European Space Agency
Press Infomation Note No. 26-94
Paris, France 30 November 1994
ON THE TRACK OF EL-NINO
Every year around Christmas, nature gives the fishermen of Peru a
break. When the westerly trade winds weaken in the southern summer,
the system of maritime streams running counterclockwise gets out of
step: the Humboldt stream, which carries cold, nutrition-rich water
northward along the coast of Chile, is pushed aside by warm northbound
water moving in from the equatorial region. For centuries, this
intermezzo has locally been called El-Nino, the Christ Child. It puts
fishermen out of work for a while, but most of the time it also brings the
long awaited rain. As autumn moves in and the southern hemisphere
begins to cool off, the trade winds regain their force, and with the cold
water the fish shoals return.
But there are times when this heavenly pause turns into hell, at least for
the fishermen. This occurs every few years in irregular intervals when El
Nino turns out to be particularly strong and the anchovies stay away
longer than usual. Because when that happens, heavy rains strike the
coastal deserts of Peru, covering the whole country with a dense layer of
green within a few weeks time. For the farmers, these are the anos de
abundancia, the year of abundance.
But such disruptions of the normal seasonal changes, which occur every
few years, not only strike the northwest coast of Latin America: on the
other side of the Pacific, in Southeast Asia and India, such seemingly
accidental mishaps have been known as long as anyone can remember.
One of the first to explore this phenomenon was Sir Gilbert Walker of
Great Britain, head of the meteorological observation service in India.
He wanted to find out why the rain-carrying monsoon did not materialize
in certain years, and he made a very interesting discovery in the process:
in normal times, atmospheric pressure over the south Pacific -- along the
eastern shore of New Zealand, which at that time was also part of the
British Empire -- was high, while the pressure above the Indian Ocean
was low, and this pressure differential boosted the monsoon. But when
the atmospheric pressure over the south Pacific weakened, the pressure
over the Indian Ocean increased, which kept the monsoon from
developing its full force, and the result was a dry year. To describe
this pressure transfer, Sir Gilbert coined the term "Southern
Oscillation", but he was unable to explain the phenomenon he had
observed.
In the eyes of today's climatologists, who, with the help of
sophisticated satellites, receive data simultaneously from all corners of
the world, this is hardly surprising: "Although Walker already took a
trans-regional approach to climate phenomena, his concept was not
sufficiently global," explained Professor Klaus Hasselmann of the
Max-Planck-Institut for Meteorology in Hamburg. "Because the same years
that bring devastating droughts to India give Peru a period of
abundance.
And by now, the Southern Oscillation, which is equally responsible for
dry years in India and the anos de abundancia in Peru, is also held
responsible for unusual dry spells in parts of the USA and Australia as
well as other irregularities of global weather," Hasselmann added.
Does this make the Pacific the "weather kitchen of the globe"? A
fascinating idea that challenges meteorologists and climatologists.
Because if it is confirmed, it might facilitate long-term weather
forecasts or at least trend analyses. To confirm this hypothesis,
scientists need as much precise information as possible to permit
mathematical modeling of the processes involved, thus giving them a
basis for comparing theory and reality. They not only need the usual
data about temperature, atmospheric pressure and wind direction and
speed, but also corresponding data on surface water layers which are in
constant exchange with the atmosphere.
In this respect, the European Space Agency's (ESA) remote sensing
satellite ERS-1 has, since its launch in July 1991, supplied valuable
information, which has helped to clause this annoying data gap. As
Professor David T. Llewellyn-Jones from the University of Leicester
explained, its instrumentation is particularly suited to the task of
measuring wind and water data simultaneously. "The sophisticated
radar system aboard this satellite permits continuous observation of the
ocean areas covered, registering the data required to determine wind
speed and direction while at the same time analyzing upwellings --
continually and globally in a 35-day pattern. A second instrument, the
ATSR (Along-Track Scanning Radiometer), scans the surface along the
satellite's path, thus providing detailed pictures of the temperature of
large distribution of water areas, with a precision of 0.5 degrees." "This
precision," says C.T. Mutlow of Rutherford Appleton Laboratory, "is
achieved by successively scanning the same ocean surface from two
different angles."
