Climate mechanisms responsible for the dramatic decline in global tropical cyclone activity

During the last three years, overall global tropical cyclone (TC) activity as measured by integrated energy metrics has declined dramatically. During 2009, analysis showed that 12 and 24 month running sums of global accumulated cyclone energy (ACE) fell to some of the lowest levels during the satellite era. This inactivity continued both below-normal ACE in the Northern and Southern Hemisphere as a whole which began in 2007. Furthermore, individual basin activity has generally been below 30-year averages during the past 3-years.

The effects of La Nina and El Nino on individual basin TC activity has been addressed in many research studies with a prominent focus on the North Atlantic teleconnections through vertical shear modulation. A consequence of the building El Nino during the summer and fall of 2009 likely led to the recent tepid Atlantic hurricane season, as expected by many seasonal forecasts. Since the global frequency of TCs generally varies between 80 and 90 each year, integrated metrics of TC activity are dependent upon the duration and intensity of each cyclone, which is a natural consequence of the genesis location and track.

The dramatic decline in global and hemispheric ACE does not contradict recent climate change / TC linkage literature but provides an additional, perhaps less publicized piece of the puzzle. Indeed, since the very strong interannual variability of global hurricane ACE is closely related to ENSO, it is suggested that the role of tropical cyclones in climate is modulated very strongly by the big movers and shakers in large-scale, global climate. Thus, it is necessary to adequately quantify the impacts of ENSO and other natural climate factors upon the reorganization and modulation of TC formation locations and preferred tracks/intensification corridors. Using a normalized ACE-per-storm metric and TC interbasin relationships, it is shown that the propensity of long-track, major intensity TCs largely dominate the atmosphere-ocean climate signals or memory footprints on longer time-scales. This hypothesis suggests that integrated global TC energy is largely a function of slowly-evolving large-scale climate.