WHILE THE SPATE of hurricane predictions announced this time of year does underscore the need to be prepared, which is a very important message; it’s also not difficult to become overwhelmed with all the terminology and statistics being thrown casually about. To this end, the following is presented as a tropical cyclone demystification primer and a one-stop synthesis of all the “pre-season” predictions – we’ll take a look back in time as well.
Of course the hurricane season officially runs for a 6-month window opening on June 1 and sun-setting on Nov. 30; yet the formation of a cyclone outside the 6-month designation is not outside the realm of possibility. We only need to point to this year as an example – Alex formed on Jan. 13 and Bonnie formed on May 27. However climatology shows early season cyclone activity does not foreshadow an active hurricane season. In fact, forecasters are predicting a “near normal” hurricane season for 2016 based predominantly on the anticipated transition from the 2015-2016 strong El Niño event to La Niña conditions within the next few months.
Because El Niño is considered the main driver for this year’s hurricane season (as usual), it’s worth diving into this phenomenon a little more. El Niño is actually a component of the El Niño Southern Oscillation (ENSO) that occurs in the Pacific Ocean basin. ENSO “warm phase” or El Niño occurs once every 2 to 7 years and generally produces atmospheric conditions that suppress the formation of tropical cyclones in the Atlantic. That’s big for us obviously. Interestingly, the term El Niño means Little Boy or Christ Child, which was coined by South American fishermen noting the appearance of unusually warm water in the Pacific Ocean occurring near Christmas. As you may have guessed by now, La Niña (the girl child) is the “cold phase” of ENSO and tends to produce atmospheric conditions more favorable for tropical cyclone development.
So how do we know when El Niño or La Niña is upon us? Traditionally, ENSO cycles were determined empirically based upon the differences in surface air pressure between Tahiti and Darwin, Australia. Today, scientists use sea surface temperature measurements along the equatorial pacific as an indicator of El Niño or La Niña (particularly in a region known as Niño 3.4). If the sea surface temperature variance is greater than or equal to +0.5° C in region Niño 3.4, then the conditions are classified as El Niño and vice versa (i.e., if the temperature variance is lower than or equal to -0.5° C, then La Niña conditions are prevalent). And finally if the temperature variance is between +0.5° C and -0.5° C, then ENSO neutral phase is dominant (neither El Niño nor La Niña).
And lastly a full-fledged El Niño or La Niña is only officially designated if the sea surface temperature thresholds are exceeded for a period of at least 5 consecutive overlapping 3-month seasons (roughly 7 months). Hence we may have El Niño or La Niña conditions, but the history books may never reveal that an El Niño or La Niña episode ever took place.
At the time this edition of Shorelines is being prepared, we have exited the 2015-16 El Niño and are in an “ENSO neutral phase” with the likelihood of La Niña conditions developing sometime within the 2016 hurricane season. While La Niña conditions have the potential to develop over the next few months, the far North Atlantic and subtropical eastern Atlantic sea surface temperatures are quite cold. These cold water anomalies tend to force atmospheric conditions that are less conducive for Atlantic hurricane formation and intensification.
Bottom line – although a “near normal” hurricane season is predicted, there is considerable uncertainty as to what ENSO conditions will exactly be during the peak of the Atlantic hurricane season from August to October (i.e., will La Niña conditions even be here?), and how Atlantic sea surface temperatures will also vary.
Now that we reviewed ENSO in great detail, there are also plenty of terms that you will likely hear or have already heard this hurricane season – cyclones, tropical storms, hurricanes, and more. For instance, Sandy in 2012 turned extratropical and Alex earlier this year started off as a subtropical cyclone – what does this mean? The following should help in our general understanding of these terms and other.
Tropical cyclone – Warm-core, atmospheric closed circulation rotating counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Tropical storm – A tropical cyclone with a maximum sustained surface wind speed ranging from 39 mph to 73 mph using the US 1-minute average.
Hurricane – A tropical cyclone with a maximum sustained surface wind speed reaching 74 mph or more.
Saffir Simpson Scale – a scale including a 1 to 5 rating based upon wind speeds, again utilizing the U.S. 1-minute average. A category 1 hurricane has winds ranging from 74 to 95 miles per hour (mph), category 2 ranges from 96 to 100 mph, category 3 ranges from 111 to 130 mph, category 4 ranges from 131 to 155 mph, and a category 5 hurricane has sustained winds exceeding 155 mph.
Major Hurricane – A hurricane reaching category 3 or higher on the Saffir Simpson Scale. Interestingly, category 5 hurricanes very rarely make landfall while maintaining their category 5 intensity – only three have ever done so in the US – the Labor Day hurricane (1935), Camille (1969), and Andrew (1992).
Now to account for some of the weather oddballs, we also need to include;
Extratropical Storm – A cold-core atmospheric cyclone deriving its energy when cold and warm air masses interact, not as part of the positive feedback loop identified with tropical storms as warm, moist air rises causing continual heat exchange. Unlike tropical storms, extratropical storms can have one or more fronts connected to them, and can occur over land or ocean. Extratropical cyclones can have winds ranging to levels associated with a tropical depression, or as strong as a hurricane and examples include blizzards and nor’easters, which often form in winter and fall months off the mid-Atlantic and drift slowly along the north Atlantic seaboard and eventually east. If it drifts back west towards land, it is called a retrograded nor’easter.
Subtropical Storm – Occurs if waters under an extratropical cyclone are warm, followed by thunderstorms that gradually build inside the storm. The storm core may subsequently and gradually go from cold to warm, and the storm will be called subtropical.
Post-tropical Cyclone – A hybrid term describing a cyclone no longer possessing the characteristics to be considered a tropical cyclone, and are further divided into either extratropical (see above) or remnant lows.
Note: Both subtropical and extratropical cyclones have the highest winds and thunderstorms a good distance away from the center, and may have frontal boundaries associated with the systems. The two (extra- and subtropical) are usually broader systems than a tropical system, but the subtropical system will produce more rain compared to an extratropical one.
What to Expect for 2016
In a recent blog post, Weather Underground’s Dr. Jeff Masters neatly summarized a total of nine different Atlantic Hurricane predictions – and that’s just scratching the surface of the nearly two dozen predictions out there (wow). However, if you’re a frequent reader of the Island Review, then you will already know our preference is to review the predictions produced by groups that make not just their forecasts 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, UK 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 before or near when the hurricane season starts and begin to crunch the numbers. As the accompanying prediction summary table indicates, we could expect 15 named cyclones, 7 of which will generate into hurricanes, with 3 of these becoming a major hurricane (on average).
The average of the predictions result in a forecast bordering on the very upper end of a near normal hurricane season, which is actually determined by looking at 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 has some great examples to support this assertion.
For instance 2012, 2011 and 2010 are tied with 1995 and 1887 for the third-most named cyclones in one year at nineteen. However the ACE Index Values were different. 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 had the highest ACE value of these past consecutive 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.
Also 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) compared to the historical 1981-2010 average of 104. Table 2 includes the ACE Index for the past 13 years and a few notes justifying each value.
In closing, by reviewing the ACE Index we can determine whether a hurricane season is termed as below normal (<68), near normal (68–106), above normal (106–168) or even hyperactive (>168). And while most experts are indeed forecasting a near normal season for 2016 (average predicted ACE Index of 105), these forecasts do not represent landfall probabilities because cyclone paths are dependent on short-term factors such as interactions with other weather systems and fluctuating steering patterns. Unfortunately and as suggested before, it only takes one cyclone to make or break a hurricane season, with 1992 being a perfect example – just 7 named cyclones, 4 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 US history until Katrina in 2005. So again be prepared and be safe.