

Opposite eddies

Jan 1, 2003

Every Gulf Stream navigator knows that there are warm-core eddies to the north and west of the stream and cold-core eddies to the south and east of the current. Navigators are also familiar with how these features rotate (clockwise for warm and counterclockwise for cold eddies). Only in the past few years, however, have researchers confirmed the existence of other types of eddies that do just the opposite.

A classic example of what experts call "anomalous" eddies lay directly astride the rhumb line in the 1994 Newport/Bermuda Race. Navigators and skippers had to rethink the way they normally handle eddies to take advantage of this anomalous feature, or, at least, avoid being slowed by it.

The favorable side of that eddy, located well south of the main body of the stream, was about 40 miles east of the rhumb line. The eddy itself was rotating clockwise with speeds of two to three knots. Normally an eddy in that area would be rotating counterclockwise with similar current velocities. In that case, the favorable track within the eddy for southbound vessels might be 40 miles west of the rhumb line.

"We knew it was there and we had at least a half-dozen boats confirm its existence after the race," said Jenifer Clark, an oceanographer for the National Ocean Service (NOS) who analyzes the Gulf Stream's thermal structure. Clark said she has observed nine such wrong-way eddies since 1991.

"They may be out there much more than we are aware, because the region is so often clouded over, and we can't always observe the surface with our satellite-mounted infrared sensors," said Clark. "These anomalous eddies usually exist in conjunction with a normally rotating eddy and a common territory with common current typically exists between them," said Clark. The NOS oceanographer said that in waters south of the main body of the stream, current in the common territory between such eddies typically runs south at two to three knots, whereas in waters north of the Gulf Stream current in such common territory typically runs north at one to two knots.Rarely seen features

Brin Ford of New Haven, Conn., who was navigator of a 47-foot sloop in the 1994 race, said he elected to sail straight through the wrong-way eddy south of the stream just to play it safe. "I've done 13 of these races, and I've never seen anything like it," said Ford. "As we got to the top of the eddy there was absolutely no question that we were set to the east and then being set in the opposite direction at the bottom, and it was definitely rotating in the wrong direction."

Race competitors also reported that they found currents in the main body of the stream to be between four and six knots in this year's race. That was considerably faster than the more typical speeds of 2.5 to 3.5 knots.

The 700-mile Newport/Bermuda Race is held in even calendar years. A similar race from Marion, Mass., is held in odd-numbered years. Navigators in both of these races often report back to Clark with Gulf Stream information and measurements.

"Wrong-way eddies," are, in many cases, located near a standard eddy; the pairs of eddies are technically referred to as double vortices. As such they are not considered rare or unusual, since double vortices can easily be induced in stratified or rotating bodies of water (including a bathtub), and they have often been observed in other bodies of water including areas near the Gulf Stream, the Pacific Ocean near the U.S. and Mexican coasts, and the Black Sea. It is well established that double vortices are generated near the Gulf Stream, but scientists are not at all certain how, according to Dr. David Sheres who researches the phenomenon for the University of Southern Mississippi.

"The Gulf Stream region is definitely one of the breeding grounds for these patterns, but we're just not sure of the mechanism," said Sheres. "Most typically, we observe double vortices in areas where there can be some sort of abrupt change, sometimes induced by weather. This might be the case in an area where you can have pulses of very high wind or a sudden influx of new water. There are many cases where if you give a body of water a strong push for a short period of time you can all of a sudden see the creation of double vortices. Or if you can cause rotation of water in one direction you may see nearby rotation in the opposite direction at the same time. It seems related to the concept that angular momentum must be conserved."

Dr. Sheres said he has been observing double vortices created in waters just west of the Santa Barbara Channel off the coast of southern California since the 1980s.

"When the wind is very strong along a north-south axis for a day or so, associated somehow with a jet of water out of the channel, we may very readily begin to observe double vortices swirling around in waters just west of the channel," he explained.

Double vortices tend to move as a unit, and they tend to push each other in one particular direction," Dr. Sheres added.

Chances are that the average mariner passing through the Gulf Stream won't be aware that he or she is encountering a double vortex. Moreover, those that do sail into one of these features stand an equal chance of encountering the well-behaved half of the eddy pair as they do of encountering a wrong-way eddy.

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