El
Niño is a charming name for the large-scale
warming of surface water of the Pacific Ocean
every 3-6 years that lasts a few months and
dramatically affects weather and climate world
wide. It is accompanied by the "Southern
Oscillation" in the atmospheric surface pressure
over the equatorial Pacific, and together they are
referred to by the somewhat less appealing acronym
ENSO... (More
basics: wikipedia).
Deviation of the sea
surface temperature from a long-term mean during
the El Niño event of 1997/8. El
Niño is expressed as the large scale
warming of 3-4 degrees Celsius seen along the
equatorial in the East Pacific
Ocean.
Colored areas in the
above map represent regions in which weather is
affected by the occurrence of El Niño in
the East Pacific.
A time series of the sea
surface temperature averaged over the eastern
tropical Pacific (called nino3 and used as an
index of El Niño activity) since 1950. The values
plotted are monthly mean temperature anomalies
from the mean seasonal cycle.
(From Ian
Eisenman.)
Irregularity and
chaos: El Niño events happen irregularly,
which makes their prediction difficult. We proposed
that the El Niño cycle may be irregular due
to low-order chaotic dynamics driven by the seasonal
cycle. In a study based on both a simple heuristic
model and a more realistic El Niño prediction
model, we suggested that the equatorial Pacific
ocean-atmosphere oscillator can go into nonlinear
resonance with the seasonal cycle. Furthermore, for
a strong enough coupling between the ocean and the
atmosphere, the system may become chaotic as a
result of irregular jumping of the coupled
ocean-atmosphere system between different nonlinear
resonances.
[see 1,
2].
The alternative to this mechanism, suggested by
other groups, is that ENSO is irregular because it
is forced by stochastic atmospheric forcing, see
references in technical papers linked above.
Two-year segments of the
observed NINO3 index (SST averaged over the
eastern equatorial Pacific) during several El
Nino events, showing that El Niño tends to peak
at the end of the calendar year.
Locking to
seasonal cycle: In spite of its irregularity, El
Niño's tends to peak near the end of the
calendar year, as seen in the above figure. We
proposed that this may be due to the seasonal
amplification of equatorial Rossby and Kelvin ocean
waves
[1,
2]
Westerly Wind Bursts:
Each major ENSO event seems to be preceded by a few
strong "westerly wind bursts" which occur in the
west equatorial pacific, last a few days, extend
over thousands of km, and seem to be responsible for
the event onset. These WWBs seem quite random, but
some further analysis we and others have done shows
that they may be related to the sea surface
temperature and therefore that their characteristics
and time of occurrence may be modulated by the large
scale ENSO signal itself
[1].
We demonstrated that this modulation strongly
affects ENSO's amplitude
[2,
3, 4]
and possibly also extends its predictability time.
good news... ;-)
 |
The effects
of the modulation of westerly wind bursts by ENSO,
demonstrating that this modulation results in a
significantly larger amplitude than purely
stochastic WWBs.
|
Controlling El
Niño...
We have used
"chaos control" methods to control the
chaotic behavior of El Niño in a model that is
used successfully for predicting El Niño events
in the Equatorial Pacific. The control is obtained by
applying small perturbations at a single point in
space, and results in El Niño events in the
model being periodic rather than chaotic. The ability
to control El Niñno in this model proves that
El Niñno's regularity in this model is indeed
due to chaos rather than other possible factors. In
addition, the investigation of the resulting periodic
behavior should hopefully improve our understanding of
El Niño's dynamics and perhaps our ability to
better predict these
events. [1]
