By DR. PETER S. DAILEY, director of atmospheric science at AIR Worldwide
November 30 marked the official end of the 2009 Atlantic hurricane season. With nine named storms, including three hurricanes, and no U.S. land-falling hurricanes, this season was the second quietest since 1995, the year the present period of above-average sea surface temperatures (SSTs) began.
This year's relative inactivity stands in sharp contrast to the 2008 season, during which hurricanes Dolly, Gustav and Ike battered the Gulf Coast, causing greater than $10 billion in insured losses.
This graph of the 2009
hurricane season shows the evolution of a "typical" season, which reflects the long-term climatological average over many decades of activity. Tropical storms, which produce winds of at least 39 mph, occur relatively frequently. By the end of the typical season, about 11 tropical storms are observed in the Atlantic. About half reach hurricane strength (with winds above 74 mph) in a typical year, and half of those reach major hurricane status (with winds of 111 mph or more).
By all standard measures, the 2009 Atlantic hurricane season was below average. After a very late start (the first named event, Ana, did not reach tropical storm status until Aug. 12), a total of nine tropical storms formed, and only three reached hurricane strength. Major hurricane formation, at two, was slightly below average.
Most significantly is that, among the hurricanes and major hurricanes that developed in 2009, none was able to reach the U.S. coastline at hurricane strength. On average, one or two hurricanes are expected to make U.S. landfall. Only about 20 percent of historical seasons failed to produce a single U.S. hurricane landfall.
WHY WAS THIS SEASON'S ACTIVITY SO LOW?
In every hurricane season since 1995, the Atlantic Ocean has been warmer than the long-term climatological average. While many believe this to be a symptom of global warming, these sea surface temperature anomalies have also been attributed to the so-called Atlantic Multidecadal Oscillation (AMO), a climate signal theorized to result in long-lived warm and cool phases in the North Atlantic Ocean.
Regardless of the underlying cause, warmer ocean temperatures provide more heat to fuel tropical cyclone activity and undoubtedly contributed strongly to the activity in seasons like 2004 and 2005. Anomalies observed this year, however, were significantly lower than those in 2004 and 2005.
On the other hand, SSTs have also been anomalously warm in the Pacific Ocean this year. This is attributed to El Niņo, a quasiperiodic phase of the El Niņo Southern Oscillation (ENSO) that is observed approximately every six to eight years. El Niņo is associated with increased wind shear in the Atlantic Ocean, which is destructive to hurricanes and inhibits their formation and ability to intensify.
During the early part of the season, when the El Niņo was not yet mature, the Atlantic was also not nearly as warm as in mid-season. In fact, parts of the tropical Atlantic were actually cooler than average.
As the season progressed, the Atlantic warmed, but by then El Niņo had matured, inducing higher levels of Atlantic wind shear and counteracting the influence of warmer sea surface temperatures. The lack of abundant fuel (the heat of the water), coupled with elevated wind shear, limited activity in what would normally be the most productive part of the season.
HURRICANE BILL AND NORTHEAST HURRICANE RISK
While there were no hurricane landfalls in the United States this season, Hurricane Bill--the strongest storm of the season, at its peak a 115-knot (132 mph) Category-4 storm--was a near-miss for the Northeast U.S.
Hurricane landfalls in the Northeast are relatively rare, occurring approximately once every 10 years. The last two were Hurricane Gloria in 1985 and Hurricane Bob in 1991, and the last major hurricane landfall in the Northeast was the Great New England Hurricane of 1938. AIR estimates that were the 1938 storm to recur today, it would cost the insurance industry $38 billion.
This graphic shows Hurricane Bill's cone of uncertainty issued by the National Hurricane Center on August 20. The westerly edge of the cone corresponded to a forecast intensity of 115 mph at landfall at Cape Cod, Mass. Had the left side of NHC's forecast been realized, Bill would have been the first major hurricane landfall in the Northeast in more than 70 years (and with landfall intensity stronger than Hurricane Ike in 2008).
Ongoing discussion in the industry about Northeast hurricane risk has focused on how the wind field in these more northerly storms differ from those remaining in tropical latitudes. Hurricanes that move into the midlatitudes often undergo a process called transitioning, whereby the cloud field and wind structure expand and take on the appearance of an extratropical cyclone. About half of transitioning hurricanes do not experience substantial changes in intensity, while the other half are roughly evenly split between those that decay and those that intensify. In some cases, the remnants of hurricanes combine with pre-existing midlatitude cyclones to cause significant damage, as was the case with Hurricane Ike last year as it moved through the Midwest.
On the other hand, some storms are so intense and fast-moving that they do not have time to adjust to the midlatitude environment. Hurricane Bill maintained a conventional hurricane structure with a roughly symmetrical wind distribution. Had it followed the track of the 1938 hurricane, Bill could have produced a wide swath of hurricane-force winds at landfall along the New York, Connecticut, and Rhode Island coastline and a wide footprint of damage from Pennsylvania to Maine.
THE LONG-TERM VIEW
The 2009 Atlantic hurricane season uniformly fell short of expectations. While many preseason forecasts called for a roughly average season, the complex feedback mechanisms that govern climate signals and the global distribution of ocean temperatures worked to inhibit hurricane activity this year.
Most notably, despite the current warm regime of sea surface temperatures in the Atlantic, not a single storm made landfall in the United States at hurricane strength, although Hurricane Bill did provide a timely reminder of latent hurricane risk in the Northeast.
The season, once again, demonstrated the uncertainty and year-to-year variability in hurricane activity--and the importance of considering diverse long-term views of risk.
January 1, 2010
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