Where have all the hurricanes gone?

Unless dramatic changes occur in the next few weeks, the 2013 Atlantic hurricane season is going to be the most inactive in almost half a century.

Although the season does not end officially until November 30, there have been just two fully-fledged hurricanes. Both Humberto and Ingrid were Category 1 storms on the Saffir-Simpson 1-5 scale, with 1 being the lowest.

Prior to the start of the season, June 1, most major weather forecasting organisations were predicting between 12 and 20 named storms with 5 to 10 hurricanes. This would have been ‘average’ to ‘above average’, as a typical season would see 12 storms and 6 hurricanes.

To date, 12 storms have formed – which is very much at the lower end of the predictions. More significantly, the formation of just two hurricanes suggests that something was seriously amiss with the predictions.

The Accumulated Cyclone Energy (ACE) index is one measure of the total destructive power of a season’s storms. The ACE index this year is just one third of normal. The index value of 24 is the lowest since 1983 (17) and the second lowest since records began in the 1950s.

Hurricane activity has been on the increase since the mid-1990s, probably as a result of rising sea surface temperatures, and the predictions for 2013 were based on several favourable factors in storm formation.

Strong trade winds and the El Nino ocean upwelling are both known to disrupt the vertical development of tropical storms. Their predicted absence for the 2013 season was likely to have favoured storm development.

Probably the key storm ‘predictor’ is sea surface temperatures. Warm waters provide the energy for developing storms. Both observed and predicted temperatures were significantly above average. So what went wrong?

It appears that wind shear, the change in winds with height, was stronger than forecast, but the key ‘spoiler’ seems to have been the lack of moisture in the lower and middle portions of the troposphere.  This is the lowest part of the atmosphere, extending to a height of around 10km, which contains the bulk of our weather.

The dryness of the air in this region has been the result of subsiding (sinking) air which has a warming and drying effect, both of which act against storm development.

Dust storms which blow from Africa’s Sahara Desert are also known to have a drying effect on the atmosphere, and vast quantities of dust are known to have blown across the Atlantic during the early part of the summer.

Climate scientists cite the example of Tropical Storm Karen which formed in the Gulf of Mexico during early October, as a system which has felt the full effects of drying air. Karen would normally have been assured a landfall over the US or Mexico. Instead, it died before reaching land, strangled by a combination of dry air and wind shear.

The season has just over four weeks to run. There is still time for a surge in activity, akin to what we have seen in the West Pacific. Here, it looked likely to be one of the quietest on record, but October has seen the development of more typhoons (seven) than in the previous nine months combined (five).
 
Either way, meteorologists and climatologists will be working to improve their prediction capabilites for future hurricane seasons.