Historical Northern Hemisphere tropical cyclone inactivity during 2007

The tropical basins of the Northern Hemisphere (NH) experienced combined historical inactivity not seen in 30 years. Whether using integrated measures such as Accumulated Cyclone Energy or the Power Dissipation Index or counts of tropical storm/hurricane days, 2007 represents a minimum during the last three decades, a period representative of increased best-track data confidence. Some remarkable facts include: the lowest September NH ACE since 1977, lowest 2006 & 2007 October NH ACE since 1976 & 1977, and the fewest NH tropical storm and hurricane-force TC days year-to-date (through Nov. 15) since 1977.

While the North Atlantic tropical basin continued the near-normal activity seen in 2006, the Eastern Pacific and Western Pacific tropical basins both experienced dramatically below normal TC activity in 2007 under a variety of metrics. Nevertheless, even in a year of relative inactivity, significant events occurred including rapid intensification episodes, two consecutive Caribbean Category 5 hurricanes (Dean and Felix), as well as Gonu and Sidr, two Category 5 Northern Indian Ocean cyclones.

Against a backdrop of global warming and the conjectured effects it may have on tropical cyclones, 2007 (and 2006) provided a definite respite from the enhanced activity of the 1990s and 2004 and 2005 -- on a hemispheric scale. Of the many factors potentially responsible, the transition from weak El Nino to La Nina conditions during the heart of the TC season seems to be a likely candidate for enhanced vertical wind shear in the main development corridors. Recent work by the authors indicates that the anomalous summer/fall TC inactivity could portend an equally anomalous winter season of 2007/2008, given the dramatically reduced workload performed by TC's in 2007.