Why a Record Hurricane Season?
By: Victor Nouhan, Climate Focal Point
As the 2005 Atlantic hurricane season draws to a close, one
cannot help to ask; why
Tropical cyclones need four basic ingredients for development.
The first ingredient are incipient disturbances, or cluster of thunderstorms,
called easterly waves. In the Atlantic basin, these originate from central
During each hurricane season in the Atlantic basin, the numbers of tropical cyclones vary due to large scale climate interactions. Three interactions have major roles in modulating the conditions (ingredients) leading to tropical cyclone development, and a fourth has a minor role.
The first, referred to as the Madden Julian Oscillation,
operates on a time scale of weeks within a hurricane season. The Madden Julian
Oscillation refers to large cluster of thunderstorms that move eastward along
the equatorial Pacific from
The second large scale climate interaction, is the EL-Nino / La-Nina (ENSO) cycle which operate on a time scale that spans an entire hurricane season, or even consecutive seasons. Tropical cyclones in the Atlantic basin are not favored during El-Nino years, since strong westerly winds in the upper atmosphere occur during these seasons, which blow in the opposite direction to the motion of incipient tropical cyclones, disrupting the outflow and even ripping apart the convective (thunderstorm) core of these systems before they can become more organized. During neutral and La-Nina years, westerly shear is less or absent allowing for greater opportunity of incipient tropical cyclones to develop.
The third large scale climate interaction is referred to
as the Multi-Decadal
The fourth large scale climate fluctuation, called Quasi-Biennial Oscillation (QBO), refers to the direction the wind is blowing in the lower to mid stratosphere over the equatorial Pacific. The direction switches from east to west and back to the east again in 24 to 30 month cycles. Seasons with the easterly (stratospheric winds blowing east to west) appear to be slightly more favorable.
Now that the players are known, what was operating during the 2005 Atlantic Hurricane season? First, MAO has been in the positive phase since about 1995, with no Atlantic hurricane season since then experiencing a below normal number of tropical cyclones. This is to say, that more incipient tropical systems have been available for development than the long term normal. Second, ENSO has been neutral, that is neither EL-Nino nor La-Nina, favorable for a low shear – tropical cyclone development environment. Third, at the onset of the 2005 Atlantic hurricane season, a strong MJO cycle provided less shear and more upper level divergence over the western Atlantic basin (especially the Caribbean) early in the season (June and July), resulting in a record number of July storms. Lastly, QBO was in the favorable easterly phase. Having every large scale climate factor being favorable in one season is very rare, with the result being a record Atlantic tropical cyclone season. This stands in stark contrast to the 1970s and 1980s which had few if any favorable large scale climate factors, and relatively few Atlantic tropical cyclones.
Lastly, one indirect consequence to this record hurricane
season was the tremendous rainfall experienced over the mid