ABSTRACT

This paper discusses the surprisingly strong prediction (by 1 August) relationship which exists between West African rainfall and US hurricane spawned destruction along the US East Coast and Peninsula Florida for the period 1950-1991. Ninety-eight percent of such damage occurs after this data. Over four-fifth of East Coast hurricane spawned damage occurs during the highest third rainfall years. The last quarter century Western Sahel drought has caused a large reduction in US East Coast hurricane damage.

1. PREDICTABILITY OF SEASONAL HURRICANE ACTIVITY FROM PRIOR AFRICAN RAINFALL DATA

We have recently observed that intense (category 3-4-5) Atlantic hurricane activity is greatly enhanced when the Western Sahel region of West Africa (Fig. 1) has above average precipitation. Similarly, intense hurricane activity is much suppressed when concurrent precipitation in this region is below average (Gray [1]). Recent analyses by Landsea and Gray [3] show very high correlations between year-to-year variations of intense hurricane days between 1949-1990 and year-to-year variations of Western Sahel rainfall. Additional analyses (Landsea [2]) show that West African rainfall data can also be used in a predictive sense to forecast variations in intense hurricane activity and US hurricane spawned destruction in subsequent seasons. We find that the rainfall which fell in the Gulf of Guinea region (Fig. 1) between August and November of the previous year, in combination with the June-July precipitation for the current year in the Western Sahel region (collectively known as the "early season combination rainfall index"), is typically a very good predictor of intense hurricane activity during the forthcoming August-October period. Figure 2 shows values for this combination rainfall index wherein rainfall during August through November of the previous year in the Gulf of Guinea region is weighted 0.3 - and June-July Western Sahel precipitation weighted 0.7. The values in Fig. 2 are expressed in terms of a standardized deviation (a "Z-score"). These combined rainfall anomaly index values, which are available by I August each year, are compared with the seasonal incidence of category 3-4-5 hurricane days. Approximately 60 percent of the subsequent seasonal variance (linear correlation of 0.78) of intense hurricane days is explained (i.e. forecast) by the combined rainfall index.

It is curious that African rainfall prior to the first of August is so highly correlated with subsequent seasonal values of intense hurricane activity. Gulf of Guinea rainfall during the prior fall season is likely related to the strength of the West African monsoon in the following year through positive feedbacks of evapotranspiration and soil moisture. Conversely, as the monsoon develops in June and July, Western Sahel rainfall is a good indication of how far north and how strong the West African monsoon trough establishes itself.

In view of this association, early season African rainfall (i.e., prior to 1 August) should be closely monitored as it is a very good indicator (hence forecast) of the amount of intense (category 3-4-5) hurricane activity likely to follow after 1 August. This is especially useful in that, historically, over 98% of all intense hurricane activity occurs during the August through November period (Fig. 3); allowing a true "forecast" to be made by I August from African rainfall information.

To normalize hurricane destruction in terms of 1990 dollars, we have utilized factors to accommodate both inflation and increased coastal property development. During the 42-year period 1949-90, there has been a total of approximately $76 billion (in equivalent 1990 dollars) of US hurricane and tropical storm spawned destruction. Of this value, approximately 10 percent was caused by a total of 61 landfalling tropical storms, 19 percent by the 40 landfalling category 1-2 hurricanes, 44 percent by 19 landfalling category 3 hurricanes, and 27 percent by a total of only 6 landfalling category 4-5 hurricanes.

The eastern US coastline can be approximately separated in two regions which experience differing landfalling hurricane responses to pre-1 August West African rainfall: The US East Coast (including the Florida Peninsula) and the US Gulf Coast (including panhandle Florida). The two regions are depicted in Fig. 4. Table 1 lists the percentage of 1949-90 US hurricane destruction in the East Coast and Gulf Coast areas which occurred during each of six seven-year groupings by decreasing rainfall amount. Note in Table 1 that the US Gulf Coast has experienced a different distribution of hurricane damage in relation to West African rainfall than has the US East Coast. Damage along the US East Coast is increased dramatically in the two wettest seven year groupings in comparison to the other five drier groupings. The US Gulf Coast damage amounts show little sensitivity to West African rainfall.

A comparative listing of West African early season rainfall prior to 1 August, ranked from highest to lowest yearly amounts, is shown in Table 2; 1955 was the wettest and 1984 the driest. Included in this rainfall ranked data are hurricane destruction values for each year in millions of 1990 dollars. Note that although the mean damage values for each region are similar, hurricane destruction on the Gulf Coast is not well associated with West African rainfall prior to 1 August whereas East Coast damage is. Most years with large amounts of hurricane destruction on the East Coast were also years when early season West African rainfall was above normal. Conversely, below average early season rainfall years were typically associated with very little hurricane spawned destruction on the East Coast and Florida Peninsula. There have been, of course, some heavy early season West African rainfall years wherein no hurricane spawned destruction occurred. These exceptions are reasonable given that hurricane formation and track statistics for individual seasons are highly variable.

