Gulf Stream Seems Not To Be Slowing Down
Gulf Stream 'is not slowing down'
BBCNews.com,
March 29, 2010
The Gulf Stream
does not appear to be slowing down, say US scientists who have used
satellites to monitor tell-tale changes in the height of the sea.
Confirming work
by other scientists using different methodologies, they found dramatic
short-term variability but no longer-term trend.
A slow-down -
dramatised in the movie The Day After Tomorrow - is projected by some
models of climate change.
The research is
published in the journal Geophysical Research Letters.
The stream is a
key process in the climate of western Europe, bringing heat northwards
from the tropics and keeping countries such as the UK 4-6C warmer than
they would otherwise be.
It forms part of a
larger movement of water, the Atlantic Meridional Overturning
Circulation, which is itself one component of the global thermohaline
system of currents.
Between 2002 and
2009, the team says, there was no trend discernible - just a lot of
variability on short timescales.
The satellite
record going back to 1993 did suggest a small increase in flow, although
the researchers cannot be sure it is significant.
"The changes
we're seeing in overturning strength are probably part of a natural
cycle," said Josh Willis from Nasa's Jet Propulsion Laboratory (JPL) in
California.
"The slight
increase in overturning since 1993 coincides with a decades-long natural
pattern of Atlantic heating and cooling."
Short measures
The first
observations suggesting the circulation was slowing down emerged in
2005, in research from the UK's National Oceanography Centre (NOC).
Using an array of
detectors across the Atlantic and comparing its readings against
historical records, scientists suggested the volume of cold water
returning southwards could have fallen by as much as 30% in half a
century - a significant decline.
The warm surface
water sinks in the Arctic and flows back southwards at the bottom of the
ocean, driving the circulation.
However, later
observations by the same team showed that the strength of the flow
varied hugely on short timescales - from one season to the next, or even
shorter.
But they have not
found any clear trend since 2004.
Rapid relief
The NOC team now
has a chain of instruments in place across the Atlantic, making
measurements continuously.
"In
four-and-a-half years of measurement, we have found there is a lot of
variability, and we're working to explain it," said NOC's Harry Bryden.
The quantities of
water involved are huge, varying between four million and 35 million
tonnes of water per second.
The array is part
of the UK-funded Rapid project, which aims to refine understanding of
potentially large climate change impacts that could happen in short
periods.
Professor
Bryden's team calculates that their system is good enough to detect a
long-term change in flow of about 20% - but it has not happened yet.
He believes the
JPL approach - using satellite altimeters, instruments that can measure
sea height precisely, and the Argo array of autonomous floating probes -
could potentially add useful data to that coming from long-term on-site
monitoring arrays.
But, he points
out: "The method concentrates only on the upper [northward] flow - it
doesn't give you much information on the returning flow southward."
Fantasy and reality
Driven by
Hollywood, a popular image of a Gulf Stream slowdown shows a sudden
catastrophic event driving snowstorms across the temperate lands of
western Europe and eastern North America.
That has always
been fantasy - as, said Josh Willis, is the idea that a slow-down would
trigger another ice age.
"But the Atlantic
overturning circulation is still an important player in today's
climate," he added.
"Some have
suggested cyclic changes in the overturning may be warming and cooling
the whole North Atlantic over the course of several decades and
affecting rainfall patterns across the US and Africa, and even the
number of hurricanes in the Atlantic."
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