In just two years (2004-2005) the United States experienced seven major hurricanes. In the nine years since then, not one major hurricane has made landfall in the US - the longest gap since records began. In an upcoming paper, NASA's Dr Tim Hall has calculated that this kind of hurricane 'drought' should only happen every 177 years. Has something changed or has the US just been extraordinarily lucky?
With the passing of the quiet 2014 season, it’s now nine years without a major hurricane landfall on the US. The last category three plus strike was Hurricane Wilma in 2005. This length of “drought” is unprecedented in the historic record at least back to 1851. Exactly how rare is such a string of years? What can we conclude from it? And how long on average do we wait to till the next major landfall?
It’s tough to answer these questions directly from HURDAT, the National Hurricane Center’s archive of hurricane data, which has no precedent for the current drought. An alternative is a statistical hurricane model that generates much larger sets of synthetic storms whose statistical properties match those of HURDAT. We’ve run such a model for 63,000 years under 1950-2012 conditions and counted the droughts of various lengths.
Figure 1 shows the average time to wait for a major landfall drought as a function of drought length. For a nine-year drought, Texas to Maine, the average wait is 177 years. Droughts are more common on subsections of the coast, as smaller targets have lower landfall odds. The process is akin to a series of independent coin tosses, where the historical probability for no major landfalls in a year is 0.60. The theoretical wait time for such a process turns out to be very close to the wait time computed from the model storm set.
How long is the wait till the next major US landfall? A long run of “heads” may be rare, but the odds in the next toss are still 1/2, independent of the current run. Analysis of the simulation set indicates an average wait of 2.6 years for a major US landfall, approximately independent of the current drought length.
Figure 1: Average waiting, AN, time for a consecutive N-year series with no major hurricane landfalls. The thick black line is for the full US Gulf-Atlantic coast, Texas to Maine, as directly counted from 63,000-years of simulation. Other colors indicate sub-regions of this coast: Texas (dark blue); North Gulf (light blue); Florida (red), Southeast (green), Mid-Atlantic (yellow), and Northeast (orange). The thin black line represents AN = (P-N-1)/(1-P), the exact result for a Bernoulli process, where P=0.60, the fraction of years with no major US landfalls in HURDAT 1851-2012. The dashed line results from directly counting HURDAT 1851-2012.
Should we conclude anything from the current major landfall drought? Has some characteristic of hurricane climate changed in a sustained way leading to fewer US major landfalls? Several observations point to the current drought being more a case of good fortune then a change in hurricane climate:
1. The seasons since 2005 have not been energetically weak, on average. The 2006-2014 accumulated cyclone energy annual mean is 97 (104 kt2), compared to a 1951-2000 mean of 93.
2. There’s been no shortage of major hurricanes. The years 2008, 2010, and 2011 had 5, 5, and 4 hurricanes reach category three status, respectively, and the nine-year period 2006-2014 averaged 2.7 major hurricanes annually, about equal to the annual average since 1950.
3. There has been no shortage of category three plus landfalls on Caribbean and Mexican coastlines. Cuba, only a few degrees latitude from Florida, has experienced five category three landfalls since 2005, well above its long-term rate.
4. There have been two US landfalls that were only a few km/hour shy of category 3: Hurricanes Gustav and Ike (Louisiana and Texas) in 2008.
A hurricane-climate shift protecting the US, even while years are active, and even while ravaging nearby Caribbean Islands, would require creative thinking. The admittedly unusual nine-year US category three plus landfall drought may well just be luck.
Dr Hall is a climate scientist at the NASA Goddard Institute for Space Studies in New York City. He specialises in hurricanes and their relationship to climate. He develops statistical hurricane and weather hazard models, and has worked as a consultant to RMS and reinsurance companies. Dr. Hall can be reached at www.linkedin.com/pub/timothy-hall/a2/709/b27/.
Other articles by Dr Hall
Posted: Monday, December 15th, 2014