What are leads and polynyas in Arctic sea ice?

In the Arctic Ocean, the continuous motion of the ice pack caused by wind and water stresses leads to the formation of cracks and open water areas known as leads and polynyas. Even in regions with permanent ice cover, these features account for approximately 10 percent of the surface area.1

Leads

Leads are linear fractures in the sea ice that can extend for hundreds of kilometers. They vary in width from a few meters to hundreds of meters and are widespread throughout the pack ice at any time of year. In winter, these open cracks typically freeze over quickly.2

Polynyas

Polynyas are semipermanent, recurrent areas of open water or thin ice within regions that would otherwise be covered by sea ice. They are generally categorized into two types:32
* Coastal polynyas: These form just beyond landfast ice, often driven by persistent offshore or katabatic winds that push ice away from the shore. Because they allow for continuous ice formation and removal, they are sometimes called "ice factories".32
* Open-ocean polynyas: These are larger, longer-lasting features that form within the ice cover, believed to be caused by the upwelling of warmer deep water.3

Polynyas are vital to the Arctic ecosystem, serving as essential feeding and overwintering grounds for marine mammals and birds.3