Cold water is denser than warm water, and so sinks. Ice melting into the ocean cools off the water, causing it to sink. This creates a current by which cold water is sinking near the poles, which pushes the water in the depths of the ocean, driving the entire ocean to circulate. This cold, deep water is then forced back up against the western shores of continents where it can be warmed up (it's always the western shores because the Earth is rotating from west to east).