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Frontiers
From Obscurity to Obsession
July/August 1998
Fifteen years ago, world climate went
haywire. The Indian monsoon failed. Fields in Australia burned in relentless
heat and drought. Crops withered in fertile parts of South America while
that continent's coastal desert bloomed unexpectedly. California got twice
its normal winter rainfall, and 18-foot waves swallowed beachfront houses.
A few reputable scientists blamed the bizarre global
pattern on the eruption of the volcano El Chichon in Mexico, calling it
the "Weather Maker of the Century." The real culprit--the greatest El
Niño up to that time--nearly got away unnoticed.
In 1997-98, by contrast, El Niño hogged headlines and became a household
word. Months before it affected the United States, El Niño forecasts
spurred not only talk but also action. Cubans harvested sugar crops early.
Peru borrowed $250 million from the World Bank in case of floods. Californians
scrambled to repair roofs. When rains finally did fall, people were ready.
El Niño leapt from obscurity to obsession because the 1982-83 experience
was a wake-up call. Scientists rallied by targeting the tropical Pacific
with intensive, coordinated studies in part funded by NSF, says Jay Fein,
director of the Atmospheric Sciences Division's Climate Dynamics Program
in the Directorate of Geosciences. A 1985-1994 initiative called TOGA (for
tropical oceans, global atmosphere) sent scientists toward the tropics to
observe how air and water turn relentless tropical sunshine into heat and
motion.
One crucial achievement of TOGA was a new network of data buoys in the
Pacific, now administered by the National Oceanic and Atmospheric Administration
(NOAA). The buoys and new NASA satellites helped make the 1997-98 El Niño
the best-monitored in history.
Just as importantly, Fein says, scientists documented unusual, overlapping
climatic rhythms in the region. Shifts in ocean characteristics have daily
and weekly timescales more typical in the atmosphere, while atmospheric
cycles exhibit longer periods favored by the ocean. As a result, Fein says,
the region boasts unique interactions.
Armed with these insights, Fein says, TOGA reached its goal--reliable El
Niño predictions. By understanding the tropics more precisely, modelers
could simplify the mathematics of Pacific interactions enough to run predictable
computer simulations. The resulting models tell scientists how El Niño
works and when it might happen.
A CENTURY'S WORK
The models solved an enigma revealed by turn-of-the-century British scientist
Gilbert Walker. Walker had noticed that global weather changed with a seesawing
of atmospheric pressure across the Pacific. In this cycle, called the Southern
Oscillation, pressure rises in Tahiti while falling in Darwin, Australia,
and vice versa. In the mid-1960s, Jacob Bjerknes, a legendary Norwegian
meteorologist who was at the University of California at Los Angeles at
the time, linked the Southern Oscillation to El Niño, a long-unheralded
Christmas warming of Peruvian coastal waters.
The equatorial Pacific surface is usually about 7 degrees Celsius warmer
in the west than in the east. Because warm oceans reduce air pressure overhead,
and air seeks low pressure, tropical winds steadily push westward across
the Pacific. The wind literally piles up warm water in the west. In response,
cold, deep ocean waters rise to the surface in the east. El Niño,
Bjerknes realized, is a periodic slackening of winds as the Southern Oscillation
lowers pressure in the east. Without strong westward winds, warm water sloshes
back toward South America, shutting off cold upwelling.
Feedback between the ocean and the atmosphere sustains El Niño.
The shift of warm water lowers air pressure over the central and eastern
Pacific, further slackening winds and releasing more warm water eastward.
Mark Cane and Stephen Zebiak of Lamont-Doherty Earth Observatory first
modeled this feedback on a computer in 1984. It was a leap forward: El Niño's
Christmas timing and its erratic two- to seven-year recurrence cycle were
now reproducible to some extent with physics and mathematics.
Computer models show that El Niño might be regulated by large-scale
internal ocean waves. The warm pool in the western Pacific sends a long,
slow wave eastward along the equator. But the El Niño-related shifting
of the warm pool also sends out the seeds of its own destruction at slightly
higher latitudes: very long westward waves that reflect off Asia and return
within about two years to cancel out the other waves moving eastward. The
equatorial feedback then tips toward the opposite of an El Niño,
in which the surface of the eastern Pacific becomes abnormally cool. This
opposite pattern is nicknamed La Niña. The cycle as a whole is often
referred to as El Niño, and the classic El Niño as a "warm
phase."
TROPICAL TROUBLE
Each warm phase is unique, Zebiak says, a painful truth for modelers. The
cycle seemed off kilter in the early 1990's: According to El Niño
scientist Lisa Goddard of Scripps Institution of Oceanography, central Pacific
warming at the surface didn't disappear as expected after the 1991-92 El
Niño. In 1994, the warm phase spread from the surface, not up from
the depths as in a classic El Niño.
