Strongest El Niño In History Dampers '97 Hurricane Season; Colorado State's Gray Says Three-Year Period Still Most Active
Wednesday, November 26, 1997
Note to Editors: Copies of Professor William Gray's
verification report for the 1997 hurricane season and
related press releases will be available this afternoon on
the World Wide Web at
http://tropical.atmos.colostate.edu/forecasts/index.html
or by calling the University Relations office at (970)
491-6432. Dr. Gray's first forecast for the 1998 hurricane
season will be issued Friday, Dec. 5.
FORT COLLINS--Despite the strongest summer El Niño event on
record, 1997 hurricane activity in the Atlantic Basin was 54
percent of the long-term average but was less than predicted by
Colorado State University's noted team of hurricane forecasters.
The team, lead by Professor William Gray, issued a report
today (Nov. 26) that outlined why the El Niño of 1997 flattened
the team's August prediction of 11 named storms, six hurricanes
and two intense hurricanes for the season. Instead, the Atlantic
Basin saw seven named storms, three hurricanes and one intense
hurricane during the season, which ends Nov. 30. On average, 9.3
tropical storms, 5.8 hurricanes and 2.2 intense hurricanes form
annually.
Although the hurricane season was below average, Gray's
statistics show that the period between 1995-1997 was still the
busiest three-year period for hurricane activity on record. The
three-year span generated 39 named storms, 23 hurricanes (13 of
which were intense) and 116 hurricane days.
"We knew going into the hurricane season that this would be
an extremely difficult year to forecast," Gray said. "The El Niño
proved to be twice as strong as any other previous record El Niño
event in history for this time of year. No one guessed that it
would grow to be so intense. And yet, despite this very extreme
weather event, we still saw hurricane activity--more than was to
be expected."
El Niño is a weather phenomenon marked by warmer-than-normal
water temperatures in the eastern Pacific Ocean off the coast of
Peru and along the equator. This rise in ocean temperatures
causes strong upper tropospheric winds to blow in a westerly
direction from the Pacific Ocean to the tropical Atlantic Ocean.
These winds typically act to shear off developing hurricanes.
Gray said that in other years with strong El Niño events,
such as 1957, 1972 and 1982, waters warmed only 2 or 3 degrees
centigrade above normal. But the El Niño of 1997 actually warmed
waters 4 or 5 degrees centigrade above normal--nearly twice as
much as the previous record El Niño of 1982-83. This rare and
extreme rise in ocean temperatures helped produce even more
intense westerly upper tropospheric winds in the Atlantic Basin,
which caused strong wind shear and prevented most easterly waves
>from Africa from forming.
Gray and his team of researchers are investigating the
possibility that the extreme El Niño this year may have been the
result of a long period of warm water accumulating in the western
Pacific, possibly left over from smaller El Niño events in 1991-
1993. The team contends this kind of warm water build-up could
only have produced the type of El Niño that emerged this year.
Despite El Niño's extreme influence over the Colorado State
team's 1997 hurricane forecast, Gray points out that factors in
the Atlantic favorable for hurricane activity were still enough
to produce seven named storms this year. These factors included
warmer sea surface temperatures in the north and tropical
Atlantic and colder sea surface temperatures in the South
Atlantic, as well as colder than normal air temperatures 54,000
feet above Singapore. Also present was the Quasi-Biennial
Oscillation, equatorial stratospheric winds at 68,000-75,000 feet
than tend to promote hurricane formation when they blow from the
west--as they did this year.
And, as predicted in the team's August forecast, El Niño
pushed many of the storms that did form in 1997 to higher
latitudes--some of them closer to the United States. Of the seven
named storms that formed in the Atlantic, six originated above 25
degrees north latitude, higher latitudes than hurricanes
typically form. Gray attributes this to the fact that while El
Niño produces strong upper-level westerly winds at lower
latitudes that block African-origin storms, it also creates
weaker upper-level westerly winds at higher latitudes that are
less able to thwart hurricane development.
Using atmospheric models, Gray and his colleagues have shown
that if the El Niño of 1997 had only been as intense as previous
record El Niño events in 1957, 1972 and 1982, those positive
factors for hurricane formation would have generated 10 named
storms, six hurricanes and three intense hurricanes--virtually on
target with the team's prediction.
"The 1997 El Niño was truly in a class by itself," Gray
said. "But I don't think it will be around to influence the 1998
hurricane season to any significant degree."
The Colorado State team's historical data shows that nine
out of the past 30 years have actually produced less hurricane
activity than in 1997. Of the nine years that were less active,
seven occurred during El Niño events. When Gray's team issues the
first forecast for the 1998 season on Dec. 5, the statistical
model will now include the extreme 1997 El Niño conditions.
The team's hurricane forecasts--issued in early December,
April, June and August--do not predict landfall and apply only to
the Atlantic Basin, which encompasses the Atlantic Ocean,
Caribbean Sea and Gulf of Mexico.
In addition to Gray, the hurricane research team includes
John Knaff, Paul Mielke and Kenneth Berry from Colorado State;
and Chris Landsea, a Colorado State graduate and a researcher at
NOAA's Hurricane Research Laboratory in Miami, Fla.
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