[Smedley]

Usually a surface current of warm water flows northward in the Atlantic toward Greenland and Europe, then cools and sinks, returning south in the deep ocean. But the fresh water, which is less dense, blocked the sinking of the cold, salty water in the North Atlantic, disrupting the currents.

How can "less dense" fresh water block the sinking of cold, salty water? Higher density fluids sink relative to other fluids. Is this New York Slimes science, or am I missing something?

[Pharmboy]

That sentence stopped me also. Perhaps the high-riding, less dense fresh water disrupted the salt water cycling...

[supercat]

How can "less dense" fresh water block the sinking of cold, salty water? Higher density fluids sink relative to other fluids. Is this New York Slimes science, or am I missing something?

If a layer of salt water sits beneath a layer of fresh water, convection will be far less effective than it would be if all water were of uniform salinity. The article's terminology is weird, however, since the salt water wouldn't be stuck on the surface but rather below.

Incidentally, ice is nearly always frozen fresh water; the act of freezing pushes out the salt. This could create variations in salinity, though in most cases natural mixing would prevent them from becoming too significant.