Dale C. S. Destin |
Recently issued forecasts for the 2015 hurricane season, which starts today, indicate another quiet season is most likely. The latest set of forecasts indicates that this season could be even quieter than 2014 and perhaps be among the quietest since 1981.
The 2015 ensemble forecast
The ensemble or mean forecast is for nine named storms (including Tropical Storm Ana), four becoming hurricanes and one becoming a major (Category 3 or higher) hurricane. On average we get twelve named storms, 6 hurricanes and 3 major hurricanes.
Our ensemble forecast is based on forecasts from Klotzbach and Gray of Colorado State University (CSU), the National Ocean Atmospheric Administration (NOAA), Saunders and Lea of Tropical Storm Risk.com (TSR), the United Kingdom Met Office (UKMET), European Centre for Medium-Range Weather Forecasts (ECMWF), Weather Services International (WSI) and Pen State University (PSU).
A better indicator of the activity for the season is the accumulated cyclone energy (ACE) index which is a measurement that takes into consideration the number, strength and duration of tropical cyclones for the season.
The ACE index ensemble forecast for this season is 52. If this forecast verifies, this hurricane season will be the tenth quietest since 1981 and the third quietest since 1995, when the hurricane season went to an active phase.
Why is a quiet season most likely?
El Nino is expected to be the main cause for a quiet season. Basically all the climate models are forecasting El Nino to persist through the hurricane season and keep atmospheric conditions difficult for tropical cyclone formation across much of the Atlantic. Cooling sea surface temperature across the tropical North Atlantic will also hinder tropical cyclone formation.
However, it is not a 100% certainty that we will have a quiet season. We are still in the time of the year when there is very little skill in predicting El Nino. Therefore, notwithstanding what is being said by the models, there is a slight chance of an above normal season; there is also a slight chance of an even quieter season than forecast; it all depends of the eventual strength of El Nino.
It must also be kept in mind that the May seasonal forecasts only possess modest skill in predicting the eventual hurricane season. The early August forecasts, which show the best skill, are the ones to look forward to next.
There is a growing consensus that we may have seen the end of a high-activity/active hurricane season era, which started in 1995, and the start of a new low-activity/quiet one, which could last for the next 20-30 years. Thus, for the next several decades, the quiet activity for the past two years could become the norm.
NOAA has indicated that the “current configuration of [sea surface temperatures] SSTs in the Atlantic Ocean… are suggestive that the [Atlantic Multi-Decadal Oscillation] AMO may no longer be in the warm phase” which is synonymous with a high-activity era.
Meanwhile, TSR has continued to indicate that “should this [there] forecast verify… it would imply that the active phase for Atlantic hurricane activity which began in 1995 has likely ended.”
According to CSU, “[the] twelve-month running average values of the [AMO] index are currently at their lowest levels since 1994, when the AMO was in a negative phase” or when we were in the last low-activity era.
This is good news for Antigua and likely the rest of the Caribbean and the wider Atlantic Basin. The fewer the number of tropical cyclones, the less often we will be impacted by tropical cyclones.
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. The probability of being hit by at least one hurricane is around 28%, based on the period 1981-2010. However, during the last low-activity era – 1962 to 1994, the probability was around 14%. While for the high-activity era – 1995 to 2014, the probability increased to around 36%.
Of some comfort, 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.
According to Klotzbach and Gray, the likely best similar/analogue years to the upcoming 2015 hurricane season are 1957, 1965, 1972, 1982, 1987 and 1997. Of these years, we were only brushed by Erika in 1997. Thus, based on similar years, the probability of Antigua being hit this year is about 15%, a decrease of 3% from the April forecast.
Don’t be caught off guard
The 2014 hurricane season produced eight named storms, six 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 each year for the hurricane season, which runs from June 1 to November 30.
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