Southern Hemisphere, we present an update of an inventory of Antarctic polynyas and discuss how coastal, glacial and deep-ocean processes affect their and distribution. Two important polynyas are examined in more detail, i.e., the North Water (NOW) polynya in the north and the Mertz Glacier polynya in the south. These case studies focus on details of the different physical processes driving their creation, maintenance and dissolution. Each of these polynyas has been the focus of dedicated in situ research programmes in recent years.