We continue to see activist journalism claiming recent hurricanes prove global warming and the need for efforts like the Paris accord. People writing these articles seem oblivious to the meteorological science pertaining to tropical storms. The intentional deception is discussed more fully in the post Media Duping Scandal.
Joseph D’Aleo of WeatherBELL comes to the rescue with a primer for the public to gain literacy on this topic.
What Made This Hurricane Season So Active in the Atlantic? summarizes for all of us what is common sense weather knowledge, with graphs and images to enhance understanding of this science. Excerpts below with my bolds.
What a hurricane season! It started very early with Arlene in April but the real action held off until the last week of August, when Hurricane Harvey flooded Texas and Louisiana. Harvey was the first hurricane to make landfall in Texas since Ike in 2008 and the first Category 4 hurricane in Texas since Carla in 1961.
(D’Aleo summarizes the sequence of Irma, Jose, Maria and Nate, then digs into the issues.)
Before the landfall of two major storms on the U.S. we had gone just short of 12 years without a major hurricane landfall, the longest such lull since the 1860s.
The quiet period came after three big years. Isabel made landfall on the Mid Atlantic in 2003. Charley, Frances, Ivan and Jeanne in 2004 and Dennis, Katrina, Rita and Wilma in 2005 all made landfall on the mainland. Emily in 2005 was another major hurricane but turned west into Mexico. 2005 holds the record for five Category 4 or greater and four Category 5 impact storms. Some speculated this was the new norm for the Atlantic before nature gave us that 12-year break.
So what causes long quiet spells and then big years like 2004 and 2005 and now 2017?
(D’Aleo then describes the historical context regarding these storms.)
Okay, major hurricanes have occurred even during cold periods, but is there a trend in the modern record?
The Accumulated Cyclone Energy index measures seasonal tropical activity.
The Accumulated Cyclone Energy index takes into account the number, duration and strength of all tropical storms in a season. The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength.
The ACE index for the Atlantic shows a cyclical behavior with no long-term trend but with spikes in 1893, 1926, 1933 and 1950 then again in 1995, 2004 and 2005. 2017 ranks 8th now with still weeks to go this season.
So what causes long breaks and then big years like 2004 and 2005 and now 2017?
OCEAN TEMPERATURE AND PRESSURE PATTERNS
The North Atlantic, like the Pacific, undergoes multi-decadal changes in ocean temperature and pressure patterns. It has long been known that when the Atlantic is in what is called its warm mode, there are more storms. Since 1995, when the current warm Atlantic mode began, we have average 14.6 named storms per year, more than five greater than the long-term 1851-2017 average.
An important factor that affects whether hurricanes affect the United States is El Niño and La Niña. When El Niños develop, more storms develop in the eastern and central Pacific, threatening Mexico, Hawaii and sometimes in weakened forms Arizona and California.
These storms enhance high-level winds that cross into the Atlantic. These winds produce shear that disrupts developing storms, causing them to weaken or dissipate and/or turn harmlessly north into the North Atlantic. Storms can still develop near the coast where the water is warm like in the Gulf and near the Gulf Stream off the southeast coast.
When La Ninas develop there are usually fewer storms in the eastern Pacific and less shear to disrupt the Atlantic storms.
In warm Atlantic years, that means trouble as the storms can track the entire basin with more time to turn into major hurricanes. Even the East Coast is more vulnerable to a landfalling hurricane. We had eight high-impact East Coast hurricanes from 1938 to 1960 and nine from 1988 to 2012.
The last important La Niña stretch was in 2010/11 to 2011/12. We avoided a major hurricane hit, though major hurricanes at sea made final landfall in the NYC metro — Irene (as a tropical storm) in 2011 and Sandy in 2012 (as a post-tropical cyclone). They caused massive flooding (from rains with Irene in upstate NY and Vermont and from a storm surge with Sandy in New York City and New Jersey).
We are still in the latest Atlantic warm period. This year, a spring attempt at an El Niño failed and La Niña-like conditions developed. Had El Niño succeeded we may have had Harvey, which developed near the Texas coast, and Nate, which came out of the bath water in the western Caribbean, but maybe Irma and Maria would have been weakened or deflected. But with La Niña conditions developing, no shear and warm Atlantic water we saw a return to big storms just as we saw in 2004 and 2005.
So when we get a year like 2017 or back-to-back bad years like 2004 and 2005, we have to accept that is how the weather works. Permadroughts ended with record wet years for Texas and California this decade. The record nearly 12-year major hurricane “drought” ended with 2017.
Joe D’Aleo is currently a senior co-chief meteorologist with WeatherBELL Analytics. Joe is a CCM, fellow of the AMS and former chair of the AMS Committee on Weather Analysis and Forecasting. He was a college professor of meteorology/climatology, the co-founder and first director of meteorology at The Weather Channel and chief meteorologist with three companies. He is the executive director of Icecap.us since 2007.