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North Atlantic May Cough up Another Out of Season Tropical Cyclone this Week
17
April, 2017
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.
Links:
Hat
tip to Vaughn
Hat
tip to Hilary
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