Gulf Stream Leaves Its Signature Seven Miles High

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“The band of diverging winds … follows the meandering Gulf Stream front.”
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Science Daily
http://www.sciencedaily.com/releases/2008/03/080320181838.htm

Gulf Stream Leaves Its Signature Seven Miles High

Cumulonimbus storm cloud forming over the warm
Gulf Stream along the Norwegian coast. The system
is sometimes called a polar low. (Credit:
iStockphoto/Erik Kolstad)

ScienceDaily (Mar. 24, 2008) – The Gulf Stream’s
impact on climate is well known, keeping Iceland
and Scotland comfortable in winter compared to
the deep-freeze of Labrador at the same latitude.
That cyclones tend to spawn over the Gulf Stream
has also been known for some time. A new study
reveals that the Gulf Stream anchors a
precipitation band with upward motions and cloud
formations that can reach 7 miles high and
penetrate the upper troposphere. The discovery,
announced by a Japan-US team of scientists, shows
that the Gulf Stream has a pathway by which to
directly affect weather and climate patterns over
the whole Northern Hemisphere, and perhaps even
world wide.

“Our findings gain even more significance by the
fact that the Gulf Stream is the upper limb of
the Atlantic portion of the ocean conveyor belt
that drives the global ocean circulation,” says
co-author Shang-Ping Xie, a research team leader
at the International Pacific Research Center in
the School of Ocean and Earth Science and
Technology, and professor of meteorology at the
University of Hawai’i at Manoa. “The conveyor
belt is predicted to slow down with global
warming, which implies that changes in the Gulf
Stream will modulate spatial patterns of future
climate change.”

Xie has been curious for some time about the
response of the atmosphere to warm currents
flowing within cold ocean water, such as the Gulf
Stream or its Pacific counterpart, the Kuroshio.
Xie says, “It has been a challenging task to
isolate the climatic influence of the Gulf Stream
from energetic weather variations by using
conventional observations, which are spatially
and temporally sporadic. Our findings were only
possible because of the availability of
high-resolution satellite data, an operational
weather analysis, and an atmospheric circulation
model.”

The first hint that these warm ocean currents
have a significant effect on the atmosphere came
from high-resolution NASA satellite data. These
images show a narrow rain band hovering
frequently over the warm flank of the currents;
wind accelerates and converges over the warm
flank and diverges and decelerates on the cold
flank.

The satellite images, however, do not allow
accurate measurements of upward motions and
divergence of air in the upper troposphere, which
are necessary to understand the link between the
current and large-scale climate. This is where
the European Center for Medium-Range Weather
Forecasts (ECMWF) analysis provided the missing
data. “It is remarkable to see how the diverging
winds 7 miles high show a structure similar to
the converging winds and the rain clouds, all
meandering with the Gulf Stream,” says lead
author Shoshiro Minobe, a professor at the
Division of Earth and Planetary Sciences at
Hokkaido University.

The upward wind velocity is strongest about the
first mile above the surface, but the Gulf
Stream-following structure is clearly visible at
4 miles and still discernible at 7 miles and
above. The band of diverging winds in the upper
troposphere follows the meandering Gulf Stream
front.

The findings from the operational weather
analysis pointed to the warm flank of the Gulf
Stream as the cause of the strong upward winds.
“We wanted more evidence, though,” says team
member Akira Kuwano-Yoshida of the Japan Agency
for Marine-Earth Science and Technology
(JAMSTEC), “and turned to the high-resolution
Atmospheric Model for the Earth Simulator (AGCM)
at JAMSTEC. We drove the model first with the
actual Gulf Stream temperatures. The model
successfully captured the rain band and the
signature in the upper troposphere. Then we
removed the sharp sea surface gradient from the
Gulf Stream front by smoothing the temperature in
the model. The narrow rain band disappeared.”

Finally, the team used outgoing longwave
radiation satellite data to measure the cloud top
temperatures. The narrow cloud band, associated
with lightning, extends 7 miles high above the
Gulf Stream meanders and has temperatures below
freezing. All this is further evidence that the
Gulf Stream influence on the atmosphere extends
far above the lower atmosphere.

The Gulf Stream’s strength has changed markedly
in the past as Earth has switched between warm
periods and ice ages. Closely linked to these
changes have been climate changes around the
globe-not only in the Atlantic, but also in the
Pacific and even in the Southern Hemisphere.
Scientists have been puzzled at how the changes
in the Atlantic thermohaline circulation (the
conveyor belt) lead to climate anomalies in other
regions in the Northern Hemisphere. The new study
discovers a direct pathway, the Gulf Stream’s
deep heating of the atmosphere. This heating
generates planetary waves that can induce quite
rapid changes in Earth’s atmospheric circulation
and alter climate over Europe and beyond by
riding on the westerly jet stream in the upper
troposphere.

Journal reference: Minobe, S., A. Kuwano-Yoshida,
N. Komori, S.-P. Xie, and R.J. Small, 2008:
Influence of the Gulf Stream on the troposphere.
Nature. March 13, 2008.

Adapted from materials provided by University of Hawaii at Manoa.

University of Hawaii at Manoa (2008, March 24).
Gulf Stream Leaves Its Signature Seven Miles
High. ScienceDaily. Retrieved March 24, 2008,
from http://www.sciencedaily.com°©
/releases/2008/03/080320181838.htm

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