Submerged in the Atlantic Ocean off the coast of Spain and Portugal are giant, salty whirlpools of warm water. These deep-water whirlpools are part of the ocean’s circulatory system, and they help drive the ocean currents that moderate Earth’s climate. Warm water ordinarily sits at the ocean’s surface, but the warm water flowing out of the Mediterranean Sea is so salty (and therefore dense) that when it enters the Atlantic Ocean at the Strait of Gibraltar, it sinks to depths of more than 1,000 meters (one-half mile) along the continental shelf. This underwater river then separates into clockwise-flowing eddies that may continue to spin westward for more than two years, often coalescing with other eddies to form giant, salty whirlpools that may stretch for hundreds of miles. Because the eddies originate from the Mediterranean Sea, scientists call them “Meddies.”
On land, the passage of a severe storm might be marked by fallen trees or swollen streams. In the ocean, a hurricane leaves a swath of cold water in its path. That trail of cold water marks the passage of Hurricane Bertha through the North Atlantic Ocean in this sea surface temperature image.
La Niña, the large area of cold water in the Pacific Ocean widely blamed for last summer's drought and often related to an increase in the number of hurricanes that make landfall, appears to be on its last legs.
A trio of globes of sea surface height anomalies shows a deep pulse of warm water—a Kelvin wave—crossing the Pacific Ocean in February 2010. Kelvin waves strengthen and maintain El Niño episodes.