The main reason for this is that warming enhances the amount of water vapor in the atmosphere, which then snows - off in the dry and cold polar region where it releases energy and warms the atmosphere. (geoengineeringwatch.org)
The temperature gradient creates atmospheric circulation, which transports heat from areas of equatorial excess to the cold polar regions. (discovermagazine.com)
Warmer waters then warm the air, which reduces the temperature difference between the cold polar region and the warmer rest of the Northern hemisphere. (sciencedaily.com)