Multi-Year Ice

Multi-year ice is sea ice that has survived at least one summer melt season, the oldest WMO stage of development, harder and far less saline than first-year ice. Summer melt and snowmelt flush brine from the upper layers, dropping bulk salinity toward 1 to 3 practical salinity units and leaving rounded hummocks and refrozen melt ponds. The desalinated, bubble-rich ice is mechanically stronger and reaches 3 m or more thick, so it carries the most negative POLARIS risk values and threatens hulls below their ice class. Its low salinity gives a distinct passive-microwave and scatterometer signature that AMSR and QuikSCAT-style sensors use to map ice age.