Colorado State University’s team of hurricane forecasters said global weather patterns continue to support above-average hurricane activity in 1997, ushering in the possibility the Atlantic Basin will experience the most active three-year hurricane span on record.
The noted team, led by Professor William Gray, released an updated forecast today that continues to predict 11 tropical storms forming in 1997, with seven hurricanes and three intense hurricanes developing. On average, 9.3 tropical storms, 5.8 hurricanes and 2.1 major hurricanes form annually.
If the team’s prediction materializes, 1997 would be the third consecutive above-average hurricane season, making the period between 1995-97 the most active three-year hurricane period in the last 120 years. This trend contrasts with 1991-94, the most inactive four-year stretch within the same period.
Gray said this recent trend of above-average hurricane activity supports the team’s theory that the Atlantic Basin is entering a new era of more tropical storms and hurricanes, generating more intense and, therefore, damaging hurricanes.
"We experienced a downturn in hurricane activity from the early 1970s to the early 1990s, when there were fewer intense hurricanes," Gray said. "Now it appears we are entering a period of increased hurricane activity similar to the late 1940s to the late 1960s, when there were many more major hurricanes making landfall along the East Atlantic coast, Florida and the Caribbean."
Although these intense hurricanes make up only one-fourth of all named storms that form, they cause about 75 percent of all tropical storm damage. Intense hurricanes have sustained winds of at least 111 mph and fall into categories 3, 4 and 5 on the Saffir-Simpson scale. Hurricanes Hugo in 1989 and Andrew in 1992 were both Category 4 when they made landfall. In comparison, storms categorized simply as hurricanes have sustained winds of 74 mph or greater.
Gray’s hurricane forecasts–issued in 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.
The recent forecast remains unchanged from the forecast Gray’s team made in April, even though one important factor that usually helps reduce hurricane activity seems to be present. The team and other meteorologists agreed that an El Nino–a period of warmer than normal water temperatures off the coast of Peru–is now in place. When El Nino is present, water temperatures reach about 27 to 29 degrees Celsius, or about 1 or 2 degrees Celsius more than normal. This rise in ocean temperature causes strong winds to blow in a westerly direction from the Pacific Ocean to the Atlantic Ocean. These strong winds typically act to shear off developing hurricanes.
But Gray and his team said most other factors used to reach their prediction point to an above-average season and are expected to offset most negative El Nino influence.
"The many other factors we look at in the Atlantic Basin are so positive for hurricane development that I do not think El Nino will play a dominant role in reducing storm activity this year," Gray added.
Other global weather features that strongly indicate an above-average hurricane season include warmer sea surface temperatures in the North Atlantic and tropical Atlantic, colder sea surface temperatures in the South Atlantic and increased rainfall in the Sahel region in West Africa. In addition, Gray’s team maintains that the equatorial stratospheric winds at 68,000-75,000 feet–known as Quasi-Biennial Oscillation–will blow from a relative westerly direction. This tends to promote the formation of especially intense hurricanes.
Gray and his colleagues also look at several other weather signals around the globe to reach their forecast. For example, Gray says that temperature readings of the air above Singapore at the 100 millibar level (54,000 feet) continue to be colder than usual. That serves as a precursor for above-normal hurricane activity for the Atlantic Ocean during the season, which began June 1.
Other factors the team examines include temperature and pressure readings in West Africa, Caribbean Sea-level pressure readings, tropospheric winds at 40,000 feet and pressure readings in the Caribbean Basin. An equally important long-range indicator of hurricane activity is the oceanic conveyor belt, the circulation of ocean water through the Pacific Rim and Indian Ocean up the west coast of Africa and into Atlantic polar regions, where it is chilled and then sinks back to lower levels before returning south. Gray believes this circulation pattern–which slowed between 1970-1994 to produce conditions that inhibited major hurricane formations–is now speeding up and therefore increasing the potential for above-average hurricane activity in the coming decades.
In addition to Gray, the research team includes Chris Landsea, Colorado State graduate and researcher at NOOA’s Hurricane Research Laboratory in Miami, Fla.; John Knaff, post-doctoral researcher at Colorado State; and statistics professors Paul Mielke and Kenneth Berry from Colorado State.