Like pretty much everywhere else in the world ocean these days, and due primarily to a rampant injection of greenhouse gasses into the Earth’s atmosphere through fossil fuel burning, the North Atlantic is now considerably warmer than during the 19th and 20th Centuries…
Warming Waters and An Angry Jet Stream
That extra heat provides more available fuel for tropical storm and hurricane formation. It increases the top potential peak intensity of the most powerful storms. And it extends the period in which such tropical cyclones are capable of forming — for sea surface temperatures of at least 70-75 degrees Fahrenheit are often necessary to fuel such systems (please also see the present science on how climate change is impacting tropical cyclones).
(Sea surface temperatures in the North Atlantic now range between 1 and 7 degrees Celsius above average for most regions. These warmer than normal sea surfaces provide more fuel for storms even as they extend the period during which tropical storm and hurricane formation is possible. Image source: Earth Nullschool.)
But it’s worth noting that warm ocean waters are not the only ingredient that add to the potential for the formation and strengthening of these powerful storms. Instability and cloud formation are often necessary to seed such systems. And the more extreme warm and cold temperature anomalies associated with wavier Jet Stream patterns inject exactly this form of instability into the middle latitudes at a higher rate than was witnessed during past decades.
Due to its proximity both to a melting Greenland and to a rapidly warming Arctic, the North Atlantic is particularly vulnerable to the production of powerful swirls of warm and cold air. Warming tropics collide with the cold air producing pools of glacial freshwater melt and the enlarging meanders of the Northern Hemisphere Jet Stream. And it’s the proliferation of these unstable vortices forming over warming waters throughout the North Atlantic that may start to generate a more and more noticeable higher incidence of both out of season cyclones and stronger storm systems.
(A persistent swirl of disorganized clouds in the Central North Atlantic — continuously re-charged by frontal systems sweeping down from Baffin Bay and feeding on warmer than normal sea surface temperatures may become the first tropical cyclone of 2017. If it later forms into a tropical storm, it will become the third out-of-season named storm to form in the Atlantic over the last 15 months. Image source: LANCE MODIS.)
Last year, extremely warm sea surface temperatures combined with this kind of observed instability to spur the formation of Hurricane Alex during January. Tropical storm Bonnie also formed out of season during May. Similar very warm ocean conditions then helped to kick-start the late November formation of Category 3 Hurricane Otto (though November is still technically hurricane season, it’s supposed to be very rare to see so strong a storm form so late in the year).
Possible April Cyclone Underlines Recent High Incidence of Out of Season Storms
Fast forward to April of 2017. According to the National Hurricane Center, there’s now a 30 percent chance that a tropical depression may form in the Central Atlantic over the next 48 hours. Ultimately, such a system could gather into the first Atlantic named storm of 2017 — Arlene. Such an event would mark the third time in just 15 months that the Atlantic basin had produced an out-of-season tropical storm or hurricane.
(A vast majority — 97 percent — of tropical storms and hurricanes in the Atlantic form during hurricane season from June 1 to November 30. That said, human forced climate change may now be in the process of providing more fuel for the formation of out-of-season storms. Image source: North Atlantic Tropical Cyclone Climatology.)
Incidence of out-of-season tropical storms or hurricanes in the Atlantic is rather rare. Over 158 years from 1851 to 2009, perhaps one such system formed, on average, each year. Moreover, these storms primarily formed during May — which by itself produced more out-of-season storms than December through April combined. And a vast majority of these systems were tropical storms — not hurricanes or major hurricanes.
In 2016 and 2017, Alex formed as a hurricane during January — which is practically unheard of. Bonnie formed during late May, which was less unusual but still out-of-season. Otto formed as a category 3 major hurricane during late November — another anomalous event. Meanwhile, if Arlene forms this April it will represent 1 out of only about 20 such systems that formed during the month in the period of 1851 through 2009.
But even if we don’t get a tropical cyclone in the middle of the North Atlantic during April of 2017, it’s becoming increasingly obvious that conditions have changed. That forecasters now need to be more alert for out-of-season tropical cyclones and to the various new weather phenomena that are now being precipitated by a warming climate.
Hat tip to Vaughn
Hat tip to Hilary