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Examples of ENSO-Society InteractionsLa Niña, El Niño, and US Atlantic Hurricane DamagesRoger A. Pielke, Jr. Center for Science and Technology Policy Research University of Colorado/CIRES Boulder, Colorado, USA Christopher W. Landsea Updated with permission from: Recent research strongly suggests that U.S. Atlantic hurricane damages are modulated by the phase of ENSO, with increased losses during La Niña events and reduced losses during El Niño events (Pielke and Landsea, 1998, 1999). Our analyses support the following statements:
2. Because damage increases with the square (or more) of wind speed, the greater intensity translates to a substantial increase in damage. The average damage in the United States per storm in El Niño years is $800 million vs. $1,600 million1 in La Niña years. On an annual basis, because the distribution of damaging events is highly skewed by a few very large losses, we suggest that the median is an appropriate measure of central tendency. However, some decision makers with an interest in expected losses (e.g., the reinsurance industry) will be interested in the mean. Decision makers should focus on variance in losses as well as central tendency because even in a relatively inactive season, a single storm can have significant impacts. This was the case of Hurricane Andrew (1992), which resulted in more than $30 billion in losses. The largest loss in the record (normalized for inflation, population, and wealth) is the 1926 Miami hurricane which caused more than $60 billion in damages. This hurricane had a second landfall in the Florida Panhandle/Alabama region, which added about $10 billion in losses. 3. The occurrence of an El Niño does not mean that there will be no hurricanes. Several El Niño years have seen large hurricane impacts. The 1997 hurricane season was quiet in terms of overall activity, and losses were minimal ($100 million). However, this is not always the case. In 1965 Hurricane Betsy resulted in more than $13 billion in normalized losses and in 1972 Hurricane Agnes caused more than $11 billion in damage. Thus, large losses are possible in any year, and three of the top five normalized storm losses occurred in neutral years (the remaining two were in La Niña years). 4. The record suggests that Niño 3.4 SSTs for the months of August-September-October provide a statistically significant indicator of damage, but the use of this relation in decision-making should be with consideration of its limitations. Common sense suggests that, with a reliable prediction of sea surface temperatures in the August-September-October period, certain decision makers might be able to derive benefits. But this raises a series of questions: How reliable? Which decision makers? What benefits? Furthermore, experience offers three reasons for decision makers to exercise caution in the use of this information. First, predictions are always uncertain, and a significant error in the prediction of SSTs might lead to costs rather than benefits, compared with a situation in which there is no prediction (Sarewitz et al., 2000). Second, these relations, while significant, provide information with which to hedge, but should not be used to "bet an entire stake." Climate patterns change. There is always uncertainty as to how closely the future will resemble the past. Third, this information will likely be of most potential value to sophisticated decision makers who can finely balance risk using probabilistic information. For an average coastal resident or community, this information might suggest accelerating preparedness plans in the face of a pending La Niña event, but improved preparedness also makes sense at any time. 5. ENSO is not the only climate factor related to US hurricane damage, there are others that sophisticated users should consider. Other environmental factors impact Atlantic hurricanes (at least partially independent of ENSO) -- such as Atlantic sea surface temperatures, the stratospheric QBO, Caribbean sea level pressures, and West African Sahel rainfall (e.g., based on the work of William Gray, 1984a,b). About 40% of the years analyzed in this study (1925 to 1995) had no significant El Niño or La Niña event occurring during the peak of the Atlantic hurricane season. Yet, substantial variations of Atlantic hurricanes and U.S. hurricane-caused damage occur in neutral years. A judicious use of the environmental factors, as controls in statistical
models, has produced skillful experimental seasonal hurricane forecasts
by Gray et al. (1993). The strong relationship between Pacific sea surface
temperatures and Atlantic hurricane damages in the United States offers a
tantalizing opportunity for the direct use to society's benefit of scientific
information about the ENSO phenomenon. It also offers an opportunity for
a closer connection between scientists and decision makers to the enrichment
of both. LinksReferences
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