As if this variability weren't enough, the cycle overall may have shifted
toward more frequent warm phases in the late 1970s. Some researchers think
that in 1997 El Niño may have returned to the benchmark behavior
of the early 1970s. El Niño warming began in deep water in the western
equatorial Pacific and eventually surfaced near Peru. But this did not yield
uniform modeling success.
"Only a couple of us got this event at long range," says climatologist
Tim Barnett of Scripps. Deep water changes showed a very strong El Niño
by June 1997. Eventually, the warming covered a larger area than the United
States. Back in late 1996, the best models predicted a weaker, later El
Niño. Others expected no El Niño at all.
The mixed success of models in 1997-98 showed an evolution, Zebiak says.
At first, models were designed to explain El Niño, and simplification
was a virtue. Cane and Zebiak had to throw out ocean changes that prevented
the model from honing in on essential patterns. "However, when you get seriously
into the prediction game," Zebiak says, "somehow all the details matter.
It becomes better to use a more complex model. But because these models
are more complex, there are more ways they can go wrong."
According to Barnett, models with sophisticated ocean circulation simulations
worked best in 1997-98. One of them, at NOAA's National Center for Environmental
Prediction, allows atmospheric circulation to interact with ocean circulation.
Another, at Scripps, couples ocean circulation with a statistical representation
of the atmosphere. But it wasn't until late spring, when observations already
showed record warming, that models predicted a large El Niño for
winter, though an official El Niño advisory had already been issued
in April. That's one reason why Michael Glantz, who monitors El Niño
impacts at the National Center for Atmospheric Research (NCAR), an NSF-supported
facility, says he's more impressed by new abilities to monitor El Niño
than by current claims of success in long-term predictions.
"One main problem," says Zebiak, "is that everybody is at a loss to explain
the speed with which this event grew." A possible reason for that speed,
Goddard says, is a thunderstorm cycle discovered by NCAR researchers in
1971. Packs of storms travel from the Indian Ocean into the Pacific every
40 to 50 days. The pulse of this stormy low-pressure air can distort ocean
layers, sending undersea waves toward Peru. In 1997, unbeknownst to the
models, strong pulses of stormy air may have added energy to El Niño.
HOOKING UP TELECONNECTIONS
Predictions may improve if modelers can gear up big global climate system
models to simulate El Niño. These models, such as one used in an
El Niño effort at NCAR led by Maurice Blackmon, now specialize in
predicting long-term climate processes because they couple everything involved--oceans,
atmosphere and land. By contrast, today's El Niño models focus on
processes with seasonal and shorter time scales to chart local sea surface
temperature. The next challenge, according to Fein, is to produce a global
climate model capable of predicting both El Niño events and associated
climatic disturbances worldwide. The promising effort at NCAR is one of
several underway in the United States, Europe and Japan.
Currently, Barnett and Lennart Bengtsson of the Max Planck Institute in
Germany use a two-step approach to predict El Niño's effects. They
model equatorial water temperatures, then use the results to run a global
climate model. But El Niño's remote effects are caused by complex
chains of events called teleconnections, and each link must be active for
the chain to work. This year, some local conditions were weak links: Warm
waters off southern Africa, for instance, may have prevented some El Niño-associated
drought.
The variable teleconnections meant that this El Niño, larger than
its 1982-83 cousin, in some places had relatively weaker impact. Having
spent 15 years catching up on their tropical knowledge, modelers now must
catch up again--this time with the rising expectations of a public watching
reliable predictions of El Niño impacts.
"If 1982-83 was the El Niño of the scientists, 1997-98 was the El
Niño of the forecast users," says Glantz. "Now they are saying, 'We
want X, Y and Z.' Now their focus is on teleconnections, on impacts."
EL NIÑO OPPORTUNITIES SPRING TO LIFE
While NSF's Atmospheric Sciences Division has
been pushing the frontiers of El Niño prediction, other
NSF-funded scientists took advantage of the advance warning of
this year's El Niño to study its effects on the world's
ecosystem.
- Jim Estes of the University of California
at Santa Cruz was able to study giant kelp along the coast
in advance of the anticipated warming. With the baseline data
in hand, Estes will continue to check for long-term effects
of El Niño on the kelp--a popular habitat for many
underwater species--for the next three years.
- In Panama, Howard Lasker of the State University
of New York at Buffalo monitored Caribbean coral reefs. Previous
El Niño warmings bleached the reefs, signaling a devastating
impact on an algae species that maintains the bright colors
in the coral.
- Peter Kimley of the University of California
at Davis watched closely the effect of El Niño on fish
in Mexican waters and monitored underwater populations with
electrical acoustical tracking devices. Stray tropical fish
were spotted as far north as Washington State as coastal water
temperatures rose.
- In California's East Mojave Desert, Phil
Rundel of the University of California at Los Angeles is studying
desert annuals. The unusual conditions brought by El Niño
have produced a bonanza of plants, both common and rare. Rundel
hopes this opportunity will lead to a better understanding
of the cues for flower germination in winter-rainfall deserts.
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