Havsis

Havsis is the Swedish term for sea ice, the frozen surface layer of ocean water that forms in polar and subpolar regions. In scientific usage, havsis refers to the layer that covers parts of seas during winter and can persist into summer in some areas. The word combines hav, meaning sea, and is, meaning ice.

Formation and structure: Havsis forms when surface seawater cools below its freezing point, with salinity affecting the exact temperature at which freezing occurs. As ice forms, salt is expelled into brine layers, creating a network of pores and channels within the ice. Ice can be classified as first-year ice, which forms and melts within a single year, or multi-year ice, which survives multiple seasons and tends to be thicker and more durable.

Distribution and seasonal cycle: Havsis is most extensive in the Arctic Ocean and nearby seas, where it exhibits a pronounced seasonal cycle with maximum extent in late winter and minimum extent in late summer. Around Antarctica, sea ice forms and expands during the austral winter, contributing to regional seasonal dynamics. The precise extent and thickness of havsis vary year by year with atmospheric temperatures, wind, and ocean conditions.

Measurement and data: Satellite observations using passive microwave sensors provide global estimates of havsis extent, while thickness is inferred from radar, altimetry, and, historically, submarine-based measurements and buoy data. Field campaigns and airborne surveys supplement satellite data to capture ice properties and deformation.

Climate context: Havsis is a key indicator of polar climate and plays a role in albedo, ocean heat exchange, and nutrient cycling. In recent decades, Arctic havsis has shown a long-term decline in both extent and thickness, with regional variability. Changes in sea ice influence ecosystems, weather patterns, and maritime activity in polar regions.