by Alberto Comendador, April 11, 2017
So it appears that the increase is due to improved/expanded reporting, not because there are in fact more tornadoes. This is essentially uncontroversial: NOAA gives a similar explanation on its website, though they get around the observation bias with a different method.
by Anthony Watts, April 5, 2017
From NOAA’s Geophysical Fluid Dynamics Laboratory, March 17, 2017
Two frequently asked questions on global warming and hurricanes are the following:
Have humans already caused a detectable increase in Atlantic hurricane activity or global tropical cyclone activity?
What changes in hurricane activity are expected for the late 21st century, given the pronounced global warming scenarios from current IPCC models?
R. Garibaldi et al., 2016
Journal of Atmospheric and Solar-Terrestrial Physics, 148, 48-52
As for the potential cause behind the downward trend, Rojo-Garibaldi et al. examined the possibility of a solar influence, performing a series of additional statistical analyses (spectral, wavelet and coherence wavelet transform) on the hurricane database, as well as a sunspot database obtained from the Solar Influences Data Analysis Center of the Solar Physics Department of the Royal Observatory of Belgium. Therein, their exploratory analyses revealed that “this decline is related to an increase in sunspot activity.”