What was done
The author examined Caribbean tropical cyclone activity over the period 1900-2008, during a time of supposedly unprecedented global warming, looking for impacts that may have been associated with it or the El Niņo-Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO).

What was learned
Klotzbach determined that "the probability of one or more hurricanes and major hurricanes tracking through the Caribbean increases dramatically from 39% and 26% in the 10 warmest ENSO years to 92% and 63% in the 10 coldest ENSO years, respectively," in harmony with the similar findings of Tartaglione et al. (2003), who additionally demonstrated that this cooling-induced response was likely due to "reductions in vertical wind shear and increases in low-level vorticity" in La Niņa conditions. This connection was also demonstrated by Klotzbach, who determined that "for the 10 warmest events since 1948, the average 200-850-mb zonal wind shear in the Caribbean was 7 m/s compared with only 3 m/s in the 10 coldest events since 1948."

The Colorado State University researcher also determined that "the impacts of ENSO are reduced slightly when the AMO is positive," and he found that "a negative AMO phase and El Niņo combine to provide large-scale climate features that are especially hostile for tropical cyclones." He reports, for example, that "29 hurricanes tracked into the Caribbean in the 10 strongest La Niņa years in a positive AMO period compared with only two hurricanes tracking through the Caribbean in the 10 strongest El Niņo years in a negative Atlantic multidecadal oscillation period."

What it means
Finding that when the ENSO and AMO are considered in combination, the compounded relationship they bear to Caribbean tropical cyclone activity is even stronger than their individual impacts, Klotzbach's work leaves little room for any significant impact of CO2-induced global warming, in and of itself, on the frequency or intensity of Caribbean hurricanes.

Reference
Tartaglione, C.A., Smith, S.R. and O'Brien, J.J. 2003. ENSO impact on hurricane landfall probabilities for the Caribbean. Journal of Climate 16: 2925-2931.