ALTHOUGH WE’RE A few weeks into 2015, perhaps it’s still not too late to entertain one last “year in review” story, which smartly leads us to our annual recap of the 2014 hurricane season (June 1-Nov. 30). The 2014 hurricane season started with a bang (or a blow) as Hurricane Arthur crossed the very eastern tip of Shackleford Banks as a Category 2 hurricane on July 3. It was the strongest hurricane to strike the U.S. since 2008. However the impacts from Arthur were very minor thanks to the rapid forward speed of the hurricane – there simply wasn’t enough time for the seas to develop along the oceanfront or the wind to pile water up on our sound and creek shorelines. After crossing Shackleford Banks, Arthur whisked away rather quietly across Pamlico Sound and into the Atlantic Ocean as the July 4 weekend festivities were as celebratory and full of visitors as usual.
From a broader perspective, most experts agree the Atlantic Ocean basin continues to be in a heightened trend of tropical cyclone activity compliments of cyclical ocean-atmosphere interactions, however, a convergence of factors helped suppress cyclone formation in 2014 (Fig. 1). Atmospheric circulation favoring dry, sinking air in the tropical Atlantic and high wind shear over the Caribbean dominated the summer 2014 weather pattern. Cyclones need moist air, not dry air to develop and when coupled with wind shear the conditions were almost exactly opposite of what cyclones need to form. And lastly, the West African monsoon season was near to below average, which made it more difficult for African easterly waves to develop. These are the low pressure systems we see all the time emerging from west Africa near the Cape Verde Islands that can subsequently develop into tropical cyclones and possibly ride the “hurricane express” across the Atlantic.
2014 Forecasts Were on the Mark
Hurricane forecasters were right on target in 2014 after a complete miss in 2013 – a near hyperactive season was predicted in 2013 but in reality the season ended as the 6th weakest since 1950. How can we objectively make all these assessments? If you’re a frequent reader of the Island Review, then you will know our personal preference is to review the predictions generated by groups that make not just their prediction public, but verify their prediction skill in the public arena as well. This really leaves us with; (1) the Tropical Meteorology Project at Colorado State University, (2) the University College London, U.K. for Tropical Storm Risk, and (3) our federal voice for climatology/meteorology matters, the National Oceanic & Atmospheric Administration (NOAA). We then take these groups’ last prediction just before hurricane season and compare the predictions to the actual results at the end of the season (Nov. 30).
As the accompanying prediction summary chart indicates (Table 1), the average prediction included 11 total cyclones (the actual was 8), four of which were predicted to generate into hurricanes (the actual was six), with two of these becoming major hurricanes (which was spot on). This means seven tropical storms were predicted and the actual number was two. Remember the term tropical cyclone refers to an atmospheric closed circulation rotating counter-clockwise in the northern hemisphere – they can develop into tropical storms and hurricanes. Major hurricanes are considered Category 3 or higher, and all of these designations are determined using different thresholds of maximum sustained surface wind (e.g. – a tropical storm develops when winds range from 39 mph to 73 mph).
As can be quickly gleaned from this prediction chart, the key elements for the 2014 hurricane season were generally lower than the 1981-2010 historical average, and maybe ever-so-slightly lower than what was predicted. Even more technically speaking, we had a “below normal” hurricane season, which is actually determined by looking at a term we haven’t discussed yet – the Accumulated Cyclone Energy Index (ACE Index). The ACE Index is simply a measurement taking a storm’s wind speed strength for each 6-hour period of its existence into account. The larger the ACE Index value, the more active the season. The ACE Index is actually one of the more revealing parameters we can use and serves as a better barometer of whether or not a hurricane season is truly “active” or not. This past decade and even this year (2014) have some great examples to support this assertion.
For instance 2012, 2011 and 2010 tied the years of 1995 and 1887 for the third-most named cyclones in one year at 19. However, the ACE Index values were different in each year of that 2012 to 2010 stretch. Why? In 2012 we had 10 of the 19 cyclones develop into hurricanes (ACE = 128), while only 7 of the 19 cyclones developed into hurricanes in 2011 (ACE = 119). In 2010 we saw the highest ACE value of these three years (ACE = 163) with 12 of the 19 cyclones developing into hurricanes, including the particularly intense and long-lasting Hurricane Igor that had an ACE value/contribution of 42 in itself. This all makes sense because again the mathematical formula takes each cyclone’s wind speed and duration into account. As an interesting note, the highest ACE Index ever recorded was roughly a decade ago in 2005 – a hurricane season punctuated by more tropical storms, total hurricanes, and Category 5 hurricanes than in any season previously recorded; and included Ophelia for North Carolina and the infamous major hurricanes of Katrina, Wilma and Rita in the Gulf of Mexico. The ACE Index was 248 (that’s not a typo). Table 2 includes the ACE Index for the past 12 years and a few notes justifying the value.
The average ACE Index is 104. This year (2014) the ACE Index was 66, which again makes a lot of sense upon further inspection. We had a relatively low amount of tropical cyclones develop, and actually hurricanes Edouard and Gonzalo accounted for over 60 percent of the ACE Index in themselves. As depicted in Table 1, the average ACE Index of all the pre-season predictions was 67 – wow, just one point of the actual value of 66. Sure, the precise number of hurricanes and tropical storms were slightly off when comparing the predictions to the actual numbers; but again in terms of overall activity, the predictions were on the mark.
In closing, the ACE Index is also used to determine whether a hurricane season is termed as below normal (<68), near normal (68-106), above normal (106-168) or even hyperactive (>168). Hence why below normal is used here as an objective term to characterize the 2014 hurricane season – again, the ACE Index for 2014 was 66. Unfortunately and bringing things back home, it only takes one cyclone to make or break a hurricane season, with 1992 being a perfect example – seven named cyclones, four of which were hurricanes, with one of those classified as major, and an ACE Index value of 75. Sounds like a very quiet year, except the one major hurricane was Andrew, which struck Florida and was the costliest natural disaster in U.S. history until Katrina in 2005. This underscores the need to be prepared for each and every hurricane season regardless if it is an “active” season or not. June and the start of the 2015 hurricane season is just a several months away – it’s never too early to start preparing.