Wouldn't more fresh water result in a smaller concentration of salt, meaning the colligative properties change? aka i believe the water would be susceptible to change in temperature based on the same change in heat.
i think they go over this in the al gore movie. as the warm water moves up from the south and cools, it becomes more dense and sinks. this helps keep the warm water flowing up from the south as it cycles. if you add fresh water it affects makes the salt water less salty and dense and it doesnt sink and the cycle slows down or stops the flow of warm water coming up from the south. i think it went something like that