Wednesday, May 05, 2010

How Can Solar Variability Affect Hurricanes?

An inverse relationship between hurricane activity over the Caribbean and the number of sunspots has recently been identified. Here we investigate this relationship using daily observations and find support for the hypothesis that changes in ultraviolet (UV) radiation are the cause. The relationship is statistically significant after accounting for annual variation in ocean heat and the El Nino cycle. A warming response in the upper troposphere to increased solar UV forcing, as measured by the Mg II core-to-wing ratio, decreases the atmosphere's convective available potential energy (CAPE) leading to a weaker cyclone. The response amplitude at a hurricane intensity of 44 m/s is 6.7 m/s +/- 2.56 m/s per 0.01 Mg II units (s.d.), which compares with 4.6 m/s estimated from the heat-engine theory using a temperature trend derived from observations. An increasing response sensitivity with increasing hurricane strength is found in the observations and in an application of the theory. Read more. Citation: 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.

Saturday, May 01, 2010

Oil Spills and Hurricanes

The oil slick over the northern Gulf of Mexico will reflect more incoming sunlight so will warm slower than the surrounding ocean. If this delayed warming continues into the hurricane season, then a tropical cyclone that visits the region might have slightly weaker winds. Reduced water evaporation from any remaining oil film at the time of the hurricane will contribute to the decrease in wind speeds assuming the high winds do not immediately disperse the oil.