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 was this season so great? To answer this question, certain basic principles must be understood regarding the development of tropical cyclones (tropical storms and hurricanes.)

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 Africa or from the inter-tropical convergence zone (where northeast trade winds converge with southeast trade winds) over the tropical Atlantic just north of the equator. The second ingredient is very warm tropical sea surface temperatures greater than 78 degrees F and preferably greater than 84 degrees F over the tropical Atlantic basin (including the Caribbean sea and Gulf of Mexico), which provides the “fuel” for storms. The third ingredient is what meteorologists refer to as a low shear environment, meaning no strong and/or opposing winds in the upper atmosphere would disrupt the core of a developing tropical cyclone. The last ingredient, is no intervening land masses to cut off the tropical moisture which would result in the weakening and demise of a tropical cyclone (dependent on where a tropical cyclone forms and tracks.)

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 Indonesia toward the South America coast. When these clusters of thunderstorms approach the western South America Coast, they not only help produce a low shear environment, but help produce divergent (also referred to as upper level diffluence or outflow) winds in the upper atmosphere over the western tropical Atlantic basin, greatly enhancing development of incipient tropical cyclones that become co-located with these areas of upper level divergence.

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 Atlantic (sea surface temperature) Oscillation (MAO), which operates, as implied by the name, on a time scale of decades. This phenomena results in the oscillation of sea surface temperature anomalies over the tropical Atlantic from negative (cooler than normal) to positive (warmer than normal) over a cycle of about 60 years. This phenomenon directly impacts the number of candidate easterly waves and tropical disturbances available to undergo development into tropical cyclones. During positive years (warmer than normal tropical Atlantic sea surface temperatures), more easterly waves form over central Africa and remain intact as they move east over the tropical Atlantic. The opposite is true for negative years.

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 Atlantic and New England states during October. Many of the remnants of tropical cyclones which impacted the southern U.S. were able to move north and merge with frontal zones greatly enhancing rainfall over the Northeast U.S.

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