Characteristics of an Annually Recurring Open‐Ocean Polynya in the Southern Ocean

Polynyas are within the sea ice cover, typically formed by wind‐driven sea ice divergence or upwelling of warm subsurface waters. They play a crucial role in ocean‐atmosphere interactions, climate regulation and marine ecosystems by substantially enhancing primary production. Open‐ocean polynyas in the Southern Ocean are rare and are typically associated with deep convection, which disrupts conventional circulation pathways and impacts regional heat and carbon budgets. The Cosmonauts Sea (30°E–60°E) is an exception, with open‐ocean polynyas forming annually. Using satellite‐derived sea ice observations, we examined the spatiotemporal variability of polynyas in this region over the past two decades. The Cosmonauts Sea polynya exhibited large spatial and interannual variability, with the largest event occurring in 2016 (139,000 km 2 ). An Argo float near the polynya recorded deep mixed layers (>400 m) and near‐complete erosion of stratification, and the presence of dense water. This event coincided with anomalously intense cyclonic wind stress curl due to synoptic scale storms and a prolonged positive Southern Annular Mode (SAM) phase (2014–2016), both generally associated with reduced sea ice concentrations. While the southward shift of the Antarctic Circumpolar Current (ACC) during 2015 acted as a preconditioning mechanism, bringing warmer water towards the polynya region and inducing upwelling by vortex stretching. Additionally, anomalously high shortwave radiative fluxes (∼+20 Wm −2 ) were observed in the summer preceding the 2016 event. The deep convective mixing observed during this event, together with the presence of dense water, indicates that the Cosmonauts Sea could be a potential dense water formation site.