A NASA satellite confirms that overturning in the North Atlantic Oceana process where surface water sinks and deep water rises due to varying water densitiesspeeds up and slows down by 20 to 30 percent over 12- to 14-year cycles. Scientists previously believed that a change of this magnitude would take hundreds of years, rather than close to a decade.
The findings were made possible by TOPEX/Poseidon satellite data that measured sea surface height in the North Atlantic.
Sirpa Hakkinen, of NASA's Goddard Space Flight Center, used computer models to link sea surface height to natural cycles of overturning in the North Atlantic. She found that when overturning increases, so does the sea surface height, and vice versa.
As the Gulf Stream moves warm surface water from the equator north through the Atlantic, the water cools, gets saltier due to evaporation, and becomes very dense. By the time it approaches the coast of Newfoundland, south of Greenland, it becomes dense enough to sink. The current then slowly travels in a kind of conveyor belt in the deep ocean back to the equator where it warms, becomes less salty, and rises. This process of sinking and rising water is called overturning.
"The close association of the sea surface height and changes in overturning provides a simple method for assessing the state of the ocean," Hakkinen said.
Using TOPEX/Poseidon sea surface height data from 1992 to 2000, Hakkinen found that between the winters of 1995 and 1996, sea surface height over the Gulf Stream decreased by about 12 centimeters. She suggests that this large, abrupt change in sea surface height can be associated with a slow-down of overturning from peaks in the early 1990s.
"They are very large changes," Hakkinen said. "People have not wanted to believe that you can have these kinds of changes over a decadal time scale. They want to think about these changes over hundreds to thousands of years. But in fact, very large changes can occur over decadal time scales."
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.”