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Solar Activity and Tropical Cyclones
Volume 11, Number 52: 24 December 2008

In an intriguing new paper, Elsner and Jagger (2008) speculate that "an increase in solar ultraviolet radiation during periods of strong solar activity will have a negative influence on tropical cyclone intensity, as the temperature near the tropopause will warm through absorption of radiation by ozone," which phenomenon, when modulated by dynamic effects in the stratosphere, "will decrease the convective available potential energy limiting the intensity of the cyclone." And they further suggest that "this effect will be most pronounced in regions of sufficient oceanic heat content and for stronger tropical cyclones."

The idea sounds logical enough; but are there any real-world data to support the concept? In an attempt designed to search for such evidence, the two Florida State University researchers conducted a number of different analyses.

First, and noting that high ocean heat content, low values of wind shear, and westerly steering currents increase the risk of Atlantic hurricanes hitting the United States, they developed a seasonal model of hurricane activity that employs sea-surface temperature (SST) as an indicator of ocean heat content, the Southern Oscillation Index (SOI) as a remote indicator of shear, and the North Atlantic Oscillation index (NAO) as an indicator of steering currents. Then, using the monthly sunspot number (SSN) for September -- which is the peak month of the hurricane season -- as a fourth covariate, they determined that the goodness-of-fit of the new four-parameter model was better than that of the three-parameter model, which implied a role of the type they had envisioned for solar activity, since they found that the probability of Atlantic hurricanes hitting the U.S. decreases with increasing solar activity after accounting for the NAO, SOI, and SST.

Elsner and Jagger then went on to develop a daily model of tropical cyclone activity for the region of the Atlantic with the greatest oceanic heat content during the hurricane season, so that "the limiting thermodynamic variable is upper atmosphere temperature rather than SST," and this model revealed an inverse relationship between solar activity and storm intensity, which is consistent with the results they obtained from their seasonal model of U.S. hurricane numbers.

Last of all, the two researchers compared seasonally-averaged (August-October) upper atmospheric temperatures with SSN over the peak hurricane season (August-September), finding "a relationship between solar activity and upper tropospheric/lower stratospheric temperatures consistent with [their] speculation that a cooler sun (fewer sunspots) results in a cooler outflow level surrounding the storm and thus greater cyclone intensity," and, of course, vice versa.

In light of these several findings, it would appear that the hypothesis of Elsner and Jagger is indeed correct, i.e., that there are "fewer intense tropical cyclones over the Caribbean and Gulf of Mexico when sunspot numbers are high," which appears to be due to "a reduction in the maximum potential intensity with a warming in the layer near the top of the hurricane."

Clearly, more such studies should be conducted over other parts of the world to assess the generality of this concept, which is particularly intriguing, since it indicates that changes in solar activity -- even over periods as short as a few months -- can measurably alter the nature of one of the most dramatic weather phenomena on earth. And if short-term run-of-the-mill variations in solar activity can do that, who knows what longer-term and potentially greater changes in solar activity might do for the climate of the world.

Sherwood, Keith and Craig Idso

Elsner, J.B. and Jagger, T.H. 2008. United States and Caribbean tropical cyclone activity related to the solar cycle. Geophysical Research Letters 35: 10.1029/2008GL034431.