Evidence Linking Solar Variability with US Hurricanes

Hurricanes, capable of filling the entire vertical extent of the troposphere, transfer large amounts of energy from the ocean to the atmosphere via latent heat released from the condensation of rising moist air. But their development and intensification are highly dependent on ambient influences. Changes in atmospheric water vapour composition (Emanuel, 1991), steering currents (Dong and Neumann, 1986), and sea surface temperatures (SST) (Holland, 1997) affect tropical cyclone intensity (for an overview, see Wang and Wu, 2004). Recently, it has been found that variations in upper-air temperature resulting from fluctuations in ultraviolet (UV) radiation from the sun can also affect tropical cyclone intensity (Elsner and Jagger, 2008 (hereafter, EJ08)).

Actually, the idea of a solar connection to tropical cyclones is almost a century and a half old (Meldrum, 1872; Poey, 1873), but the relationship has recently been re-examined. EJ08 show that the annual US hurricane counts are significantly related to solar activity. The relationship results from fewer intense tropical cyclones over the Caribbean and Gulf of Mexico when there are many sunspots. The finding is in accordance with the heatengine theory of hurricanes, which predicts a reduction in the maximum potential intensity in response to warming in the atmospheric layer near the top of the hurricane. An active sun warms the lower stratosphere and upper troposphere through ozone absorption of additional UV radiation. In short, increased solar activity – associated with more sunspots – means more UV radiation reaching the Earth’s upper atmosphere. The extra radiation warms the air aloft and decreases the temperature differential between high and low elevations resulting in less available potential energy for hurricane intensification (Elsner et al., 2010). But increased solar activity also contributes to warming of the ocean and altering of the atmospheric circulations, thus complicating the role solar variability plays in modulating hurricane activity.

The purpose of this paper is to (1) further examine the evidence for a sun–US hurricane relationship on the interannual time scale, and (2) show that the relationship is detectable in data that pre-dates the modern record. In doing so, the study provides additional evidence that the relationship is likely the result of changes in upper-level temperature caused by variation in UV radiation. This treatment is statistical, and no attempt is made to directly address the hypothesized causal mechanisms, as is done in Elsner et al. (2010).

The paper begins in Section 2 with background information about solar variability and its possible relationship with hurricane activity. In Section 3, the data used in the study are described. In Section 4, we examine the sun–hurricane relationship using data since 1851. The intra-seasonal variability in solar activity is proposed as a better way to describe the sun–hurricane relationship. This is shown using a table of ranked years and, more comprehensively, with a Poisson regression model. In Section 5, the technique of ranking years according to the magnitude of the change in intra-seasonal solar activity is used on a set of US hurricane counts dating back to 1749. The results are consistent in showing an increase in US hurricane probability for years with large sunspot number (SSN) anomalies, featuring relatively low numbers of sunspots during the hurricane season and high numbers immediately before and after the season.

Robert E. Hodges and James B. Elsner. 2011. Evidence Linking Solar Variability with US Hurricanes. International Journal of Climatology. Volume 31, Issue 13, pages 1897–1907