Daily tropical cyclone intensity response to solar ultraviolet radiation
J. B. Elsner, T. H. Jagger, and R. E. Hodges
GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L09701, doi:10.1029/2010GL043091, 2010
Abstract: 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 rather than changes in other concomitant solar and cosmic variations are the cause.
The relationship is statistically significant after accounting for annual variation in ocean heat and the El Niño cycle. A warming response in the upper troposphere to increased solar UV forcing as measured by the Mg II index (core‐to‐wing ratio) decreases the atmosphere’s convective available potential energy leading to a weaker cyclone. The response amplitude at a cyclone intensity of 44 m s−1 is 6.7 ± 2.56 m s−1 per 0.01 Mg II units (s.d.), which compares with 4.6 m s−1 estimated from the heatengine theory using a temperature trend derived from observations. The increasing hurricane response sensitivity with increasing strength is found in the observations and in an application of the theory. 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,
Figure 2. Upper air temperature and tropical cyclone intensity response to variations in solar UV radiation. (a) Change in 50 hPa temperature for a change in Mg II index given the Mg II index exceeds the given percentile. The first point to the left is the ordinary least
squares regression coefficient of temperature on Mg II index using all but the lowest 10% of the Mg II values. The next point is the regression coefficient after removing the lowest
20% of the values, and so on. The point‐wise one standarderror band is shown in grey and is computed using a sandwich estimator to account for the autocorrelation in the daily values. (b) Observed and theoretical response of tropical cyclone intensity to variations in solar UV radiation (Mg II index). The observed response is a change in a percentile of tropical cyclone wind speed for all values of Mg II index. The theoretical response is the change in a percentile of wind speed for a set of temperature responses to Mg II index values exceeding a given Mg II index percentile. The solid curve (circles) and the 90% confidence band is based on a bootstrap resampling of the daily data. The dotted curve (squares) is based on equation (2) with the temperature response estimated from NCEP reanalysis data.
Here we show compelling evidence that the relationship between hurricane intensity and solar activity on the daily time scale is physically linked to changes in atmospheric temperature near the top of the cyclone induced by UV radiation. This new finding sheds light on the problem of forecasting hurricane intensification. The overall greater
sensitivity of the response found in the tropical cyclone wind data compared with the heat‐engine theory and temperature data might result from the tropical cyclones themselves warming the temperature aloft and thus dampening the temperature‐UV relationship [Swanson, 2008]. It is noted the theoretical results reflect a change in the maximum potential intensity of a particular tropical cyclone while the observational results reflect a change in the daily maximum wind speed over all tropical cyclones in the
region. Since a tropical cyclone plays a role in moistening the stratosphere [Romps and Kuang, 2009] and since the dissipation of the cyclone’s energy occurs through ocean
mixing and atmospheric transport, a tropical cyclone can act to amplify the effect on the Earth’s climate of a relatively small change in solar output. On longer time scales it is
noted that a portion of the variation in tropical SST’s (0.08 ± 0.2 K) lags the Schwabe cycle by 1 to 3 years, which is roughly equal to the time required for the upper 100mlayer of the ocean to reach radiative equilibrium [White et al., 1997].
Leif Svalgaard who brought this paper to my attention, has a full copy available for review here
He also mentions:
But see also: http://www.leif.org/research/MgII%20Calibration.pdf
Their MgII proxy for UV [see their Figure 1 is not correct. It is unclear how much that influences the result. Also, I have alerted the lead author. He said he would ‘look into it’.