Strongest El Niã?o in History Dampers ’97 Hurricane Season;Colorado State’s Gray Says Three-Year Period Still Most Active

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 Niñ’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.