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Reason for low hurricane activity this season

WAYNE NEELY

Published: Oct 23, 2013

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There have been some red faces among hurricane forecasters.  After dire warnings that this season would be treacherous, with up to 11 hurricanes, so far this has been one of the quietest hurricane seasons for more than 60 years.

There have been two small hurricanes and 10 tropical storms, which carry less intense winds than a hurricane.  This has come as a relief to residents of hurricane-prone areas here in The Bahamas, as well as the insurance industry, which has paid large compensations in recent years.  Federal meteorologists, academic researchers and private companies each called for a busy year back in the spring.  All the soothsaying signs were in place – warm seas, weak shear winds and so on.  Even as the season stayed relatively calm through August, they called for a late surge.  In other words, forecasters are finding – yet again – that there’s still a lot to learn about the complex dynamics, ever-changing, interrelated weather phenomena that create hurricanes.

2013 hurricane season ranks as one of the least intense

There have been other years where the season delivered two or fewer hurricanes the entire season.  However, most of these were prior to the satellite era, and the accuracy of this data is considered to be questionable.  This season so far and 1982 are the only years, since weather satellites were launched in the early 1960s, in which there were only two hurricanes.

The preseason predictions were all dire, using phrases like “extremely active” and “above normal” to describe the forecast for the 2013 Atlantic hurricane season.  The National Oceanic and Atmospheric Administration (NOAA) predicted that seven to 11 hurricanes would form, while AccuWeather predicted eight.  Despite projections to the contrary, the 2013 Atlantic hurricane season has largely been a dud – and there’s no sign that will change any time soon.  It’s the first year in recent memory that every major hurricane forecast has busted after pointing to “above-normal activity”.

Some meteorologists called it a “meteorological mystery”.  Why have there been so few storms so far this year?  First of all, there have been storms – 12 named storms so far, which is about average for this time of year.  But those storms have been weaker than average – only two, Humberto and Ingrid, were classified as hurricanes – and none of them have yet made landfall.  Historically, the worst part of the Atlantic hurricane season stretches from the last part of August through September and October.

Nearly one year after Hurricane Sandy lashed The Bahamas, the 2013 North Atlantic hurricane season has not produced a single land-falling hurricane in The Bahamas nor anywhere within this region.  Instead of having above-average storm activity, as the seasonal hurricane outlooks unanimously called for, the season has been quiet – notable for its inactivity.  The tropical season doesn’t officially end until November 30, but it would take a barrage of late-season storms to bring the season up to average levels, let alone above average, something that forecasters say is unlikely.  While the season does not end until late November, time is running out for the season, much to the relief of those living in coastal areas.

“It’s not only quiet, but it’s got the potential to be near record quiet for the Atlantic Basin,” Chris Landsea, a meteorologist and science and operations officer at the National Hurricane Center in Miami, said.

So far, there have been just 12 named storms, two of which have been hurricanes, and none that have been major hurricanes.  While pre-season outlooks rarely, if ever, have pinpoint accuracy, they don’t usually miss by such a great margin.  In May, NOAA forecast that there would likely be between 13 to 20 named storms (with sustained winds of at least 39 mph).  Of those storms, NOAA projected that between seven to 11 would achieve hurricane status (winds of at least 74 mph); and that three to four would become major hurricanes of category three, four or five intensity (winds of at least 111 mph).  To put it in a plain and simple language: This did not happen.

There has not been a major hurricane in either the North Atlantic Basin or the Eastern Pacific this year, something that hasn’t happened since 1968, according to Philip Klotzbach, a hurricane researcher and seasonal forecaster at Colorado State University.  He further stated, “This is by far the worst seasonal forecast bust that our project has had, and it’s not an easy answer this year.  Typically, if our forecast busts, it’s due to either El Niño developing when we didn’t expect it to, or alternatively, El Niño not developing when we thought it would.

