Albedo is the fraction of solar energy (shortwave radiation) reflected from
the Earth back into space. It is a measure of the reflectivity of the earth's
surface. Ice, especially with snow on top of it, has a high albedo: most sunlight
hitting the surface bounces back towards space. Water is much more absorbent
and less reflective. So, if there is a lot of water, more solar radiation is
absorbed by the ocean than when ice dominates.
Albedo is not important at high latitudes in winter: there is hardly any incoming
sunlight to worry about. It becomes important in spring and summer when the
radiation entering through leads
can greatly increase
the melt rate of the sea ice.