Why does melting sea ice create a fresher surface layer?

Melting sea ice creates a fresher surface layer because sea ice is composed primarily of pure water, having expelled most of its salt content during the freezing process.12

When seawater freezes, salt is rejected from the ice matrix as brine, leaving the resulting ice with a significantly lower salinity than the surrounding ocean. Over time, through processes such as gravity drainage and flushing, sea ice becomes even further desalinated. When this relatively salt-free ice melts during warmer months, it releases this trapped fresh water back into the upper layers of the ocean. This addition of fresh water dilutes the local seawater, effectively lowering the salinity and creating a fresher surface layer.1234