Review article: Arctic polynyas in a changing climate

Abstract. The Arctic is the fastest changing region in the world. Its atmosphere is warming 3–4 times the global average; the ocean is warming through increased advection of heat from the Atlantic and Pacific; the sea ice cover is disappearing, thinning, and becoming more mobile. The interactions between ocean, ice and atmosphere are also evolving. These changes are manifest in polynyas, openings in the sea ice that are crucial for exchanges of heat, moisture, momentum and gases between ocean and atmosphere, but also for ecosystems, Indigenous communities, and navigation. We here review how polynyas and their impact on the climate system have changed throughout the satellite record, focussing on the last twenty years. The trend is of fewer but larger polynyas that open for longer, unless they depend on ice arches (in which case they are shorter-lived). Many regions have switched from ''latent heat'' polynyas, only driven by dynamics, to ''hybrid'', where the heat from ocean or atmosphere matters. Consistent with that switch and with an overall warming and wetting of the atmosphere, moisture fluxes from polynyas in winter seem to be decreasing despite an increase in polynya area. How this will impact the low altitude clouds, generated by polynyas and that help maintain polynyas, has not been studied. Similarly, sea ice production in polynyas is decreasing at the pan-Arctic scale. In the ocean's Eurasian basin, process studies suggest a conflict between this switch to hybrid polynyas (suppressing dense water formation) and Atlantification (favouring it). In the Amerasian basin, where upwelling of warm water is necessary for dense water formation, this should be enhanced, but it also facilitates the inflow of warm water into the Greenland glacial fjords. In all components of the climate system, large uncertainties remain because of the lack of direct observations and the lack of continuous time series in the atmosphere, ice, and ocean.