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September-October 2010

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Weather Queries

Have two hurricanes ever crossed paths? If so, what happened?

Tedd Beegle
Boulder, Colorado

Tropical cyclones (which include both tropical storms and their stronger progeny, hurricanes) occasionally collide, not in the manner of two cars heading toward each other in a game of “chicken,” but rather by spiraling slowly inward toward each other.

Sakuhei Fujiwhara, a noted Japanese meteorologist, studied the interaction among multiple vortices (swirls) in bodies of water during the early 1920s. He noticed that if two vortices of the same size, strength, and direction of spin approach each other, they spin about a common point halfway between them, as this point moves off in a constant direction. The Fujiwhara Effect, the tendency of two nearby tropical cyclones to rotate cyclonically about each other (in a counterclockwise direction), is named after him.

A study of tropical cyclones from 1945–1981 concluded that the Fujiwhara Effect occurs only occasionally, an average of 1.5 times per year in the western North Pacific but only about once every three years in the North Atlantic. Large tropical storms interact only if they come within about 1400 kilometers of each other. For smaller storms, the critical separation distance for interaction is less. Of all interacting cyclones, about 70 percent orbited about each other, with the orbital speed greater for smaller separations.

Two tropical cyclones approaching each other may or may not collide, as shown in the diagram in Figure 1. The letter A designates the initial positions of two storms. As they come nearer, the circulation of each storm influences the track of the other, and the track of each becomes an arc about a central point labeled M. The motion depicted here is relative to M, but bear in mind that M is moving with respect to the Earth's surface. At some later time, the cyclones reach point C, when they begin to orbit about each other. Mutual orbiting is in progress at point O. If the two storms are similar in size, they continue in orbit and eventually escape each other (E). If one is considerably larger and more intense than the other, the two storms can merge, the larger one engulfing the smaller one. Merging does not imply strengthening. The opposite is more likely because the flow at the edge of one storm opposes the flow at the adjacent edge of the other. In 1995, Hurricane Iris merged with much weaker Tropical Storm Karen and absorbed it.

The high winds and towering waves in hurricanes, especially slow-moving ones, mix ocean water near the surface, causing a reduction in sea-surface temperature. If a later hurricane follows a track similar to that of an earlier hurricane and thereby encounters cooler surface water, it may weaken. High sea-surface temperatures are essential in maintaining a hurricane's strength through the supply of warm, saturated air at the base of the storm.

Figure 1. Potential paths of two interacting tropical cyclones. The initial positions of the two cyclones are shown at A. As their separation decreases, each affects the trajectory of the other. At point C, they begin to orbit about a central point (centroid) at M. If the two cyclones are roughly equal in size and strength, they complete their orbit about each other and exit at E. If one is significantly larger and stronger than the other, they may spiral toward each other from O and eventually merge near M.

Figure 1. Potential paths of two interacting tropical cyclones. The initial positions of the two cyclones are shown at A. As their separation decreases, each affects the trajectory of the other. At point C, they begin to orbit about a central point (centroid) at M. If the two cyclones are roughly equal in size and strength, they complete their orbit about each other and exit at E. If one is significantly larger and stronger than the other, they may spiral toward each other from O and eventually merge near M.

Does an El Niño affect the frequency of hurricanes?

Sue Gruning
Roseland, New Jersey

This is a 17-year old question, whose answer is clearer today than when it was first posed. Yes, El Niño does affect hurricane frequency. El Niño refers to anomalously warm surface water in the central and eastern tropical Pacific Ocean. Its opposite, cooler than normal water in the same location, is called La Niña. These conditions are part of what is called the El Niño Southern Oscillation (ENSO), an irregular swing in sea-surface temperature from warmer to cooler than normal, and back again, that recurs every two to seven years. ENSO is associated with a seesaw in surface pressure between the western and eastern tropical Pacific. During an El Niño, a weakening of the trade winds suppresses upwelling of cold water off the coast of South America and results in rising sea-surface temperature from there westward. For more than a century, it has been known that torrential rains sometimes follow in the coastal deserts of southern Ecuador and northern Peru, particularly during a pronounced El Niño. Only since 1969 has it been recognized that El Niño and, more generally, ENSO can affect weather patterns in places far removed from the tropical Pacific Ocean. One effect is a reduction in the number of hurricanes in the tropical North Atlantic Ocean during El Niño years.

A study of hurricanes striking the U.S. coastline between 1900 and 1997 (Bulletin of the American Meteorological Society, Vol. 79, No. 11, 1998, pp. 2477–2482) found that during an El Niño year, the probability of two or more hurricanes making landfall in the United States is 28 percent. The probability of this happening during neutral years (neither El Niño nor La Niña in progress) is 48 percent. The probability during La Niña years is 66 percent. The same study compared the annual average number of named tropical cyclones in the Atlantic and Eastern Pacific Oceans when El Niño was in progress with the annual average for all years (Table 1).

Table 1. Annual average number of named tropical cyclones by category in the Atlantic and Eastern Pacific Oceans during El Niño years and all years, period considered: 1900–1997

 

Atlantic

Eastern Pacific

Average

El Niño average

Average

El Niño average

Named storms

9.4

7.1

16.7

17.6

Hurricanes

5.8

4.0

9.8

10.0

Intense hurricanes

2.5

1.5

4.8

5.5

In the North Atlantic, it is clear that tropical storms and hurricanes are significantly less likely in El Niño years than overall. In the Eastern Pacific, these storms are a little more likely in El Niño years than overall. In the Atlantic, the reason is that vertical shear (the difference between the wind near the surface and high troposphere) is greater during El Niño years than in other years. Strong shear is the nemesis of hurricanes; it tends to disrupt the circulation about the eye. Evidence also points to a somewhat drier and more stable atmosphere over the tropical Atlantic during El Niño years.

The tendency for fewer Atlantic hurricanes during El Niño certainly held true during the 2009 hurricane season. There were nine named storms, of wich only three became hurricanes, and two became intense hurricanes. Tropical Storm Claudette, the only storm to make landfall in the United States, caused minor wind and water damage.

Caption: In the July/August issue of Weatherwise, the Figure 5 image in the “Weather Queries” section on page 47 was corrupted. The correct image is reprinted here.

Caption: In the July/August issue of Weatherwise, the Figure 5 image in the “Weather Queries” section on page 47 was corrupted. The correct image is reprinted here.

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author in care of Weatherwise, Taylor & Francis, 325 Chestnut Street, Suite 800, Philadelphia, PA 19106.

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