A surprising return

One of the “pumps” that helps drive the ocean’s global circulation suddenly switched on again last winter for the first time this decade. The “pump” is in the western North Atlantic Ocean, where pools of cold, dense water form in winter and sink beneath less-dense warmer waters. The sinking water feeds into the lower limb of a global system of currents often described as the Great Ocean Conveyor. The finding surprised scientists who had been wondering if global warming was inhibiting the pump and did not foresee any indications that it would turn back on. (Illustration by Jack Cook, Woods Hole Oceanographic Institution)