This year we were treated to an
excellent example, yet this begs the next question. Why do we see so much actual variation from
year to year? I do not think enough
actual variation exists in the Gulf Stream
itself to matter much.
Certainly continental weather can
be drawn into the Northeast. Yet the
effect practically disappeared fro a couple of years.
Add this to your kitbag of weather
theory and know it will never be enough.
by Staff Writers
This image, taken by NASA's Terra satellite in March 2003, shows a much
colder North America than Europe --even at
equal latitudes. White represents areas with more than 50 percent snow cover.
NASA's Aqua satellite also measured water temperatures. Water between 0 and -15
degrees Celsius is in pink, while water between -15 and -28 degrees Celsius is
in purple. Credit: NASA/Goddard Space Flight Center Scientific Visualization
Studio; George Riggs (NASA/SSAI).
If you're sitting on a bench in New York City 's
Central Park in
winter, you're probably freezing. After all, the average temperature in January
is 32 degrees Fahrenheit. But if you were just across the pond in Porto , Portugal ,
which shares New York
Throughout northern Europe, average winter temperatures are at least 10
degrees Fahrenheit warmer than similar latitudes on the northeastern coast of
the United States and the
eastern coast of Canada Pacific Northwest .
Researchers at the California Institute of Technology (Caltech) have
now found a mechanism that helps explain these chillier winters-and the culprit
is warm water off the eastern coasts of these continents.
"These warm ocean waters off the eastern coast actually make it
cold in winter-it's counterintuitive," says Tapio Schneider, the Frank J.
Gilloon Professor of Environmental Science and Engineering.
Schneider and Yohai Kaspi, a postdoctoral fellow at Caltech, describe
their work in a paper published in the March 31 issue of the journal Nature.
Using computer simulations of the atmosphere, the researchers found
that the warm water off an eastern coast will heat the air above it and lead to
the formation of atmospheric waves, drawing cold air from the northern polar
region. The cold air forms a plume just to the west of the warm water.
In the case of the Atlantic Ocean, this means the frigid air ends up
right over the northeastern United States
and eastern Canada
For decades, the conventional explanation for the cross-oceanic
temperature difference was that the Gulf Stream delivers warm water from the
Gulf of Mexico to northern Europe . But in
2002, research showed that ocean currents aren't capable of transporting that
much heat, instead contributing only up to 10 percent of the warming.
Kaspi's and Schneider's work reveals a mechanism that helps create a
temperature contrast not by warming Europe, but by cooling the eastern United States Gulf Stream that causes
this cooling.
In the northern hemisphere, the subtropical ocean currents circulate in
a clockwise direction, bringing an influx of warm water from low latitudes into
the western part of the ocean. These warm waters heat the air above it.
"It's not that the warm Gulf Stream waters substantially heat up Europe ," Kaspi says. "But the existence of the
Gulf Stream near the U.S.
coast is causing the cooling of the northeastern United States
The researchers' computer model simulates a simplified, ocean-covered
Earth with a warm region to
mimic the coastal reservoir of warm water in the Gulf
Stream . The simulations show that such a warm spot produces
so-called Rossby waves.
Generally speaking, Rossby waves are large atmospheric waves-with
wavelengths that stretch for more than 1,000 miles. They form when the path of
moving air is deflected due to Earth's rotation, a phenomenon known as the
Coriolis effect. In a way similar to how gravity is the force that produces
water waves on the surface of a pond, the Coriolis force is responsible for
Rossby waves.
In the simulations, the warm water produces stationary Rossby waves, in
which the peaks and valleys of the waves don't move, but the waves still
transfer energy. In the northern hemisphere, the stationary Rossby waves cause
air to circulate in a clockwise direction just to the west of the warm region.
To the east of the warm region, the air swirls in the counterclockwise
direction. These motions draw in cold air from the north, balancing the heating
over the warm ocean waters.
To gain insight into the mechanisms that control the atmospheric
dynamics, the researchers speed up Earth's rotation in the simulations. In
those cases, the plume of cold air gets bigger-which is consistent with it
being a stationary Rossby-wave plume. Most other atmospheric features would get
smaller if the planet were to spin faster.
Although it's long been known that a heat source could produce Rossby
waves, which can then form plumes, this is the first time anyone has shown how
the mechanism causes cooling that extends west of the heat source. According to
the researchers, the cooling effect could account for 30 to 50 percent of the
temperature difference across oceans.
This process also explains why the cold region is just as big for
both North America and Asia , despite the
continents being so different in topography and size.
The Rossby-wave induced cooling depends on heating air over warm ocean
water. Since the warm currents along western ocean boundaries in both the
Pacific and Atlantic are similar, the
resulting cold region to their west would be similar as well.
The next step, Schneider says, is to build simulations that more
realistically reflect what happens on Earth. Future simulations would
incorporate more complex features like continents and cloud feedbacks.
The research described in the Nature paper, "Winter cold of
eastern continental
boundaries induced by warm ocean waters," was funded by the NOAA Climate
and Global Change Postdoctoral Fellowship, administrated by the University
Corporation for Atmospheric Research; a David and Lucille Packard Fellowship;
and the National Science Foundation.
No comments:
Post a Comment