http://en.wikipedia.org/wiki/Arctic_Ocean#Salinity_and_temperature

In large parts of the Arctic Ocean, the top layer (about 50 m) is of lower 
salinity and lower temperature than the rest. It remains relatively stable, 
because the salinity effect on density is bigger than the temperature 
effect. It is fed by the freshwater input of the big Siberian and Canadian 
streams (Ob, Yenissey, Lena, MacKenzie), the water of which quasi floats on 
the saltier, denser, deeper ocean water. Between this lower salinity layer 
and the bulk of the ocean lies the so called halocline, in which both 
salinity and temperature are rising with increasing depth. Any convection 
eddies caused by the temperature difference between the cold ocean surface 
and the warmer depth stop at this thermocline, leaving only heat conduction 
as upward heat transport mechanism, which is orders of magnitude smaller. 
Without this insulation effect, there would be much less Arctic sea ice. The 
salinity and temperature pattern of the Arctic Ocean can be quite complex, 
being dependent on the different flows into and out of the Arctic region