22 Aug 2014

Why is the 2014 Atlantic hurricane season so quiet?

As the statistical peak of the Atlantic hurricane season approaches in the coming weeks, the notable lack of storm activity in the Atlantic ocean continues.

It’s not completely unexpected, with the US weather service NOAA giving a 70 per cent chance of a below normal season in its August update, increased from 50 per cent in the prediction it made in May.

So far, there have only been two named storms, Arthur and Bertha – both of which grew in strength to become hurricanes. 

hurricane_arthur_NASA_wp

In fact the remnants of Hurricane Bertha travelled across the Atlantic ocean, giving the UK some unseasonably wet and windy weather around a week ago, causing flash flooding and some disruption.

As I write this, it looks as though the third named storm of the season will form in the coming days in the Caribbean, but it look unlikely to reach hurricane status over land.

So why has this season been so quiet? As is always the case in meteorology, in order for something to happen, a number of factors need to come together at the right time, and hurricanes are no different.

Sea surface temperatures

One of the main requirements for a hurricane to form is for sea surface temperatures to be 26C or higher. This provides a fuel source for the storm to grow and become more powerful, drawing up tropical moisture into the atmosphere.

However, this season, sea surface temperatures have been below normal in the central Atlantic ocean by around 1-2C in places.

atlantic_sstanomaly_NOAA_wp

So while the water is still warm enough for storms to form, the potential supply of energy for any storms to thrive upon is reduced.

Presence of dry air

Hurricanes are vigourous tropical areas of low pressure, which thrive on a supply of tropical moisture from the warm ocean waters that I mentioned above.

As the warm water over the ocean evaporates, it is transported upwards and condenses into storm clouds, releasing large amounts of latent heat energy in the process

Moisture needs to remains present through a great depth of the atmosphere in order for potential hurricanes to survive.

However, this year there has been a persistent occurrence of dry air over the middle of the Atlantic ocean, fed by Saharan dust being blown off the west coast of Africa.

dust_atlantic_NASA_wp

It is this dry, dusty air that has acted as an inhibitor to tropical waves (the seeds of hurricanes) that may have blossomed into hurricanes had this dry air not been so prevalent.

Wind shear

Hurricanes form when clusters of thunderstorms clump together and start to spin around a central point of circulation.

However, in order for this to happen, the winds through the depth of the atmosphere shouldn’t vary too much in speed and direction with increasing height – known as wind shear.

If wind speed and direction vary too much with increasing height in the atmosphere, it has a tug of war effect on storm clouds, tearing them apart before they can start spinning.

Although the season has been quiet so far this year, as the statistical peak of the season approaches in the coming weeks, the US will be hammering home its hurricane preparedness message to ensure that when a hurricane does form, people are ready.

I’ll be keeping a close eye on the potential for hurricanes in the coming weeks and posting updates on Twitter – @liamdutton

Images: Nasa, NOAA

Tweets by @liamdutton

6 reader comments

  1. Veronique Thevoz says:

    The elephant in the room is intense geoengineering (in the form of solar radiation management), which has been cooling the eastern part of the US, exacerbating droughts in California as well as playing havoc with the jet stream. Controlling hurricanes is just one step towards owning the weather. Why are the media so silent on such topics?

  2. James Edwards says:

    Liam, you say that the sea temps were 1º-2º below normal this season. How did you arrival at a ‘normal’ temperature ?

    I ask this because with sea temps rising with global warming where do you begin to define what is normal. I have oceanographic charts compiled by that excellent scientific observer Lt Mathew Maury which served mariners for generations but now because of 100 years of man’s industrial activity seem to becoming less useful as the temperatures rise.

    In the fifties the Caribbean maxim was

    “June, too soon, July, stand by, August, look out you must, September, remember and October, all over”

    But now it seems we can have hurricanes in November and December, and in the Atlantic south of the ITZ, unheard of when I was at sea.

    OK, back to the original question, over what period of time are you using to calculate ‘normal’ and how these figures obtained.

    Many thanks for introducing the subject,

    (In the fifties I was taught by an ex Raf met man that the Atlantic and Pacific islands were the main ‘triggers’ for the genesis of TRSs’ and you only had to look at the chart to see that this was likely, has this reasoning been completely discarded ? )

  3. Fred Moore says:

    Beyond atmospheric and marine forces and constraints, hurricanes are relatively free moving machines which can thus OPTIMISE the ENTROPY gradient along their final trajectory.
    To do this they stepwise seek a limited path differential towards High Entropy conditions in the same way the car engines are most efficient between the lowest air temperature and the highest temperature of the engine block.

    The best high entropy waters are usually at the mouth of the Gulf Rivers, particularly the Mississip.

    The last 2 years this plot has been confounded by the BP oil spill and low entropy oil dispersant interventions which have left hurricanes hunting the wastewater outlets along the US seabord with little prospect of landfall due to the dispersant content circulating into the Gulf Stream.

    If the cost of the dispersants were competetive to average hurrican damage then you would have a 100% hurricanne mitigation system. Something to ponder.

    However if the US did some research on 1-2acre Engineered wetland systems at major marine effluent points along farms, factories, rivers and bays a cheaper and more resilient long term hurricane abatement program is certainly possible.

    However anything that clean and green is just a thorn in the side of the American dream. I think Congress prefers the drama of the Big ‘canes .. as in ‘speak softly and carry big sticks’.
    It’s one helluva way to run a country!

  4. norcalguy101 says:

    I just have to laugh at the three posts below. All three embrace the paradigm that human intervention is a fundamental cause for the decreased hurricane activity, with one post, James Edwards, refusing to recognize that cooler ocean surface temperatures have developed throughout the world’s oceans.

    Why?

    Because the sun’s solar activity virtually shut down for a whole year from 2008 to 2009. The latest solar maximum for solar sunspot cycle 24 in 2013 was approximately 70, or about 50% less than the previous solar sunspot maximum of approximately 140 in 2001.

    The fact there has been virtually no hurricane activity in the Atlantic for the past two years is clear evidence we are now experiencing a cooling of the earth, and the only reason is the lack of solar activity.

    1. James Edwards says:

      “cooler ocean surface temperatures have developed throughout the worlds’s oceans.” He said, as the sea levels continue to rise with unprecedented ice melt.

      1. norcalguy101 says:

        Um, James Edwards, you cannot produce any legitimate data that indicates sea levels are rising.

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