“This year, El Niño was very well behaved (e.g., neutral conditions like we forecast with both our June and August outlooks), and Atlantic sea surface temperatures were above average.  Also, vertical shear values were near average across most of the main development regions of the North Atlantic.  It seems like the primary factors that destroyed this season were dry mid-level air and subsidence.  The combination of sinking motion and dry air aloft choked the waves this year.  Also, we tended to have a lot of upper-level lows in the eastern tropical Atlantic which caused early season recurvature.” Klotzbach, who along with Professor William Gray pioneered the art of forecasting the severity of hurricane seasons, said future outlooks will need to incorporate more variables.

“We will be looking at ways to be able to incorporate more moisture data into our models, in hopes to not make a similar mistake in future years,” he said.

Bryan Norcross, a hurricane specialist at the Weather Channel, shared similar sentiments as Klotzbach.  He said, “The bottom line is, we don’t know for sure why this season has produced fewer named storms than expected, and, especially, why the ones that have developed have been so weak.  We can point to the strength and orientation of the typical summertime high-pressure system over the Atlantic.   It was unusually farther east this year and the circulation was oriented so it brought dry air off of Europe and Africa out over the Atlantic.

“Also, coincident with the high being farther east than normal, the dip in the jet stream that is often over the eastern part of the U.S. was also farther east over the western Atlantic.  That put unfavorable upper winds over the ocean region for a good part of the summer.”

He further noted, “Tropical development has been dramatically down all over the world, so there might also be some larger forces are at work.  Is the global decrease in tropical activity a coincidence with each part of the world being affected by local factors, just a fluke, or a trend related to a warmer atmosphere?  We don’t have answers yet, or a good explanation.”

The conditions

The major reasons for the negligible number of storms are surprising given some of the favorable conditions that exist that would normally fuel tropical cyclones.  The conditions for an above-average tropical cyclone season in theory were ripe for an active season.  The ocean waters throughout the North Atlantic were warmer than average, the trade winds were initially lighter, and there was no El Niño event in the Pacific to ramp up high altitude winds that can tear emerging storms apart.

The three main features that enhanced support for a low activity season this year were: large areas of sinking air, frequent plumes of dry, dusty air coming off the Sahara Desert, and above-average wind shear in the upper atmosphere.  The dry, stable and at times dusty air blowing from Africa was choking storms instead of allowing them to grow.  On top of that, shifts in the jet stream – the same river of air that some blamed for the wild weather in 2011 and 2012 – have caused dry air and wind shear, which interfere with the storms formation, said Gerry Bell of NOAA’s Climate Prediction Center.  None of those features were part of their initial calculations in making seasonal projections.  Researchers are now looking into whether they can be predicted well in advance like other variables, such as El Niño and La Niña events.

During this year in the North Atlantic, you had air sinking throughout a pretty large depth of the atmosphere.  This is significant because sinking air inhibits storm formation by causing air to become drier and more stable, thereby preventing the growth of thunderstorms that require rich and moist, unstable air in order to grow and thrive.  Tropical weather systems depend on a plentiful supply of warm, moist air to form and intensify, and when these storms ingest exceptionally dry air, as many of the storms have this year, they can die out or dissipate in a matter of hours or days.  One notable example of this was, Tropical Storm Karen, which formed in the Gulf of Mexico, where landfall in the U.S. or Mexico is under normal circumstances virtually assured, but died before reaching land because of the presence of dry air and strong wind shear.

Some of the dry air across the Atlantic Basin came from expansive areas of dry and dusty air that originated in the scorching Sahara Desert.  However, such outbreaks of dusty air are fairly typical during the early part of the hurricane season (mainly May through late July).  What has been unusual has been the broad expanse of sinking and drying Sahara air throughout the North Atlantic basin, lasting as far as late August and early September.

