What was done
The authors used the "improved extended reconstructed" sea surface temperature (SST) data described by Smith and Reynolds (2004) for the period 1854-2006 to examine historical temperature changes over the global ocean, after which they regressed vertical wind shear -- "calculated as the magnitude of the vector difference between winds at 200 mb and 850 mb during the Atlantic hurricane season (June to November), using NCEP-NCAR reanalysis data" - onto a temporal variation of global warming defined by the SST data.

What was learned
Wang and Lee report discovering that warming of the surface of the global ocean is typically associated with a secular increase of tropospheric vertical wind shear in the main development region (MDR) for Atlantic hurricanes, and that the long-term increased wind shear of that region has coincided with a weak but robust downward trend in U.S. landfalling hurricanes. However, this relationship has a pattern to it, whereby local ocean warming in the Atlantic MDR actually reduces the vertical wind shear there, while "warmings in the tropical Pacific and Indian Oceans produce an opposite effect, i.e., they increase the vertical wind shear in the MDR for Atlantic hurricanes."

What it means
The two researchers conclude that "the tropical oceans compete with one another for their impacts on the vertical wind shear over the MDR for Atlantic hurricanes," and they say that to this point in time, "warmings in the tropical Pacific and Indian Oceans win the competition and produce increased wind shear which reduces U.S. landfalling hurricanes." As for the years and decades ahead, they say that "whether future global warming increases the vertical wind shear in the MDR for Atlantic hurricanes will depend on the relative role induced by secular warmings over the tropical oceans." Hence, it is by no means clear whether further global warming, due to any cause, will lead to an increase or decrease in U.S. landfalling hurricanes. All we can say is that up to this point in time, global warming has had a weak negative impact on their numbers.

Reference
Smith, T.M. and Reynolds, R.W. 2004. Improved extended reconstruction of SST (1854-1997). Journal of Climate 17: 2466-2477.