Fluctuations in North Atlantic Hurricane Frequency

The annual record of hurricane activity in the North Atlantic basin for the period 1886–1996 is examined from the perspective of time series analysis. Singular spectrum analysis combined with the maximum entropy method is used on the time series of annual hurricane occurrences over the entire basin to extract the dominant modes of oscillation. The annual frequency of hurricanes is modulated on the biennial, semidecadal, and near-decadal timescales. The biennial and semidecadal oscillations correspond to two well-known physical forcings in the local and global climate. These include a shift in tropical stratospheric winds between an east and west phase quasi-biennial oscillation (QBO) and a shift in equatorial Pacific Ocean temperatures between a warm and cold phase El Niño–Southern Oscillation (ENSO). These climate signals have previously been implicated in modulating interannual hurricane activity in the North Atlantic and elsewhere. The near-decadal oscillation is a new finding. Separate analyses on tropical-only (TO) and baroclinically enhanced (BE) hurricane frequencies show that the two components are largely complementary with respect to their frequency spectra. The spectrum of TO hurricanes is dominated by timescales associated with ENSO and the QBO, while the near-decadal timescale dominates the spectrum of BE hurricanes. Speculations as to the cause of the near-decadal oscillation of BE hurricanes center on changes in Atlantic SSTs possibly through changes in evaporation rates. Specifically, cross-correlation analysis points to solar activity as a possible explanation. “The hurricanes . . . used to come every seven years, or every five years, but they have become more frequent following the settlement of the Antilles.” —Jean Baptiste Du Terte