1990 Hurricane Alert
A BREAK in Africa's western Sahel drought might not seem to mean much for the Caribbean Basin and eastern North America. But this year it brought them an unexpectedly intense hurricane season, including mighty Hugo. Hurricane forecaster William M. Gray of Colorado State University in Fort Collins, Colo., warns that, if the western Sahel summer season has undergone a basic change from dry to wet, the 1990s are ``likely to see the return of more intense Atlantic hurricanes as were experienced in the 1950s and 1960s.''
Robert Sheets, who heads the National Hurricane Center in Miami, agrees that ``we are probably in a new cycle.'' He adds: ``Unfortunately, during the past two decades, the coastal population has increased.... If we do return to the [earlier] pattern ... we are going to see a lot more damage.''
Dr. Gary has identified several atmospheric factors that influence Atlantic hurricane activity. He plugs them into a forecasting equation developed from 40 years of hurricane data. This does not foretell individual storms. It does estimate the number of tropical storms that intensify enough to be given names during a hurricane season that runs from June 1 to Nov. 30. It estimates how many storms will become hurricanes and the season's total number of storm days - days when storms are active.
Gray also forecasts what he calls the season's Hurricane Destructive Potential (HDP) index. This, roughly, is based on the sum of the squares of a hurricane's maximum wind speeds. The square of the wind speed is a direct measure of how hard the wind pushes against objects.
Gray has been right on the money in forecasting these quantities for several years. But he failed miserably this season. He blames the unanticipated Sahel rainfall. For the second year, it was abundant. In fact, the rains in 1988-89 have been heavier than in any two-year consecutive period since 1966-67.
He expected nine named storms of which four would be hurricanes, 35 storm days with 15 hurricane days, and an HDP index of 40. Instead, the season that closed Nov. 30 saw 11 named storms including seven hurricanes, 67 storm days with 35 hurricane days, and an HDP index of 108! Hugo's strength and persistence alone were enough to blow the forecast.
Gray has explained right along that his scheme can predict only ``about half of the total variability in Atlantic seasonal hurricane activity.'' He has found some of the factors that affect that variability and is still groping for others. Sahel rainfall seems to be a key influence not yet in his calculations.
He explains that many tropical storms and hurricanes - especially the vigorous ones - arise from disturbances that move eastward from Africa. The atmospheric conditions that affect western Sahel rainfall also influence these disturbances. When there is Sahel drought, this storm-breeding activity weakens. It is stronger when the Sahel has substantial rainfall, as it did during the previous cycle of vigorous hurricane activity.
Looking ahead to next year, and assuming that the Sahel drought has indeed broken, Gray says this combines with other factors to foreshadow an above-average hurricane season. Looking ahead generally, he warns, ``We have likely entered into a new multidecadal period of more intense West African-spawned hurricanes.''
Seen in this perspective, Hugo was more a warning than a rare event.