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, Greenbelt, Md., 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 heats, 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.
The study, which appears in the July 15 issue of Journal of Geophysical Research-Oceans, supports the idea that ocean overturning drives the decadal cycles of the Gulf Stream current, as opposed to winds. With careful modeling simulations, Hakkinen extrapolated data back to the 1950s to find overturning shifts on a near decadal time scale.
"These kinds of occurrences can be expected every 12 to 14 years," she said. "It appears that in the last 50 years there have been three large-scale changes in the overturning."
When a lot of warm water moves northward, the overturning process initially speeds up, and that pulls the current along at a faster rate, and increases the sea surface height along the current. But eventually, as warm water starts to build up in the north near Newfoundland, it has a negative effect on overturning. When enough warm water collects up north, it becomes less dense, and slows down the circulation, which in turn lowers the sea surface height. The cycle starts anew as the warm water that has pooled north eventually cools again and begins to sink.
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 90s.
By closely examining these heights, Hakkinen's results showed that the rate of overturn not only increases and decreases according to near-decadal cycles, but that rate varies by as much as 20 to 30 percent from the annual mean.
"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."
"The altimeter has provided a useful tool to study and monitor the ocean circulation on spatial scales from 100 to 1000's of kilometers and temporal scales from days to years and beyond with the increasing record length," she said.
The Topex/Poseidon satellite was launched on August 10, 1992. A joint effort between NASA and France's National Center for Space Studies, this satellite mission measures global sea level every 10 days. This mission allows scientists to chart the height of the seas across ocean basins with an accuracy of less than 4 centimeters (1.5 inches), affording a unique view of ocean phenomena such as El Niño and La Niña. NASA's Jet Propulsion Laboratory manages the mission.
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