The answer is; Formation of sea ice increases the salinity of the oceans
When ice form in water, this water is ‘osmotically isolated’ (in solid form) hence making liquid water become lesser in quantity than before. Since the number of dissolved salts remain more or less the same and the liquid water in which it is dissolved has reduced, then the water becomes more concentrated (because they are now more ppm of salts in each cubic amount of the water).