In the North Atlantic Ocean in winter, the contrast between frigid, dry winter air and warm water draws heat from the ocean into the atmosphere and leaves ocean water colder and denser. The cold waters (blue arrows) sink and feeds into the lower limb of a global system of currents often described as the Great Ocean Conveyor. To replace the down-flowing water, warm surface waters (red arrows) from the tropics are pulled northward along the Conveyor’s upper limb. The ocean releases heat to the atmosphere, keeping winters in the North Atlantic region warmer than they would be otherwise. The sinking cold waters also draw down the man-made buildup of carbon dioxide from air to surface waters and eventually into the depths, where the greenhouse gas can be stored for centuries and offset global warming.
(Illustration by Jack Cook, Woods Hole Oceanographic Institution)