Discussion and Summary

An explanation for this geographic difference in the SSN-hurricane intensity relationship centers on the difference in the limiting factors associated with tropical cyclone intensity. As mentioned over the western Atlantic cloud top temperature appears to be the limiting factor, whereas over the eastern and central tropical Atlantic, the limiting factor in the thermodynamic potential for intensity is oceanic heat content. Since there is a direct relationship between SST and MPI, an active sun increases the shortwave flux to the ocean raising the heat content and increasing the potential for tropical cyclone intensification. In fact, a time series model of Atlantic SST contains a component corresponding to the solar cycle (Elsner et al. 2008) with Aug-Oct SST values generally higher (lower) during years of high (low) sunspot numbers.

To better understand and predict how global warming affects hurricanes it is necessary to consider the full range of natural factors responsible for variations in tropical cyclone activity. Here we identify a 10-year cycle that explains a significant portion of the inter-annual variability in tropical cyclone frequency after accounting for SST, shear, and steering. The cycle is negatively correlated with solar activity. Daily SSN is significantly negatively correlated to tropical cyclone intensity over the Caribbean especially for storms near their MPI. The variation in tropical cyclone frequency related to the solar cycle is explained by the heat-engine theory of tropical cyclones that for a constant ocean heat content implies an inverse relationship between storm intensity and cloud top temperature and therefore the amount of UV radiation absorbed by ozone.

This natural co-variability between solar and tropical cyclone activity needs to be incorporated into forecasts of hurricanes that look decades ahead. Since the dissipation of the cyclone's energy occurs through ocean mixing and atmospheric transport, tropical cyclones act as an amplifying mechanism by which small changes in the sun's output can appreciably alter the climate. The discovery has critical implications for life and property throughout the Caribbean, Mexico, and portions of the United States.

Acknowledgments We acknowledge the SIDC for the sunspot data and the NHC for the tropical cyclone data. Partial support for this study was provided by the National Science Foundation (ATM-0435628) and the Risk Prediction Initiative (RPI-05001). The views expressed within are those of the authors and do not reflect those of the funding agencies. All statistical modeling was done using R (R Development Core Team 2006).

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