La Niña, El Niño, and US Atlantic Hurricane Damages
Roger A. Pielke, Jr.and
Christopher W. Landsea
National Center for Atmospheric Research
Boulder, CO. USA
rogerp@ucar.edu
Recent research (Pielke and Landsea, submitted, and Pielke and Landsea 1998) suggests strongly 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.
Our present under analysis supports the following conclusions:
- La Niña means a greater frequency of damaging storms and more damage per storm. During cold events in the Pacific, the odds are significantly higher that the U.S. will experience greater impacts because of a larger number of tropical cyclones and higher intensities for each storm.
Because damage increases with the square (or more) of wind speed, the greater intensity translates to a substantial increase in damage. The average damage per storm in El Niño years is $800 million vs. $1,600 million 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, although some decision makers with an interest in expected losses (e.g., reinsurance) will be interested in the mean. Decision makers should focus on variance in losses as well as central tendency, as even in a relatively inactive season a single storm can have significant impacts, for example Andrew (1992), with more than $30 billion in losses. The largest normalized loss in the record is more than $60 billion in damages due to the 1926 Miami hurricane, which had a second landfall in the Florida Panhandle/Alabama with about $10 billion in losses.
- El Niño does not mean no hurricanes, as several El Niño years have seen large 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 Betsy resulted in more than $13 billion in normalized losses and in 1972 Agnes had more than $11 billion. Large losses are possible in any year, and 3 of the top five normalized storm losses occurred in neutral years (the other 2 were in La Niña years).
- The record suggests that ASO Niño 3.4 SSTs provide a statistically significant indicator of damage, but the use of this relation in decision making should be in consideration of its limitations.
Research strongly suggests that with a reliable prediction of ASO sea surface temperatures, certain decision makers might be able to derive benefits. However, we offer 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 SST might lead to costs rather than benefits, compared with a situation with no prediction.
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 probabalistic information. For an average coastal resident or community, this information might suggest accelerating preparedness plans with a pending La Niña event, but improved preparedness makes sense at any time.
- ENSO is not the only climate factor related to US hurricane damage, there are others that sophisticated users should consider (e.g., based on the work of William Gray and colleagues).
About 40% of the years analyzed in this study 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. Other environmental factors impact Atlantic hurricanes (at least partially independent of ENSO) -- such as the Atlantic sea surface temperatures, the stratospheric quasi-biennial oscillation, Caribbean sea level pressures, and West Sahel rainfall.
Judicious use of these environmental controls in statistical models has produced skillful experimental seasonal hurricane forecasts by Gray et al. The strong relation of Pacific sea surface temperatures and Atlantic hurricane damages in the United States offers a tantalizing opportunity for the direct use of scientific information about the ENSO phenomenon to society s benefit. It also offers an opportunity for a closer connection of scientists and decision makers to the enrichment of both.
write to
Roger A. Pielke, Jr.
Also see Pielke and
Landsea 1998 for details on normalized damages.