North Atlantic decadal variability and the formation of tropical storms and hurricanes

Both the annual number of Atlantic tropical storms forming south of 23.5°N and of Atlantic major hurricanes increased between the 1970's/1980's and 1995–2000. These increases are coincident with a multi‐decadal warming in North Atlantic SST suggesting that the high activity of 1995–2000 may persist for the next ∼10 to 40 years. However, during 1950–2000 strong decadal oscillations are superimposed on the multi‐decadal changes in both SST and tropical storms (positive SST anomalies, increased storm activity). We appear to be entering a negative phase of the decadal SST signal implying that tropical storm, and most likely major hurricane, activity may be reduced in the next several years rather than remain at the very high 1995–2000 level when both signals were in their positive phase. Tropical storm activity during 2001 and 2002 is less than the expected only from the multi‐decadal signal but for 2002 the main cause may be El Niño.