What was done
Noting that the El Niņo-Southern Oscillation (ENSO) "has been shown in many previous papers to impact seasonal levels of Atlantic basin tropical cyclone activity," and, more specifically, that "El Niņo reduces U.S. hurricane landfall numbers (Bove et al., 1998; Elsner and Jagger, 2004, 2006; Smith et al., 2007)," the author "revisits this relationship by examining a longer period (1900-2009) than has been examined in earlier analyses."

What was learned
Klotzbach confirmed that "Atlantic basin hurricane activity is significantly reduced in El Niņo years compared with La Niņa years," and that "the largest impacts of ENSO on large-scale climate fields were shown to be in the Caribbean, with smaller signals observed over the remainder of the tropical Atlantic." He also determined that "the large-scale field that appears to be impacted the most by the phase of ENSO is the 200-850-mb vertical [wind] shear field, with considerably more shear present in El Niņo years, especially over the Caribbean." And as would thus be expected, he reports that hurricane "landfalling frequency along the U.S. coastline is less in El Niņo years as well," citing the fact that "a total of 19 major hurricanes made landfall along the U.S. coastline in the 15 coldest ENSO events, compared with only 3 major hurricanes in the 15 warmest ENSO events." And he adds that, "in general, El Niņo-La Niņa relationships are stronger in the negative phase of the Atlantic Multidecadal Oscillation (AMO) than in the positive phase of the AMO."

What it means
In light of the author's several significant findings, there would appear to be little room for any significant impact of CO2-induced global warming, in and of itself, on the frequency or intensity of Atlantic basin hurricanes.

References
Bove, M.C., O'Brien, J.J., Elsner, J.B., Landsea, C.W. and Niu, X. 1998. Effect of El Niņo on U.S. landfalling hurricanes, revisited. Bulletin of the American Meteorological Society 79: 2477-2482.

Elsner, J.B. and Jagger, T.H. 2004. A hierarchical Bayesian approach to seasonal hurricane modeling. Journal of Climate 17: 2813-2827.

Elsner, J.B. and Jagger, T.H. 2006. Prediction models for annual U.S. hurricane counts. Journal of Climate 19: 2935-2952.

Smith, S.R., Brolley, J., O'Brien, J.J. and Tartaglione, C.A. 2007. ENSO's impact on regional U.S. hurricane activity. Journal of Climate 20: 1404-1414.