Solar impacts on hurricanes

I'm in Tucson for the American Meteorological Society's 29th Conference on Hurricanes and Tropical Meteorology. This is the premier scientific conference on hurricanes, and is held only once every two years, so pretty much all of the world's greatest hurricane experts are here. One of the more intriguing posters presented at Tuesday's poster session was titled, 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.

Previous research
The findings presented at this week's conference build upon earlier work published by Elsner et al. (2008) and Elsner et al. (2010). The first of these studies found that for every 100 extra sunspots in September, the temperature of the atmosphere at 16 km altitude over the Caribbean and Gulf of Mexico increased by about 0.5°C, and the number of hurricanes in this region was reduced by 26%. Interestingly, a reduction of hurricanes over the eastern Atlantic off the coast of Africa was not observed during solar maxima, which the authors attributed to the fact that hurricanes in this region are limited by sea surface temperature, not instability. Solar maximum brings a small increase in sea surface temperature to the globe, aiding hurricane development in regions where sea surface temperature is the limiting factor. The second of these studies (Elsner et al., 2010) computed that for a Category 2 hurricane affecting the U.S. during the most active 30% range of the solar cycle, the resultant heating of the upper troposphere would cause a 19% decrease in the stability, lowering the hurricane's winds by 10% (10 mph.) Stronger hurricanes would be affected even more, with a potential wind speed reduction of 23 mph for the most powerful hurricanes. The 27-day rotation period of the sun causes a change in UV light even larger than the change observed during the 11-year sunspot cycle, so perhaps we should be monitoring the phase of the sun's rotation to look for more favorable periods for hurricane formation.

Commentary
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. My next post, which may not be until Monday, I'll discuss 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. Also, El Niño now appears to be over, as sea surface temperatures in the Eastern Pacific have crossed the threshold into neutral territory.

References
Elsner, J. B., and T. H. Jagger, 2008, United States and Caribbean tropical cyclone activity related to the solar cycle, Geophys. Res. Lett., 35, L18705, doi:10.1029/2008GL034431.

Elsner, J. B., T. H. Jagger, and R. E. Hodges, 2010, Daily tropical cyclone intensity response to solar ultraviolet radiation, Geophys. Res. Lett., 37, L09701, doi:10.1029/2010GL043091.

Lean, J.L., 2009, Cycles and trends in solar irradiance and climate", Wiley Interdisciplinary Reviews: Climate Change, Volume 1, Issue 1, Pages 111-122 Published Online: 22 Dec 2009

Jeff Masters