Another instrument that proved to be particularly valuable in observing El
Nino was the radar altimeter aboard the ERS-1 satellite. It permitted
comprehensive measuring of sea level changes, which previously could
be registered only by a number of coastal stations. The average sea
level indicates the cold and warm ocean currents almost like a map. With
the help of these data, scientists in 1991/92 were able to monitor an El-
Nino Southern Oscillation practically in real time. Since then, American
scientists even succeeded in proving traces of an El-Nino anomaly that
had occured ten years earlier, in 1982/83, and is considered to be the
strongest El-Nino of this century. Its early development was, almost
accidentally, observed by scientists from the Woods Hole Oceanographic
Institution and from Duke University in North Carolina, Aboard the
research ship "Conrad", which had been sent by the Lamont-Doherty
Geological Observatory on an expedition along the equator, they
registered, south of Hawaii, the starting point of their journey, a water
temperature of 27 degrees Celsius -- three degrees above the expected
value.
Normally, seasonal wind systems close to the equator push warmer and
thus lighter Pacific surface water westward to the coast of Southeast
Asia. The atmosphere there heats up, creating a rather durable low-
pressure area. At the same time, the air along the coast of Chile and
Peru is cooled off by the cold Humboldt stream and drops to the ground,
thus creating an equally durable high-pressure system. Professor
Hasselmann explained the modern view of an El-Nino anomaly as
follows: "This pressure differential between the western and the eastern
Pacific rim supports the trade winds, which propel the ocean currents
during most of the year. If the low-pressure system over the western
Pacific is reduced precisely at a time when the trade winds weaken
anyhow, the wind direction can, in the extreme case, be reversed and
move warm water towards the east."
In the early eighties, these patterns had not yet been fully explored so
that the scientists aboard the "Conrad" could not assess the implications
of their observations. As the warm surface water reached the west coast
of South America, the marine food chain was massively disrupted. Fish
shoals moved to other areas, and even the marine birds who normally
breed in large numbers on the coastal islands, were decimated or moved
on in search of other hunting grounds. The desert area along the South
America Pacific coast experienced extremely heavy rains, causing
numerous floods and mud-slides. And thousands of miles to the north,
the western United States also registered record precipitation. At the
same time, the region on the western Pacific rim (Australia, Indonesia,
the Philippines and India) were devastated by a lengthy drought, and
even in far-off Africa, the already difficult situation deteriorated
further. In its 1994 "Report to the Nation", the U.S. National Oceanic
and Atmospheric Administration (NOAA) evaluated the overall economic
damage attributable to El-Nino at more than 8 billion dollars worldwide.
In view of such global effects, forecasting El-Nino is of great ecological
and economic interest -- not only for South America. The first results
have been rather encouraging. Once an El-Nino event has been identified,
it is, for instance, now possible to forecast its development over
an 18-month period with the help of climate models. This can be used to
optimize crop planning in Peru: more rice in heavy El-Nino years with
strong rains, more cotton in years with little precipitation. Still
lacking is a system that will permit forecasting of El-Nino anomalies --
probably because not all aspects of a possibly self-regulating system of
interactions between the atmosphere and the ocean have yet been uncovered.
The new insights into the "aftermath" of the last major El-Nino anomaly,
which, to some extent, were gained with the help of ERS-1 data, can help
close this gap. It appears that there is a link between that event and
the rise of the surface temperature of the northern Pacific in 1992/93.
In the context of a long-term observation between 1986 and 1993, American
scientists at the Naval Research Laboratory in Mississippi and at the
University of Boulder, Colorado, first registered a slight rise of the
sea level along the U.S. west coast, which, beginning in the early
nineties, was accompanied by a warming-up of the northern Pacific beyond
the 35th parallel. This amounts to a northward drift of the Kuroshio
stream, which carries warm, salt-rich water from the western Pacific
south of the Japanese islands to the east, making it the equivalent of
the Gulf Stream in the Atlantic.
Still open is the question whether the extreme rainfalls in the
Mississippi area in the summer of 1993 were related to the migration of
the Kuroshio stream and the resulting high-temperature zone in the
northern Pacific. But the parallels to the effects of an El-Nino anomaly
are striking. Still unanswered is also the question whether -- and if
so, how -- the long-term aftermath of such anomalies can in turn spawn
the development of the next anomaly. Early next year, ESA will launch
its new satellite ERS-2. The data supplied by ERS-2 from 1995 to 1998
might bring the much-needed clarification, which is indispensible for a
reliable prognosis of such events.
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- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
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