Trade winds' slowdown backs warming theory
- *Trade winds' slowdown backs warming theory*
- Carl T. Hall, Chronicle Science Writer
Thursday, May 4, 2006
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Climate scientists have documented a pronounced slowdown in the Pacific
Ocean atmospheric system that drives the trade winds, a prediction of
global warming theory that appears to be coming true.
A study released today in the journal Nature suggests that the movement
of moisture and heat across the tropical Pacific has tapered off by 3.5
percent since the mid-1800s, when such records begin, and appears likely
to ease by another 10 percent this century.
That could have wide repercussions for weather and sea life throughout
the Pacific region, although it's hard for anyone to be certain at this
early stage what effect the slowing of the winds would have.
Possibilities include more El Niño-like conditions, stronger hurricanes
and less upwelling of nutrient-rich cold water from the deep Pacific.
Weather generally may become more variable -- and harder to predict.
The new research was led by Gabriel Vecchi, a scientist affiliated with
the 69-institution consortium known as the University Corporation for
Atmospheric Research but working as a visiting scientist at the National
Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics
Laboratory in Princeton, N.J. Co-authors included scientists at the
University of Miami and at the same NOAA facility.
They focused on the giant system known as the "Walker circulation,"
named in honor of Sir Gilbert Walker, the late British scientist who was
one of the first to trace connections among widely scattered weather
events. The system is a kind of heat engine that drives half the world's
climate. The Walker circulation works like a seesaw in which warm, moist
air rises in the western Pacific, becomes drier at high elevation and
displaces eastward, where heavy air sinks and returns westward. The
phenomenon thus generates west-to-east air currents high up, and
east-to-west trade winds near the ocean surface -- a great climatic
wheel centered on the equator.
Global warming theorists have long expected to see a slowdown in this
system, but until now had been unable to see convincing evidence.
The latest study relied mostly on sea-level barometric pressure, drawing
on data collected by sailors plying the Pacific for more than a century.
Results pointed to a clear slowdown in the intensity of the Pacific heat
pump, particularly during the past 30 years, essentially matching the
results expected from computer climate models.
The scientists said the only explanation that fit all the data was the
temperature increase attributed to the release of heat-trapping
greenhouse gases, chiefly carbon dioxide, into the atmosphere.
Rising temperatures are said to cause more water vapor to enter the
lower atmosphere from the ocean -- a 7 percent increase for every 1.8
Fahrenheit degree (1 degree Celsius) temperature increase. Only some of
this added moisture falls back as rain. The extra moisture serves as a
drag in the bottom of the system, leading to slower wind speeds.
Because ocean winds drive currents, Vecchi said one clear impact may be
a reduced Pacific upwelling, potentially reducing the biological
productivity of ocean regions affected.
As for the impact on land, it's anybody's guess. One possibility may be
generally wilder weather -- bigger storms, drier droughts and stronger
hurricanes feeding off the warmer, wetter tropical Pacific.
"You are directly changing a basic engine of weather and climate," said
Daniel Kammen, co-director of the Institute of the Environment at UC
Berkeley. "The range of potential effects is broad, and none of our
climate models tells us what's unpredictable. If you mess the system up
too much, you will get surprises."
A relaxing of the Walker circulation appears to fit into a larger
pattern of rapid climate change that the models actually do predict,
further reason to suspect those models may be working as intended.
The circular flow is what drives ocean currents and sets the background
against which El Niños are measured.
El Niño, named after the Christ child by Southern Hemisphere fishing
communities because it tends to show up around Christmas every three to
seven years, is marked by the periodic warming of ocean waters in the
eastern Pacific and a slackening of trade winds. The flip side also
happens, a phenomenon known as La Niña.
In effect, the gradual changes now being documented seem to be tipping
the system slightly more toward the El Niño state.
"We don't know how this is going to play out," Kelly Redmond, deputy
director of the Western Regional Climate Center in Reno, said Wednesday.
"What this does is very slightly make the system overall look a little
more El Niño-like, chronically."
El Niños generally are associated with wetter conditions in the
Southwestern United States and drier conditions to the north. Northern
California sits roughly in the middle -- making the impact here
particularly hard to gauge.
Jan Null, consulting meteorologist at Golden Gate Weather Services, said
any change in the Pacific's air-flow pattern may be compensated by a
change in some other part of the system yet to be pinpointed.
"The Earth seems to have a way of balancing things," he said. "We're
just now finding this one piece of the puzzle, and there are other
pieces we are still looking for. Then we can try to put the puzzle
Scientists have detected a slowdown in the movement of heat and moisture
across the Pacific Ocean, one of the predicted effects of global climate
1 Evaporation from warm ocean moistens lower atmosphere
2 Trade winds carry moisture west
3 Moist air rises and feeds rain
4 Dry air cools and sinks
1 Atmospheric moisture increases strongly
2 Rainfall increases more slowly than moisture; to compensate, winds slow
Source: Gabriel Vecchi, UCAR
John Blanchard / The Chronicle
E-mail Carl Hall at chall@....
©2006 San Francisco Chronicle
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