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New research from the May hurricane conference of the American Meteorological Society sheds new light just ahead of the start of the season June first.

Evidence linking solar variability with USA hurricanes, by Robert Hodges and Jim Elsner of Florida State University. They showed that the probability of three or more hurricanes hitting the U.S. during a hurricane season with warmer than average sea surface temperatures increases dramatically during minima in the 11-year sunspot cycle. The odds increase from 20% to 40% for years when the sunspot activity is in the lower 25% of the sunspot cycle, compared to years in the upper 25% of the cycle. Near the peak of the sunspot cycle, the odds of at least one hurricane hitting the U.S. are just 25%, but at solar minimum, the odds increase sharply to 64%. The authors studied the period 1851 - 2008, and controlled for other variables such as changes in sea surface temperature and El Niño. Such a large impact of the sun on hurricanes might seem surprising, given that the change in solar energy at all light wavelengths is only about 0.1%. This relatively small change causes just a 0.1°C change in Earth's mean surface temperature between the peak of the 11-year sunspot cycle (high solar activity) and the minimum of the sunspot cycle (where we are now.) However, variation in radiation between extrema of the solar cycle can be 10% or more in portions of the UV range (Elsner et al., 2008.) The strong change in UV light causes globally averaged temperature swings in the lower stratosphere of 0.4°C between the minimum and maximum of the sunspot cycle--four times as great as the difference measured at Earth's surface (Lean, 2009). This sensitivity of the stratosphere to UV light is due to the fact the ozone layer is located in the stratosphere. Ozone absorbs a large amount of UV light, causing the stratosphere to heat up when solar activity is high. The authors speculate that a warmer stratosphere then heats up the upper troposphere, making the atmosphere more stable. An unstable atmosphere--with hot temperatures at the surface and cold conditions in the upper troposphere--are conducive for stronger hurricanes. Thus, we would expect to see reductions in hurricanes during the peak of the sunspot cycle.
Considering that this year we are at the deepest solar minimum in more than a century, this research gives us yet another reason to expect a severe Atlantic hurricane season this year. The sea surface temperatures in the Atlantic hurricane main development region, which set an all-time record last month for the warmest monthly anomaly for the 100+ years we have records.

Angela Colbert of the University of Miami/RSMAS showed how different weather and climate patterns affect the Azores-Bermuda High, and thus the tracks of Atlantic hurricanes. She divided storms into straight-moving storms that move straight west-northwest through the Caribbean, recurving landfalling hurricane that hit the east coast of the U.S., and recurving ocean storms that miss land. Roughly 1/3 of all hurricanes between 1950 - 2009 fell into each of these three categories. These proportions stayed pretty constant during La Niña and neutral years, but El Niño caused a weakening of the high, resulting in far fewer hurricanes hitting the U.S. East Coast. These storms instead recurved out to sea.

Jim Kossin of the University of Wisconsin separated all Atlantic storms from 1950 - 2007 into 4 clusters, based on genesis location. Two of the clusters were more northerly-forming storms that tended to be less tropical in nature--Gulf of Mexico storms, and storms off the U.S. East Coast that tended to recurve. The other two clusters were more southerly tropical-origin systems--ones that tended to form in the Caribbean, and storms that form near the Cape Verde Island region off the coast of Africa. The more tropical Cape Verde and Caribbean storms dominated major hurricane frequency by a factor of four. In mid-1980s, there was an abrupt shift to more of these more dangerous tropical type storms--ten years prior to the active hurricane period that began in 1995. It is unknown what caused this shift. The shift is unlikely to be a result of measurement error, since we had good satellite imagery then. Independent of any trends in frequency, this shift caused an increase in intensity metrics of Atlantic hurricanes. A doubling of these tropical systems has also occurred since 1950. Interestingly, there has been no change in the number of Gulf of Mexico storms, and a slight increase in storms forming off of the U.S. East Coast. Since slight changes in track can make a big difference in what SSTs and atmospheric environment a storm sees, there is a lot of natural "noise" in the system that will make it difficult to get a clear sense of when climate change is having a substantial impact on hurricane intensity.

Bin Wang of the University of Miami studied the global number of storm days from 1965 - 2008, which should be a less sensitive quantity to data problems than the number of storms or their intensity. Storm days were defined as any day when a tropical cyclone of tropical depression strength or greater existed. However, there are still some data problems, as evidenced by a sharp drop in storm days observed in the North Indian Ocean beginning in 1978. Dr. Wang found that there was no global trend in storm days. The Atlantic was the only individual basin that showed an increase in storm days.

Greg Holland of NCAR looked at the distribution of the strongest hurricanes over time by using a mathematical description of the historical hurricane data. His analysis showed that during the period 1995 - 2008, we probably had about a 30% increase in Category 5 storms in the Atlantic, and an 18% increase in Category 4 hurricanes. Using a climate model, he predicted that by the years 2045 - 2055, we should see a 60% increase in Cat 5s, 32% increase in Cat 4s, and 16% increase in Cat 3s in the Atlantic.