Landfall on the US East Coast during individual years is dependent upon factors other than early season West African rainfall and hence, an association in individual seasons cannot be expected. However, when averaged over several years, very clear signals emerge. For example, total normalized hurricane destruction on the East Coast during the ten years with the greatest early season rainfall during 1949-1990 was $23.70 billion; this versus $0.30 billion for the ten driest early season years. This difference amounts to a ratio of 79 to 1. By contrast, differences in Gulf Coast destruction during the ten wettest versus the ten driest years in West Africa are little more than two to one. Therefore, it is during the very wet early seasons in West Africa that East Coast destruction goes up so dramatically.

It is remarkable that the probability of East Coast and Florida hurricane destruction should go up so dramatically during those years of high West African rainfall prior to 1 August. If hurricane destruction information were analyzed for the Caribbean region, we are confident that similar wet versus dry hurricane destruction would also be found. We are working to understand why West African rainfall anomalies greater than 0.4 standardized deviations above normal are associated with increased amounts of intense hurricane activity and potential US coastal hurricane destruction over those when West African rainfall is normal or below normal.

The comparatively poor association between West African rainfall and hurricane destruction along the US Gulf Coast and Florida Panhandle region is shown in Table 2; large amounts of hurricane destruction can occur along the Gulf Coast during very dry West African conditions. Notably, Hurricane Alicia (1983) caused 1990 normalized hurricane destruction of $2.71 billion in the Galveston- Houston region during what was the driest early season Western Sahel rainy season of the last 42 years. Intense hurricane activity along the Gulf Coast typically occurs earlier in the season than the East Coast (see Landsea [2]). For example, 7 of 13 (i.e., 54 percent) of landfalling Gulf Coast category 3-4-5 hurricanes between 1949-90 made landfall before 21 August while only 1 of 12 (8 percent) East Coast intense hurricane made landfall by this date. Whereas, category 4 Hurricane Audrey (1957) made landfall on the Gulf Coast on 27 June, the earliest East Coast-Florida intense hurricane to make landfall (between 1949-90) was Connie on August 12, 1955. Conversely, during the same period, only two intense hurricanes (15 percent of the total) made landfall along the Gulf Coast after September 12 while six (50 percent) made landfall after this date along the US East Coast.

We may generalize that some intense hurricanes which make landfall along the Gulf Coast form in climatological environments which are notably different from those of hurricanes which make landfall along the US East Coast. In particular, hurricane activity affecting the Gulf Coast is much less related to West African rainfall than is hurricane activity along the US East Coast. Illustrative examples include intense Gulf Hurricanes Audrey (1957) and Alicia (1983) which did not develop from African waves, as do nearly all East Coast intense landfalling hurricanes. Typically, intense hurricanes landfalling in the Gulf are highly dependent upon meteorological conditions in tile western Caribbean Basin and Gulf of Mexico which are much farther downwind from West Africa and hence, are less responsive to circulation characteristics associated with African rainfall. Therefore, it is circulation features in the western Caribbean and Gulf of Mexico which tend to be primarily responsible for the intensification of powerful and destructive hurricanes making landfall along the Gulf coastline while African rainfall is much less of a contributing feature.

Given the juxtaposition of West Africa and the tropical Atlantic and the fact that the majority of Atlantic hurricanes develop from easterly waves from West Africa, it is not surprising that variable aspects of African summer monsoon rainfall and Atlantic intense hurricane activity might be related. But what is new and surprising is the fact that they are so well related and that there is a multi-month predictive link between rainfall and subsequent hurricane activity. Figure 5 shows an illustration of this relationship. As the majority of hurricane related damage is due to the comparatively infrequent but very powerful storms, it becomes important that we carefully examine and fully appreciate both the annual and inter-decadal variations of the occurrence of intense hurricanes. We believe that the strong precursor signals in seasonal rainfall prior to 1 August each year are a consequence of the strength and latitude of the West African monsoon trough as it becomes established in June and July. A strong monsoon at a high latitude position in June-July is conducive to continued abundant rainfall for the remainder of the summer. Also, the delayed influence of precipitation during the late summer and fall of the prior year through vegetation and soil moisture processes on the amount of later available water vapor appears to also be a significant precursor influence.

This research has been supported by a NOAA Climate Grant. The author thanks Richard Taft and William Thorson for their very expert assistance in computer processing of West African rainfall.

[1] Gray, W. M. Strong association between West African rainfall and U.S. landfalling intense hurricanes. Science, 1990, 249, 1251-1256.

[2] Landsea, C. W. West African monsoonal rainfall and intense hurricane associations. Dept. of Atmos. Sci. Paper No. 484, Colo. State Univ., Ft. Collins, CO, 80523, 280 pp, 1991.

[3] Landsea, C. W. and W. M. Gray. The strong association between Western Sahelian monsoon rainfall and intense Atlantic hurricanes. J. Climate, 1992, 5, 435-453.

[4] Landsea, C. W. ,W. M. Gray, P. W. Mielke, Jr., and K. J. Berry. Multidecadal variations of Sahel monsoon rainfall and U.S. landfalling intense hurricanes. J. Climate, 1992, 5, in press. 189