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.
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.
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.
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.
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