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Feature March 10, 2005

A Tale of Two El Ninos

"It was the best of times, it was the worst of times.." The
same event can have dramatically different outcomes. For
some, an El Nino means a welcome respite from bitter winter
weather. For others, it can bring lashing rains and
devastating floods.

All El Ninos don't run the same course either. Though the El
Nino of 2002-03 affected climate around the world, it was
much milder than the unusually large El Nino of 1997-98.
What made one so much stronger than the other?

University of Maryland researcher Eric Hackert and three
colleagues at the university's Earth System Science
Interdisciplinary Center took a close look at these two El
Ninos to find out. Using satellite data of ocean temperature
and sea-surface height along with computer models, they
analyzed the two events to determine how they were alike
and, more importantly, how they were different. One of the
things they discovered was that a particular kind of wave
helped make one El Nino much stronger and longer lasting
than the other.

Hackert and his colleagues first studied how the two events
began to see if something in their initial states might have
foreshadowed the difference in their ultimate intensity. "In
many respects they started the same," says Hackert. "With
these initial conditions, the prediction for both of these
scenarios would have been for mild events."

The striking differences between the two El Ninos became
evident after the winds, what scientists call forcing, came
into play. Winds drive ocean currents and excite waves. In
an El Nino, a breakdown in the easterly (east to west) trade
wind system spawns two different kinds of waves: Kelvin
waves and Rossby waves. Kelvin waves travel from west to
east along the equator. Rossby waves move in the opposite
direction from east to west on either side of the equator.
These waves create a change in ocean circulation.

When Hackert and his colleagues dissected ocean height
satellite data to isolate the individual effects of the
Kelvin and Rossby waves, they began to see differences in
how the two El Ninos developed.

They found that the Kelvin wave component for the two El
Ninos was similar at first but then strengthened for the
1997 El Nino and weakened for the 2002 El Nino. Even more
striking was the influence of the Rossby waves. They had
little effect on the development of the 2002 El Nino but
made a large contribution to the strength and duration of
the 1997 El Nino.

"Rossby waves alone contributed up to half of the sea
surface temperature signal in the central Pacific during the
key period of the build-up of the 1997 event and served to
sustain the warm temperatures during the spring of 1998 in
the far eastern Pacific," says Hackert. "One-third of the
total sea level signal-the seesaw effect with sea level down
in west and up in the east--is accounted for by Rossby
waves."

"Our study is the first time that anyone has actually broken
the components into Kelvin and Rossby waves and done data
assimilation to try to separate the role of the two," says
Hackert. "It is an example of how satellite data in
combination with numerical ocean models can be used to
investigate theories on how an El Nino develops." For their
study, Hackert and his colleagues used sea-surface
temperature data from National Oceanic and Atmospheric
Administration satellites and sea-surface height
measurements from the Topex/Poseiden and Jason satellites,
joint missions of NASA and the French space agency.

"This was just one step toward improving our understanding
of how an El Nino works and what makes the atmosphere and
the ocean couple to create a strong event," Hackert says.

Media contact:

Alan Buis (818) 354-0474
Jet Propulsion Laboratory, Pasadena, Calif.

"baalke" wrote in message
oups.com

All El Ninos don't run the same course too, neither. Though the El
Nino of 2002-03 affected climate around the world, it was
much milder than the unusually large El Nino of 1997-98.
What made one so much stronger than the other?

Might one hazard a guess that the sea ice in the Arctic was greater for
the latter? (I.E. the former.)

"In many respects they started the same,"

Well, they would wouldn't they!

The striking differences between the two El Ninos became
evident after wind.... forcing, came into play.

The Kelvin wave component for the two El Ninos was similar at first but
then strengthened for the 1997 El Nino and weakened for the 2002 El Nino.

But if there is a relationship with sea ice in the Antarctic the Kelvin
Wave component drops off as the sea ice increases?

The Kelvin wave is an inverse constant?

Even more striking was the influence of the Rossby waves. They had little
effect on the development of the 2002 El Nino but made a large contribution to the strength and duration of
the 1997 El Nino.

I'm a little flumoxed he

If the sea ice is extensive is the Rossby wave more evident?

If so how is it instrumental -or is it just a symptom?

The alledged Kelvin wave has a heat input potential being at or near sea
level. (If you believe that you will believe anything:

http://www.oc.nps.navy.mil/webmodules/ENSO/kelvin.html)

"Rossby waves alone contributed up to half of the sea
surface temperature

"One-third of the total sea level signal -the seesaw effect with sea level
down in west and up in the east -is accounted for by Rossby waves."

If you believe that you will believe everything:

http://www.soc.soton.ac.uk/JRD/SAT/R...ssbyintro.html

"...at mid-latitudes (say, 30 degrees N or S) one such wave may take
several months - or even years - to cross the Pacific Ocean."

"Our study is the first time that anyone has actually broken the components
into Kelvin and Rossby waves and done data assimilation to try to separate
the role of the two," says Hackert.

One wonders why.

"It is an example of how satellite data in combination with numerical ocean
models can be used to investigate theories on how an El Nino develops."

Is it... er.. I mean it.. er... is?

"This was just one step toward improving our understanding of how an El Nino
works and what makes the atmosphere and the ocean couple to create a strong event,"

The only question one can come up with is?:
How many steps will it take in the right direction to ..

..... well.. you know..

................................................li ke the obvious ...



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