WASHINGTON — As Hurricane Irma
barrels dangerously toward Florida, scientists say that a perfect mix
of meteorological conditions has conspired over the past week to make
the storm unusually large and powerful.
“You need just the right ingredients for a hurricane of this magnitude to last for so long,” said Phil Klotzbach, an atmospheric scientist at Colorado State University. “And Irma has had them all.”
Weather forecasters had already expected
this summer to be an active hurricane season in the Atlantic Ocean
because of warmer-than-average ocean surface temperatures, which provide
fuel for hurricanes, as well as weaker-than-average wind shear, which
can help to dissipate storms.
But even in that context, Irma was special.
Irma
initially developed near Cape Verde off Africa’s coast on August 30,
and a ridge of high pressure kept the storm from wandering off
harmlessly into the cooler northern ocean. Instead, it was pushed
inexorably westward, with plenty of time to intensify over the warm
Atlantic waters.
On its journey west, Irma underwent at least six cycles called “eyewall replacement,”
when the eye of the hurricane naturally contracts. While storms can
weaken during this process, they also can quickly intensify and expand
in size once the cycle is complete. And Irma did not weaken — it just
grew and grew.
Graphic
A ‘500-Year Flood’ Could Happen Again Sooner Than You Think. Here’s Why.
Terms like “500-year flood” and “100-year flood” are
used as shorthand by government officials and actuaries, but they can
confuse the public.
Ultimately,
Irma persisted for three consecutive days in the Atlantic Ocean as a
Category 5 storm, a record since scientists have been using satellites
to watch storms. It also had maximum sustained winds recorded at more
than 185 miles per hour for 37 straight hours — beating the record set
by Super Typhoon Haiyan in the Pacific in 2013, according to Dr. Klotzbach.
So
far, there has been little to slow down Irma. The hurricane has
encountered almost no wind shear to weaken it. And, while Irma passed
over and devastated several Caribbean islands, including the Virgin Islands and St. Martin, those land masses were not large enough to make it lose momentum.
On Friday, the National Hurricane Center downgraded Irma slightly to a Category 4 storm
as it neared Cuba, with maximum sustained winds of 155 miles per hour.
But as the storm passes over unusually warm, deep waters in the Straits
of Florida, it’s expected to maintain its intensity or possibly even
strengthen before making landfall.
And
because Irma now encompasses such a large area — with hurricane-force
winds extending out 70 miles from the center — meteorologists say that
the storm poses extremely grave risks as it heads toward Florida.
For
comparison, Hurricane Andrew, which made landfall in South Florida as a
Category 5 storm in 1992 after quickly intensifying near the Bahamas,
was a relatively compact storm, with hurricane-force winds extending 25
miles from its center. Hurricane Andrew killed 65 people, destroyed
63,000 homes and brought $26.5 billion in economic losses.
The Cost of Hurricane Harvey: Only One Recent Storm Comes Close
Hurricane
Irma is expected to affect a much broader swath of Florida, and its
sheer size means it could potentially create a much larger storm surge
along the coast, pushing water inland and causing severe flooding.
Central and South Florida have grown at a rapid pace since Andrew, putting more people and property in harm’s way.
“It’s flawed to use Andrew as an analogue,” said J. Marshall Shepherd,
director of the atmospheric sciences program at the University of
Georgia. “You have to look at both the magnitude of the storm and the
size. Everything we’re seeing indicates that Irma will be extremely
dangerous.”
As
for climate change, scientists say they are still working to tease out
exactly what role warmer temperatures are playing in recent hurricane
activity.
According to Thomas Knutson,
a research meteorologist at the National Oceanic and Atmospheric
Administration, current climate models suggest that a rise in global
temperatures could potentially lead to fewer hurricanes in the Atlantic
basin, but those that do form would be more intense, thanks to warmer
water near the ocean surface, and bring heavier rainfall because of
increased moisture in the air.
But,
to date, it has been difficult to detect clear trends against a
background of natural variability. Ocean temperatures are certainly
rising because of global warming. But over the past century, the
Atlantic Ocean has also gone through periods of relatively little
hurricane activity, as in the 1960s and ’70s, as well as periods of high
activity, as occurred after 1995. Researchers suspect
those cycles are linked to fluctuations in wind shear, which can be
affected by events like El Niño, as well as by ocean events like the
Atlantic Multidecadal Oscillation, a cycle of fluctuations in sea
surface temperature that may be affected by ocean currents and possibly
air pollution.
“But even if we can’t detect it yet, the models indicate that there should be some effect there,” Dr. Knutson said.
Posts
No comments:
Post a Comment