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May
31, 2007 "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 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 "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 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 "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 "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." The National Science
Foundation
and the Purdue Research Foundation funded this research. The
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