No Long-term Trend in Atlantic Hurricane Numbers

A paper in the Journal of Geophysical Research authored by a team of leading hurricane researchers (Villarini et al., 2011) examined the historical record of tropical storm observations from the Atlantic Ocean, focusing on the number of tropical storms whose entire lifetime was less than two days. The authors' termed these very minor storms "shorties." The identification of shorties is one element of the tropical cyclone record that could be significantly impacted by changing observational methodologies and technologies, as short-duration storms are presently identified much more readily than they were prior to the satellite era, which really began around 1966.

If the Atlantic tropical cyclone history is divided up into "shorties" and those that are longer lasting, something very interesting emerges. Over the entire record, there is a big upward trend in the number of "shorties" but there is no trend in the annual number of "longies" (Figure 1)

This agrees with work done by Landsea (2007) who did a bias-corrected time series of tropical storms, subtropical storms, and hurricanes to take into account undercounts before the advent of geostationary satellite imagery in 1966 and new technology available since about 2002.

Villarini et al. conclude their paper by saying:
"Based on our results, it appears that the long-term record of the basin-wide shorties is sufficiently contaminated by spurious components to mask any climatically induced variation within the raw data. Moreover, based on these results and those of Vecchi and Knutson [2008] it is unlikely that a homogeneous record of Atlantic tropical storm counts would contain a statistically significant positive trend since the late 1800s. Our results provide a context for interpreting studies exploring trend behavior in the North Atlantic tropical storm activity starting prior to the 1940s. In particular, the conclusions of certain studies reporting large secular increases in North Atlantic tropical storm activity in which shorties are included [e.g., Holland and Webster, 2007; Mann et al., 2007] could be affected by what we interpret as likely spurious nonphysical trends unless an alternative physical explanation can be uncovered for the pronounced increase in shorties starting from the middle of the 20th century."

Improved technologies have improved our ability to detect and warn of both violent local storms and hurricanes. It appears very likely that this better detection has been responsible for apparent trends in frequencies of these storms frequently attributed to global warming.

Additional References
Holland, G.J., Webster, P.J., 2007: Heightened tropical cyclone activity in the North Atlantic: Natural variability or climate trend? Philos. Trans. Roy. Soc. Math. Phys. Eng. Sci., 365A, 2695-

Landsea, C. W., 2007, Counting Atlantic Tropical Cyclones Back to 1900, EOS, Transactions American Geophysical Union, 88, 18, 197-208

Mann, M.E., , T. A. Sabbatelli, and U. Neu, 2007: Evidence for a modest undercount bias in early historical Atlantic tropical cyclone counts. Geophysical Research Letters, 34, L22707, doi:10.1029/ 2007GL031781.

Vecchi, Gabriel A., Thomas R. Knutson, 2008: On Estimates of Historical North Atlantic Tropical Cyclone Activity*. J. Climate, 21, 3580-3600. doi: 10.1175/2008JCLI2178.1