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After six years that set a record for Atlantic Basin tropical storms and hurricane activity, William Gray and his associates at Colorado State University are calling for a season slightly below the long-term average.
For 2001, they forecast nine named storms, five hurricanes and two major (Saffir-Simpson category 3-4-5) hurricanes. This compares with a 1950-90 average of 9.3 named storms, 5.8 hurricanes and 2.2 major hurricanes a year, which is only half the average seasonal activity of the last three years.
Gray, a professor of atmospheric science at Colorado State, warned that while the 2001 season seems likely to be milder than five of the past six years, it should still exceed storm activity in the surprisingly quiet period of 1970-94, when population and property grew at a rapid pace along the southeast U.S. coast and the Florida Peninsula.
"We’re not as confident about this forecast as we were about last year’s, but we anticipate a weak to moderate El Niño event beginning next summer, and that and several other indicators should produce a significant reduction in next year’s Atlantic Basin (North Atlantic, Caribbean Sea and Gulf of Mexico) storms," Gray said.
Gray and his colleagues believe that an El Niño, a body of relatively warm, equatorial water in the eastern Pacific, is due in 2001. A characteristic of El Niño is westerly winds in the upper troposphere that act to shear the tops off potential forming Atlantic easterly (barometric) waves, preventing them from growing into named storms or hurricanes.
"Most of us agree that a weak to moderate El Niño event should be anticipated for next summer," Gray said. "This should be an inhibiting factor in 2001.
"On the contrary, the Atlantic sea surface temperatures will tend to enhance next year’s prospects for U.S. hurricane landfall."
This occurs because relatively warm sea surface temperatures in the North Atlantic are a proxy measure of the strength of the Atlantic Ocean thermohaline circulation system, an ocean current that moves warm, salty water from the tropics to the North Atlantic. This circulation has been running strongly since 1995. Historically, a strong thermohaline circulation system has signaled more major hurricanes making landfall on the East Coast and Florida peninsula and more weaker cyclones making landfall along the U.S. Gulf Coast. Running strongly since 1995, the thermohaline circulation system, however, is unlikely to overcome the El Niño and other, lesser inhibiting factors. These include:
- The Quasi-Biennial Oscillation, equatorial stratospheric winds that reverse direction every 26-30 months. Colorado State researchers think this will be a weak constraining influence next year on Atlantic Basin hurricanes.
- A measure of rainfall in the Western Sahel and Gulf of Guinea regions of West Africa is suggestive of an inhibiting hurricane effect.
- The October-November 2000 strength of a ridge of high pressure over the Azores Islands is likely to have an inhibiting effect on next year’s hurricanes in the Atlantic Basin.
Last year, the Colorado State team’s final update in August called for 11 named storms, seven hurricanes and three intense hurricanes. The 2000 season wrapped up Nov. 30 with 15 named storms, eight hurricanes and three intense hurricanes.
Gray estimates the probability of one or more major (Saffir-Simpson 3-4-5) hurricanes making landfall along the entire U.S. coastline in 2001 at 63 percent (the past century’s average is 52 percent). For the U.S. East Coast and Florida peninsula, the 2001 average will be 43 percent (vs. 31 percent for the past century). The Gulf Coast, from the Florida Panhandle westward to Brownsville, has a major storm landfall probability of 36 percent compared with 30 percent for the century.
Because of the difficulty of estimating landfall probabilities for Caribbean land masses, Gray offered no probabilities but said next year the chances of a major storm coming ashore in this region are about average.
Gray maintains the United States has been especially fortunate in recent years with regard to major hurricane landfall. From 1900-2000, there have been 218 major storms in the Atlantic Basin, and 73 (roughly one-third) have come ashore along the U.S. coastline. In the years 1995-2000, the Atlantic Basin has seen 23 major hurricanes, but only three have come ashore. Had the typical one-in-three ratio held, seven or eight storms could have been expected to reach land.
"We have been lucky that upper-level trough frequency over the U.S. East Coast has been above average," Gray said. "This has led to the steering of most major Atlantic Basin hurricanes away from the mainland."
The situation on the East Coast and Florida peninsula is even more skewed. The first six decades of the 20th century have averaged 3.4 times more landfalling major hurricanes per year than have the last four decades.
"Climatology will eventually right itself and we must expect a substantial increase in landfalling major hurricanes on the U.S. East Coast and peninsular Florida in the coming few decades," he said. "Due to the great increase in coastal population and property values in the southeast in recent years, we should expect to see hurricane damage at levels never before experienced."
Gray and co-authors Chris Landsea, Paul Mielke Jr., Kenneth Berry and Eric Blake apply statistical regression in addition to searching for specific past years with analogous conditions to make their forecasts. In the case of 2001, the "analog years" are 1951, 1953, 1957, 1963 and 1965, which had weak to moderate El Niños. These years did not suppress hurricane activity as much as did the stronger El Niños of the 1970s through the mid-1990s.
"We believe we are in a new era when El Niños will not suppress hurricane activity as they have in the recent past," Gray said.
"We assume that the climate will continue to behave in the future as it has in the past century, and that prior, or precursor, years with conditions similar to the present will produce similar future storm activity," Gray said. "However, since our forecasts involve statistical regression schemes, there will be years in which they are not superior to climatology (i.e., long-term averages).
"What we believe is that 2001 will prove to be less active than the very busy seasons of 1995, 1996, 1998, 1999 and 2000, but more active than the average of the 25-year period of relatively low activity we saw beginning in 1970."
GRAY RESEARCH TEAM
HURRICANE FORECAST FOR 2001 SEASON
|Named Storms (9.3)*||9|
|Named Storm Days (46.9)||45|
|Hurricane Days (23.7)||20|
|Intense Hurricanes (2.2)||2|
|Intense Hurricane Days (4.7)||4|
|Hurricane Destruction Potential (70.6)**||65|
|Maximum Potential Destruction (61.7)||60|
|Net Tropical Cyclone Activity (100%)||90|
* Number in ( ) represents average year totals based on 1950-1990 data.
** Hurricane Destruction Potential measures a hurricane’s potential for wind- and ocean-surge damage. Tropical Storm, Hurricane and Intense Hurricane Days are four six-hour-long periods where storms attain wind speeds appropriate to their category on the Saffir-Simpson scale.