Thursday, September 21, 2017

Hurricanes: A perfect storm of chance and climate change?

[BBC] The succession of intense and deadly tropical cyclones that have barreled across the Atlantic in recent weeks have left many people wondering if a threshold of some sort has been crossed. Is this chain of hurricanes evidence of some significant new frontier in our changing climate?
The answer is mostly no, but with worrying undertones of yes.
The first thing to note about this season is that it shows the power of science and weather forecasting.
Every year, the US National Oceanic and Atmospheric Administration (Noaa) puts out a hurricane forecast for the season that runs from 1 June to 30 November for the north Atlantic, Caribbean and the Gulf of Mexico.
In August, Noaa updated its predictions, stating that there would be 14-19 named storms and of these, 5-9 would become hurricanes.
To date, we've had seven cyclones that have gained category three status or stronger. So this season is unusual but not unprecedented. The bigger picture shows that between 1981 and 2010 the average was six hurricanes per season. 
What has happened this year is that a number of natural variable factors have come together and helped boost the number and power of these cyclones. In the background, climate change has loaded the dice.  
This season has been particularly warm in the region of the Atlantic where hurricanes form with temperatures 0.5 to 1.0C above average, according to Noaa.
A wetter and stronger monsoon in West Africa and a natural cycle called the Atlantic Multi-Decadal Oscillation (AMO) have also helped boost the energy available for the massive, swirling heat engines that hurricanes become.
Another factor that has helped so many storms form is the lack of wind shear.
Some scientists believe that because this is a neutral El Niño year, there has been less wind shear which tends to break these storms apart.
"There haven't been the upper-level wind flow or lower pockets of moisture that can often erode these storms; all these factors have come together this year," said Prof Richard Allan from the department of meteorology at the University of Reading, UK.
"It relates primarily to the weather, but also the slower fluctuations in the ocean that you get from year to year have produced an unusually strong hurricane season."
That view is echoed by Kerry Emanuel, an eminent atmospheric scientist at the Massachusetts Institute of Technology (MIT), US.
"The number of hurricanes in the Atlantic varies a great deal from year to year for reasons that have to do with natural climate fluctuations like El Niño and also just plain random variability," he told BBC News.  Read More