Three weeks ago, James Kossin published a major study in the scientific journal Nature: “Hurricane intensification along United States coast suppressed during active hurricane periods” (http://www.nature.com/nature/journal/v541/n7637/full/nature20783.html). This paper has gotten a bit of press in various insurer publications as over the last decade near term hurricane risk has dramatically influenced the insurance industry in hurricane-prone states. In short, Kossin’s paper provides valuable insight into rapid hurricane intensification during active and inactive periods of hurricane activity in the Atlantic basin (known as the warm and cold phase of the Atlantic Multi-Decadal Oscillation [AMO]). The findings suggest that approaching hurricanes are more likely to intensify before landfall during colder phases of the AMO and are more likely to weaken before landfall during warmer phases. Suggesting that during times when the sea surface temperatures are anomalously warm in the main development region, regions along the U.S. coast experience higher then normal vertical wind shear. Furthermore, the local sea surface temperatures are substantially cooler than the main development region which tend to inhibit intensification and form a “protective barrier” along the United States coastline during this period.
My very first thought as I read about this “protective barrier” was, finally, someone has scientifically proven the existence of HAARP and electromagnetic capabilities of the ionosphere that conspiracy theories suggest can also be used in weather modification and more. (i.e., a “protective barrier” created to control the weather (hurricane landfall))! (If you’re unfamiliar with the HAARP conspiracy theories that abound, then you won’t get this reference, but it’s one of the top questions meteorologists get asked, and you can learn more at http://www.nbcnews.com/science/weird-science/conspiracy-theories-abound-u-s-military-closes-haarp-n112576.)
How the Science has Changed
In all seriousness, to truly understand the impact of Kossin’s paper, it’s important to review how our understanding of active and inactive phases of the Atlantic Ocean has evolved and influenced the insurance industry. Since the 1960’s, the late Dr. Gray showed distinctly differing environmental conditions for tropical cyclone development. But not until the marked increase beginning in 1995 did insurers begin to notice what drives this activity. After the costly 2004 and 2005 hurricane seasons, a dozen and half papers highlighted a link to increased hurricane activity due to a warming world and/or the current phase of the AMO. About the same time, catastrophe modeling companies were being pressured to adjust for this increased frequency and severity that had impacted the insurance industry since 1995. Modeling companies incorporated a range of statistical analyses, and in some cases expert elicitation, to make assessment on future hurricane landfalling risk.
Mother Nature, however, does not always follow weather trends; and since 2005, the U.S. has entered what some have called a “U.S. landfall hurricane drought.” While overall named storm activity remains elevated across the Atlantic basin with some very active years since 2005 (2010, for example), many hurricanes have not made landfall as expected, which is actually influencing the long term landfall rates across the U.S. A few recent research papers suggest that scientists never settled on the exact impact the warmer phase of the AMO might have had on U.S. landfalls. Some of these papers suggest that in a warmer world, African dust and/or North Atlantic sea surface temperatures could actually encourage hurricanes to form further east or recurve before impacting North America, resulting in a lower U.S. hurricane landfall rate.
But Kossin’s paper provides new evidence that the science is not settled, and there is a lot to learn about hurricanes and the rate at which they make landfall. In general, the correlation between the number of hurricanes that develop in the Atlantic basin and number of hurricanes that make landfall is weak. This new study in Nature could explain part of this relationship as it highlights that elevated wind shear (changing wind speeds with height) and cooler sea-surface temperatures along the U.S. coast during the warm active phase of the AMO create conditions that tend to weaken hurricanes as they approach the U.S. coast. Conversely, the cool phase of the AMO makes approaching hurricanes more likely to intensify. This might conflict with ideas that some scientists suggested prior to 2004 and 2005, that the warmer phase of the AMO was correlated to more landfalls and possibly stronger landfalling storms (and fewer landfalls during the cooler phase of the AMO).
The Hurricane “Drought” can be Explained
Kossin suggests in his paper’s closing that this new research could possibly explain the recent “drought” of major hurricane landfalls. However, in late 2015 another research paper funded by Risk Prediction Initiative (“The Arbitrary Definition of the current Atlantic Major Hurricane Landfall Drought”) suggested the major hurricane drought is more a function of the definition of a major hurricane (defined as a Category 3 or higher on the Saffir-Simpson Scale) and the uncertainty in wind estimates. As an example, Ike (2008) made landfall as a high-end Category 2, but could just as easily have been classified as a low-end Category 3 at landfall. For contrast, consider a storm like Sandy (2012), which was not even classified as a hurricane at landfall, but still resulted in large wind and storm surge insurance losses.
Image created by Roger Pielke Jr. who has been keeping track of the days between major hurricane landfalls.
It’s important to remember that only 78 major hurricanes have made landfall since 1900 (117 years), which results in a particularly small sample size, and that there are likely many other factors that affect intensification before landfall. Hurricanes can experience higher levels of disruptive wind shear while turning north, which could be due to a large scale trough of low pressure or drawing in dry air from landmasses. Hurricanes can also send large waves and surge ahead of the storm causing warm water to mix with cooler water and deplete important energy from the storm as it approaches land. These vacillating factors highlight the complex interactions hurricanes may experience during a life cycle. These factors could also explain why many hurricanes have not intensified prior to U.S. landfall (with the exception of Charley  and Humberto ).
Future Hurricane Landfall Rate Models
As always, new research will continue to become available and catastrophe modeling companies will continue to judge how the science best fits into catastrophe models. This new research should not discredit near term view modeling work. Hindsight is 20/20, and as Hall, T.M., and K. Hereid, 2015: The frequency and duration of U.S. hurricane droughts paper suggests several observations point to the current drought being more a case of good luck than any shift in hurricane climate. Therefore this luck could have swayed higher then normal insured losses in a completely different direction over the last 10 years. These facts highlight why it is important to continuously evaluate new science. Modeling companies need to ensure they avoid knee-jerk reactions to industry demands or speculative science. Seasonal hurricane forecasts are improving, and with time, multidecadal sea surface patterns and other variables could help clarify the cause of reduced landfall rates, not just in the Atlantic basin, but in other basins near high exposure. El Niño/Southern Oscillation (“ENSO”) and other variables could also be worked into the landfall rate catalogs produced by model vendors.
As pointed out in this blog, the science might not be settled on hurricane landfall rates and what drives these rates during particular climatic conditions. Although it is short by climate standards, the U.S. landfall hurricane record is one of the best natural catastrophe databases insurers have to understand frequency and severity. There is a reason why catastrophe model companies calibrate their models to the longest period of record available.
There is plenty of room for debate about near term hurricane landfall rate models, the drivers of this risk, and what position an insurer should take regarding rate models. Are they a good idea or a bad idea? Should a blended approach be used? Those are interesting questions and a worthy topic, which is why I will be moderating the session, “Is Near Term vs. Long Term View of Hurricane Risk Over?” at the February RAA Cat Management Conference. We hope this subject generates lively discussion and hope to see you there!