The El Niño Southern Oscillation (ENSO)
Corey J Gabriel
"El Niño has taught two lessons that will endure. The first is that largescale variability such as El Niño is not a disaster, anomaly, or cruel twist of
fate; it is how Earth works. To mature and live harmoniously in the Earth
system, human culture must adapt to Earth's rhythms and use natural
variability to its advantage.“
Richard Barber
Duke University
The Walker Circulation
Typical conditions: westerly trade winds
across the Pacific, resulting in substantial
upwelling, and intense convective towers in
the western Pacific. The circulation closes
with easterly winds aloft, and sinking air in
the eastern Pacific.
ENSO Events in the Equatorial Pacific
Figure 3: The peak of a strong El Niño (warm) event
from December 1997. Warm water, typically confined to
the western Pacific, has spread east, towards South
America (top panel). In response, the Walker Circulation
has weakened, resulting in the cessation of westerly trade
winds. Normally weak winds in the western Pacific are
now strong easterly winds, which push warm water
towards the East (bottom panel). Source:
http://www.pmel.noaa.gov/tao/elnino
Figure 4: The peak of a strong La Niña (cold) event from
December 1998. Anomalously strong westerly trade
winds advect warm water farther into the western Pacific.
In response, robust upwelling occurs in the equatorial
eastern and central Pacific. During the height of a strong
La Niña event, SSTs along the equator may decrease by
up to 4 °C. La Niña events typically have a duration of
between 9 and 12 months. Source:
http://www.pmel.noaa.gov/tao/elnino
ENSO: Kelvin and Rossby Waves
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Walker circulation weakens.
Tradewinds cease to pile up water in the
west
Mass surplus created near the equator
Downwelling Kelvin Wave forced to
propagate eastward with equator as
wave guide (speed ~3m/s, 70 days to
cross Pacific). Deepens thermocline,
warms eastern Pacific
Reflects westward as an upwelling
Rossby Wave after reaching the eastern
boundary. (speed~0.9m/s, 200 days to
cross Pacific). This reverses the
warming.
This is a huge simplification: ENSO
actually involves a long series of windstress anomalies
Monitoring the El Niño-Southern Oscillation (ENSO):
Figure 1: NINO regions in the equatorial Pacific.
Source:
http://iri.columbia.edu/climate/ENSO/backgroun
d/monitoring.html
Figure 2: Locations of moored buoys in the TAO
Array. Source
http://iri.columbia.edu/climate/ENSO/backgroun
d/monitoring.html
The ENSO phase impacts global temperature and precipitation patterns in
remote regions of the globe through robust teleconnections.
*Teleconnection pattern: a
characteristic and persistent
synoptic pattern that arises as
a result of internal climate
dynamics.
*Changes in SST and
precipitation patterns in the
equatorial Pacific, as a result
of the ENSO cycle, cause
changes in the position and
strength of the jet stream,
altering storm tracks, as well
as precipitation and
temperature patterns across
the globe.
*Remote temperature and
precipitation response are
shown here. Teleconnection
responses to an El Nino
(warm) event and La Nina
(cold) event are seasonally
dependent. Source:
linkingweatherandclimate.co
m/ENSO/tele.php.
Mean-State of ENSO in each CMIP3 Model:
*General trend toward more
uniform SST pattern across the
Pacific, in experiments forced
by a 1% per year increase in
CO2 concentration.
*A more El-Niño-like mean
state may emerge, reflecting a
general weakening of the
tropical circulation..
*Negative feedbacks will tend
to stabilize the western Pacific
warm pool, while surface
temperature and convection
will increase across the central
and eastern Pacific. Source:
http://www.ipcc.ch/publication
s_and_data/ar4/wg1/en/ch10s1
0-3-5-3.html
References
Barnett, T. P.D. W. Pierce, M. Latif, D. Dommenget, R. Saravanan, Interdecadal interactions between the tropics and
midlatitudes in the Pacific basin, Geophys. Res. Lett., 26, 615–618, 1999.
Lowell Stott, Christopher Poulsen, Steve Lund, and Robert Thunell Super ENSO and Global Climate Oscillations at
Millennial Time Scales Science 12 July 2002: 297 (5579), 222-226.
Trenberth, Kevin E., 1997: The Definition of El Niño. Bull. Amer. Meteor. Soc., 78, 2771–2777.
Zebiak, Stephen E., Mark A. Cane, 1987: A Model El Niño–Southern Oscillation. Mon.Wea. Rev., 115, 2262–2278