Distinct modes of multidecadal variability in North Atlantic tropical storm frequency and season length

Analysis of total annual tropical cyclone (TC) counts and season length from 1855 to 2005 revealed three distinct modes of long-term (> decadal) variability. An approximately bidecadal mode of small amplitude was most clearly observed in the TC frequency and even more clearly in the season length, suggesting that increased frequency is partially associated with longer seasons. Also apparent in both TC frequency and season length was the 65-year Atlantic Multidecadal Oscillation (AMO). Underlying both of these modes was a positive linear trend (~5 days per decade) of increasing season length from the early 20th century to the present. This trend explained about half of the variability in season length. Sea surface temperature (SST) was strongly positively correlated with tropical cyclone frequency at time scales > 50 years, consistent with the AMO. At the bidecadal scale, however, SST was strongly negatively correlated with both TC frequency and season length, suggesting that controls other than SST oscillate and exerting a small (~2 TC per year) influence on the bidecadal time scale.