By Dale C. S. Destin |
Early forecasts just issued for the upcoming 2015 Atlantic hurricane season indicate another quiet season is likely. The forecasts indicate that the 2015 season could be even quieter than 2014 and perhaps be the quietest since the middle of the 20th century.
Early 2015 consensus forecast
The consensus based on forecasts from Klotzbach and Gray of Colorado State University and Saunders and Lea of Tropical Storm Risk.com (TSR) calls for nine named storms, four becoming hurricanes and two becoming major (Category 3 or higher) hurricanes.
A better indicator of the activity for the season is the accumulated cyclone energy (ACE) index which is a measurement of the strength and duration of each tropical cyclone. Summing together the ACE of each cyclone, provides a snapshot of how active the season is likely to be outside of just the number of storms.
This year, the consensus forecast calls for an ACE index total of 48. If this forecast pans out, the 2015 hurricane season would be quieter than last year’s and the third quietest since 1995, when the hurricane season went from a quiet phase to an active one.
End of Atlantic active phase?
Around 1995, the Atlantic hurricane season went from a quiet phase, when the average annual number of named storms increased from 9 to 15. Questions are now being raised in the tropical cyclone community as to whether we have come to the end of this active phase.
According to Saunders and Lea, “should the…forecast for 2015 verify, it would mean that the ACE index total for 2013-2015 was easily the lowest 3-year total since 1992-1994, and it would imply that the active phase of Atlantic hurricane activity which began in 1995 has likely ended”.
However, before we burst open the bubblies in celebration of fewer hurricanes for the next few decades, it must be stressed that the ability to accurately forecast an upcoming hurricane season from April, over one and a half months before the start of the hurricane season, is very low. The next set of forecasts, with better skill, will by out by June 1.
Factors pointing to a quiet season
Although the April seasonal forecasts are very low skilled, there are two already occurring phenomena present that are notorious for producing quiet Atlantic hurricane seasons. These are El Nino and a positive North Atlantic Oscillation (NAO). These are the main factors cited for the projected quiet season.
Probability of Antigua being hit by a hurricane
The probability of Antigua being hit by a hurricane annually appears to vary depending on the phase of the Atlantic. Based on the period 1981-2010, the climatological probability of being hit by at least one hurricane is around 28%. However, for the Atlantic hurricane season quiet phase – 1962 to 1994, the probability was around 14%. While for the active phase – 1995 to present, the probability increased to around 36%.
Interestingly, based on ENSO record dating back to 1950, we have never been hit by a hurricane during an El Nino episode that has occurred over the whole or part of a hurricane season.
On the other hand, we have been hit by nine hurricanes during 24 La Nina episodes. By Poisson distribution, this equates to around a 31% probability of us getting hit by at least one hurricane during La Nina.
According to Klotzbach and Gray, the likely best similar/analogue years to the upcoming 2015 hurricane season are 1957, 1987, 1991, 1993 and 2014. Of these years, we were only hit in 2014 by Gonzalo. Thus based on similar years, the probability of Antigua being hit this year is about 18%.
Good news and bad news
There is an Antiguan saying: “de same stick that hit the wild goat will also hit the tame one.” El Nino and the positive NAO will likely “hit” the hurricane season, resulting in suppressed activity. However, these same phenomena will likely “hit” our rainfall activity, resulting in suppressed rainfall and continued drought.
2014 hurricane season and lessons learnt
The 2014 hurricane season produced eight named storms, three hurricanes and two major hurricanes. The ACE index total was 66, the fourth lowest since 1995. It was a quiet year for many but not Antigua as we were hit by Hurricane Gonzalo.
Gonzalo serves as a perfect reminder that notwithstanding a quiet season, it only takes one hurricane to make it an active season for us. Hence, quiet season or not, the same preparations are required.
Our next blog on this topic will be on June 2.
Follow us also @anumetservice, facebook and tumblr to keep updated with weather & climate info for the protection of life, property, livelihood & the enhancement of the economy.