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Ocean Surface Topography from Space
Simultaneous Measurements from Two Altimeters Bring Eddies into Focus
December 01, 2007

TOPEX/Jason-1 data on top and TOPEX/Jason-1 combined with ERS-1 data below
This image shows the data from TOPEX/Jason-1 (top), and the combined data from TOPEX/Jason-1 and ERS-1 (bottom).
Image credit: NASA/JPL-Caltech/Chelton, et al, 2007

More than half the variation in sea surface height across Earth's ocean is due to eddies, according to a recent study by Ocean Surface Topography Science Team members. These eddies, they concluded, are responsible for much of the westward energy propagation in the ocean. This new view of ocean dynamics comes from combining satellite data from multiple missions to reveal changes in the ocean surface in greater detail than is possible with a single instrument.

Dudley Chelton and three of his colleagues at Oregon State University analyzed ten years of sea surface measurements from two different sources: the TOPEX/Poseidon altimeter and the successive European Space Agency's ERS altimeters. The higher resolution of this merged data set showed the presence of many individual, slow-moving eddy-like features, between 5 to 40 centimeters in amplitude and 100 to 200 kilometers in diameter.

These features are too small to be resolved by the measurements from one instrument alone. For example, the ground tracks of the TOPEX/Poseidon and Jason altimeters are 2.8 degrees of longitude apart (about 315 kilometers at the equator). But smaller features do show up clearly in the merged data set Dudley and his colleagues used for their study, which was produced by the European data center Aviso.

An image of the ocean.
With the combined data, they were able to map the rotational speed and direction of these mesoscale eddies (cyclonic or anti-cyclonic, corresponding to negative and positive sea surface heights, respectively); their trajectories across the ocean; their lifetimes; and their propagation speeds and directions. "The observed eddies have a strong tendency for purely westward propagation at speeds similar to those of Rossby waves," says Dudley. Rossby waves are very large, planetary-scale waves that travel west across the Pacific. Scientists describe them as "linear," which refers to the mathematical equations used to describe their motion. The equations for nonlinear features are more complicated.

"Only 25 percent of the eddy trajectories we observed deviated by more than plus or minus 10 degrees from due west," Dudley says. "Significantly, however, the cyclonic and anticyclonic eddies have preferences for slight pole ward and equator ward deflection, respectively." This is consistent, he says, with what theories about large nonlinear eddies predict.

This is an image of Dudley Chelton, Oceanographer
Dudley Chelton, Oceanographer
"The most remarkable and dynamically important result from this analysis is the determination that the vast majority of the observed mesoscale features are significantly nonlinear. This was determined from the fact that their rotational speeds are faster than their propagation speeds, which is a commonly used measure of nonlinearity," Dudley says.

"The nonlinearity of the observed eddies distinguishes these features from linear Rossby waves. The distinction is important since nonlinear eddies can trap water parcels and are thus able to transport momentum, heat, mass and other water properties, including nutrients and chlorophyll that are vitally important to ocean biology."

"Another important conclusion from this analysis is that measurements from two or more simultaneously operating altimeters are critical to resolving these mesoscale features," he continues. "While there were hints of their existence in earlier studies, measurements from a single altimeter simply cannot resolve these features that have diameters smaller than the track spacing in the TOPEX/Poseidon. We are in the process of analyzing the data from the time period when four altimeters were operating simultaneously. From this, we will find out how much additional information is unresolved in the sea level fields constructed from just two altimeters."

"The merged altimeter data from two simultaneously operating altimeters has introduced a new paradigm about the ubiquitous westward propagation in the world ocean," according to Dudley. "Much of what has been interpreted by many, including us, as Rossby waves over the past decade is actually nonlinear eddies."

A paper summarizing the results of their study, "Global observations of large oceanic eddies," appeared in Geophysical Research Letters, Vol.34. Dudley and his co-authors have subsequently extended their analysis to include the merged Jason and Envisat data, thus resulting in a 15-year record of eddy mapping from altimetry.

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