The warm tropic water mentioned here is the Gulf Stream. If the Gulf Stream ends it's travels up the east coast of the USA that will cause enormous changes in the water temperature along with all the fisheries that count on it, including the Sargasso Sea. The Sargasso Sea is an entire ecosystem that will be lost if the temperatures of the Atlantic Ocean change.
...Another concern among scientists (click here) is that melting sea-ice is affecting a major ocean current in the Arctic - the Beaufort Gyre.
Freshwater is less dense than salty seawater. The researchers said a sudden influx of freshwater from the Arctic Ocean into the northern Atlantic Ocean could alter the strength of the current.
This is because the force pushing water down the eastern coast of continental North America will be reduced, resulting in a smaller volume of warmer tropic waters from equatorial regions being displaced towards western Europe.
Models suggest the reduction in warmer waters heading towards western Europe will result in lower temperatures in the region. This, in turn, would also affect weather patterns in the global climate system.
