Saline water is more dense than fresh water.
Greenland Ice Core measurements going back thousands of years indicate that the North Atlantic SST patterns typically fluctuate back and forth on 20-50 year time scales and appear to fit this hypothesis. Warmer than normal North Atlantic SSTs occur with a stronger Atlantic thermohaline circulation and colder SSTs with a weaker circulation. A faster thermohaline circulation causes larger amounts of warm upper ocean mass to be advected to higher latitudes. These back and forth shifts are hypothesized to result for natually occurring North Atlantic salinity variations on time scales of 20-50 years. Figure 2 shows North Atlantic SST anomalies over the last 100 years.