This season, the dry air made a significant difference and simply squashed all the other enhancing factors that looked good for an active season.  The area where most tropical storms and hurricanes form had the driest mid-to-lower atmospheric conditions during the August 1 to September 30 period since reliable records began in 1970.  Today, Saharan dry air is a staple of hurricane forecasting.  And this year, the Saharan winds blew farther southwest than usual, right into the zone where African monsoons blown into the Atlantic from tropical waves that become hurricanes – the notorious Cape Verde-type storms.  Meteorologists in West African nations called August the driest they had seen in quite a long time.

In addition to the dry air, an above-average wind shear was another significant reason for the low activity.  Wind shear is the difference in wind speed and direction between the ocean surface and the mid-to-upper atmosphere.  Strong wind shear can knock storms off balance or decapitate the thunderstorms accompanying the storms, essentially tearing them apart and allowing dry air to enter their circulation.  Landsea said that NOAA’s seasonal outlooks focused on the other pieces of the puzzle that argued in favor of an above-average to average season, namely the absence of El Niño and the presence of warm sea surface temperatures.  The seasonal outlook won’t be correct every time, Landsea said.

The danger of complacency

No one’s complaining about this “apparent” good luck, but there is one notable downside and that is “complacency”.  Anywhere you live, there are verbal accounts of past major storms that live on in people’s memories.  But the longer you go without a significant event, the harder it becomes to convince people to take action when one finally threatens.  It is a 100 percent certainty that it will happen again, but there will be lots of people who won’t know what they’re in for if they choose to ride it out.  Scientists fear this streak of good luck is leading to more severe cases of “hurricane amnesia”, which can complicate emergency preparation efforts the next time a monster storm threatens some area of The Bahamas.  This is significant because when a predicted active hurricane season doesn’t happen often, you certainly become a little more lax.

In the 21 years since the last category five storm Hurricane Andrew struck The Bahamas, tens of thousands of people have moved to the coastal areas of The Bahamas, many of whom have never before experienced a major hurricane.  These residents may be less willing to evacuate their homes before the next major storm strikes.  Unfortunately, people who have never experienced a major storm don’t really know what they’re in for by staying in their homes.  Unfortunately, the “fear of complacency” grows as the major hurricane gap lengthens.  One statistic that Bahamians should remember is that The Bahamas on average gets brushed or hit by a hurricane once every three years, and gets hit by a major hurricane (category three or higher on the Saffir-Simpson hurricane wind scale) once every 12 years.  Going by the records, it seems like we are more than overdue for a major hurricane.  There are three Bahamian islands ranked in the top 10 effects from tropical systems of all cities, islands and countries in the North Atlantic Basin – Andros, Abaco and Grand Bahama.  Even without a major hurricane reaching land in quite a while, The Bahamas has seen its fair share of damaging storms in recent years.  Hurricanes Michelle, Frances, Jeanne, Wilma, Irene and even Sandy have all reminded us to respect these violent creatures of nature called hurricanes.

I often get asked what role global warming is playing in a given hurricane season – whether it’s an abnormally quiet one or an abnormally active one.  It seems as if whichever one it is, global warming always seems to get the blame.  It is never accurate to correlate a hurricane season – and certainly not a specific hurricane – with global warming.  Regardless of where you stand in the debate, natural inter-seasonal variability is so large that subtle signals due to climate change are overwhelmed.  That is not to say that trends might not show up in long-term averages and climatology, but one should not cherry-pick individual seasons and storms to make a case in the debate.  When an ocean basin kicks up a fuss on one part of the globe, usually another ocean basin is quiet.  Nature tends to balance itself that way.  This year, according to the ratings, storm activity in all the world’s ocean basins is below normal.  This adds even more to the mystery and to the global debate on the impact of global warming on the intensity and frequency of hurricanes.

• Wayne Neely is an international speaker, best-selling author, lecturer on hurricanes, and a meteorologist.  Traveling extensively, Neely addresses critical issues affecting all aspects of hurricanes, especially Bahamian hurricanes, which are one of his central areas of expertise.  Neely is also a certified Bahamian meteorologist working at the Department of Meteorology in Nassau, Bahamas for the last 23 years.  He has written eight books on hurricanes.


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