Antarctic coastal polynyas in the global climate system

Coastal polynyas describe regions of persistent open water within the sea-ice pack. In this Review, we outline the critical importance of Antarctic coastal polynyas in the Earth system (including for the atmosphere, sea-ice, ocean and biosphere) and outline their past, present and future changes. Strong offshore winds are the primary force opening coastal polynyas, varying on synoptic timescales to influence polynya existence and size. The exposed ocean surface ventilates heat to the atmosphere, allowing sea surface cooling and frazil ice formation. Frazil ice increases the salinity of surface waters, ultimately sinking as dense shelf water that drives the southern limb of the global ocean overturning circulation. Light and nutrient availability in coastal polynyas also encourages high primary productivity, making them critical aspects of the Antarctic marine food web. Coastal polynya strength and location varies through time, most notably at glacial–interglacial timescales owing to changes in continental shelf available for polynya formation. Predicting the future evolution of Antarctic coastal polynyas is challenged by inadequate model resolution and poorly constrained processes and behaviours, but there are indications that activity will decline with warming. A coordinated and expanded campaign of in situ measurements, as well as new satellite-based observations that use intelligent algorithms, would improve coupled atmosphere–sea-ice–ocean models and, thereby, enhance knowledge of Antarctic coastal polynyas. Antarctic coastal polynyas have a critical role in the Earth system, influencing the atmosphere, hydrosphere, cryosphere and biosphere. This Review outlines the importance of Antarctic coastal polynyas and documents their changes over time.