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Topic Name: Cyclones and hurricanes play an important role in the ocean circulation patterns
Category: Environmental Fluid Mechanics
Research persons: Matthew Huber , Steven R. Jayne, Ryan L. Sriver
Location: Earth and Atmospheric Sciences Purdue University,550 Stadium Mall Drive, United States
Details
University researchers have found evidence that tropical cyclones and hurricanes
play an important role in the ocean circulation patterns that transport heat and
maintain the climate of North America and Europe.These findings support a 2001 theory by Kerry Emanuel, a professor of
atmospheric science at Massachusetts Institute of Technology, and suggest that
there is an additional factor to be included in climate models that may change
predictions of future climate scenarios .
"It was thought that hurricanes occurred over too short of a time period
and over too small of an area to affect the global system," said Matthew
Huber, the Purdue University professor of earth and atmospheric sciences who led
the research group. "This research provides evidence that hurricanes play
an important role and may be one of the missing pieces in the climate modeling
puzzle."
The research also showed that hurricanes cool the tropics, forming in
response to higher temperatures and acting as a thermostat for the area, Huber
said.
"Warm water fuels hurricanes, which have been shown to leave cold water
in their wake," said Huber, who also is a member of the Purdue Climate
Change Research Center at Discovery Park.
"I like to say the good news is that hurricanes function like a
thermostat for the tropics, and the bad news is that hurricanes function like a
thermostat for the tropics. The logical conclusion of this finding, taking into
account past research into the impact of rising temperatures on cyclone and
hurricane intensity, is that as the world and the tropics warm, there will be an
increase in the integrated intensity of hurricanes."
Movies such as "The Day After Tomorrow" brought into the spotlight
information about the ocean conveyer belt and its impact on climate. The upper
part of the conveyer belt travels from the south to the north, passing through
the Pacific Ocean and Indian oceans and past warmer latitudes warming the water
brought to North America and Europe, Huber said.
In the tropical oceans, this pattern must be reversed; warm, buoyant water
must be mixed downward, and cold, dense water must be mixed upward. This
process, called vertical mixing, plays an important role in the conveyer belt's
circulation. It was known that this mixing occurred, but the cause was not
well-understood, said Ryan Sriver, the paper's lead author and a Purdue graduate
student.
"Climate models today use what is called 'background mixing' to solve
this problem," he said. "They represent the mixing as an average of
the total amount that is needed and apply it over these regions consistently.
However, we believe this mixing is not consistent; it is not everywhere all of
the time. It is sporadic and happens over a small area for a limited amount of
time."
In some areas of the world, such as the equator, there are no cyclones, and
no mixing occurs.
"If cyclones were added to models in place of the background mixing,
there would be zero mixing at the equator," Huber said. "This is very
important because it is well-known that to get El Niño right in a climate
model, the background mixing at the equator must be greatly reduced. Our data
has a beautiful no-mixing zone right where there should be no mixing."
This explains some of the mystery of the observed temperatures from the
distant past during a greenhouse climate. The poles were much warmer than today,
about 82 degrees Fahrenheit, but the tropics were not much warmer than the
present, he said.
"Using the best, most comprehensive models in existence, we could not
obtain results that matched this past climate that we know existed," Huber
said. "We knew a basic, fundamental process that cooled the tropics was
missing from the models."
The results of the study, being published in the May 31 issue of Nature, are
consistent with providing all of the mixing necessary to match what is needed in
climate models.
"Our results suggest that this is the missing mixing and it is a vital
part of ocean circulation," Huber said.
Steven Jayne, an assistant scientist at Woods Hole Oceanographic Institution
in Massachusetts, said Huber and Sriver present strong evidence for a
cyclone-driven heat pump.
"It is remarkable how closely the amount of mixing generated by the
cyclones and the location of this mixing matches what appears to be needed to
improve climate models," Jayne said. "People suspected these
connections, but no one had done the necessary detailed calculations. It means
there may be another feedback loop in the climate system, and that is
significant."
Huber and Sriver studied the cooling effects of hurricanes from 1981 to the
present using the cold wakes that follow a hurricane.
"These cold wakes can be easily observed," Sriver said. "The
typical size is about 200 kilometers across and about 1,000 kilometers long, or
about as big as the Eastern Seaboard."
The researchers used surface temperature data during the cold wakes to obtain
an estimate of the cooling in the tropics due to cyclones and hurricanes. The
data analyzed was provided by the National Center for Atmospheric Research, the
National Oceanic and Atmospheric Administration, and the National Aeronautics
and Space Administration. The team then examined the process that leads to this
cooling and evaluated the ocean water mixing.
"Multiple studies have shown that tropical cyclones are an
excellent source of inertial oscillations, or internal waves that cause mixing
in the upper layer of the ocean," Huber said. "It is like putting
sugar in a cup of coffee. The sweetened coffee is more dense and will form a
layer at the bottom of the cup. It needs to be stirred or agitated somehow to
bring the sweet layer up to mix with the rest. The same thing is needed to mix
ocean water. Dense water hangs out at bottom unless something stirs it up.
Cyclones stir it up in addition to other processes."
Tropical cyclones cause waves below the surface of the ocean that break, just
like what can be seen on the beach shore. When the waves break, the top layer of
water curls into the bottom layer and water of different densities and
temperatures mix, Huber said.
"Warm, fresh water is less dense than cold, salty water, so the cold
water sinks, and this drives the conveyer belt," Huber said. "However,
cold, salty water rises in the Pacific, and there has been no complete
explanation for this. Cyclones and hurricanes appear to pump warm water down and
bring cold water to the surface. Mixing down buoyant, warm water lessens the
density of the cold water and allows it to rise "
The study did not examine deep ocean mixing, but it is reasonable to
speculate that warm water pumped down joins the ocean circulation and becomes a
part of the upper limb of the conveyer belt where dense water makes it up to the
surface, Huber said.
Huber and Sriver plan to incorporate their findings into a climate model for
further testing.
"Current predictions are based on tropical ocean mixing remaining
constant or decreasing with warmer temps," Huber said. "This evidence
suggests the opposite is true, and upper ocean tropical mixing increases with
warmer temperatures. This has major implications for oceanography and climate as
a new factor that had not been included in previous predictions."
About Researchers:
Matthew Huber
Assistant Professor Earth and Atmospheric Sciences Purdue University
huberm@purdue.edu
Steven R. Jayne
Associate Scientist
Physical Oceanography
Office Phone: +1 508 289 3520
sjayne@whoi.edu
Personal Site
WHOI Mailing Address:
Woods Hole Oceanographic Institution
Clark 313A, MS#21
Woods Hole, Ma. 02543
Ryan L. Sriver
(rsriver@purdue.edu)
http://roskilde.eas.purdue.edu/~rsriver
Department of Earth and Atmospheric Sciences
Purdue University
550 Stadium Mall Drive
West Lafayette, IN 47907 - 2051
Office Phone: (765) 494-0652
Funded
The National Science Foundation
and the Purdue Research Foundation
funded
this research. The Purdue Cyber Center and
the Office of Information Technology
at Purdue provided computational resources and support.
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