The El-Nino Southern Oscillation
(ENSO)
El Nino Impacts
Tropical
La Nina Impacts
M. D. Eastin
Outline
History
Observed Structure and Evolution
What Causes an El-Nino?
ENSO Forecasting
Global Impacts
Tropical
M. D. Eastin
ENSO
History
• In the 1600s, Peruvian fisherman noticed their fish harvests failed
every few years due to warmer-than-normal waters (upwelling
provides nutrient-rich cold water for fish). The warming always
occurred in December, so the phenomena was named El Nino, in
reference to the Christ child.
• In 1899 the Indian Monsoon failed, leading to severe drought and
famine. This lead Gilbert Walker, head of the Indian Met. Service,
to search for a way to predict the monsoon. He identified a peculiar
surface pressure oscillation: when the pressure is high over the
maritime continent (Indonesia and Darwin), surface pressures are
low over India and the central southern Pacific (Tahiti). He referred
to this as the Southern Oscillation.
Sir Gilbert Walker
• In 1969, UCLA professor Jacob Bjerknes first recognized that El Nino
and the Southern Oscillation were actually manifestations of the same
physical phenomena that results from unstable interactions between the
ocean and atmosphere, and referred to it as ENSO
• The 1982-83 El Nino event was the first to receive significant public
and research interest → a relatively new (not well understood) phenomena
Tropical
Jacob Bjerknes
M. D. Eastin
ENSO: Observed Structure
Normal Conditions or La Nina:
• Strong easterly winds induce upwelling of cold water in the equatorial eastern Pacific
• Shallow oceanic thermocline in the east Pacific (due to upwelling)
• Warm SSTs confined to western Pacific with a deep thermocline
• Low pressure and convection in west Pacific
• High pressure and subsidence (clear air) in east Pacific
Tropical
M. D. Eastin
ENSO: Observed Structure
El-Nino Conditions:
• Weaker easterly winds result in less upwelling of cold water
• Warm SSTs “spread” to east Pacific (also solar heating not offset by upwelling)
• Increase in the east Pacific thermocline
• Low pressure and convection shifts to the east Pacific
• High pressure and subsidence shifts to the west Pacific
Tropical
M. D. Eastin
ENSO: Observed Structure
ENSO Indices
• Based on observed SST anomalies (difference from the long term mean) in the
equatorial Pacific in four regions (observation from TAO moored buoys)
• Based on surface pressure differences between Tahiti and (minus) Darwin, called
the Southern Oscillation Index (SOI)
Nino 3.4
Nino 3
Most highly correlated
with eastward shift
of convection
Largest variability
in SSTs over an
average ENSO cycle
Nino 4
Most highly correlated
with global
weather patterns
SOI
Most highly
correlated with
Nino 3.4 SST
Tropical
Darwin
SOI
Tahiti
Nino 1+2
Region that often first
warms during the
onset of an El Nino
M. D. Eastin
ENSO: Observed Structure
NOAA’s Multivariate ENSO Index (MEI)
• Combines normalized anomalies of SST (in Nino3.4), surface pressures (the SOI),
surface winds, surface air temperatures, and cloud fraction to obtain a “composite”
view of the state of ENSO
• Definitions:
El Nino = Standardized Departures > +1.0
La Nina = Standardized Departures < -1.0
82-83
97-98
El Nino
La Nina
88
98-99
10-11
Source: http://www.cdc.noaa.gov/ENSO/enso.mei_index.html
Tropical
M. D. Eastin
ENSO: Observed Structure
The 1995-1996 La-Nina Event
January 1995 – December 1996
[Animation]
Tropical
M. D. Eastin
ENSO: Observed Structure
The 1997-1998 El-Nino Event
January 1997 – December 1998
[Animation]
Tropical
M. D. Eastin
What Causes El Nino?
Triggering Mechanism
• Not well understood
• Deep thermocline in the western Pacific believed to be a necessary (not sufficient) condition
• “Westerly wind bursts” (WWBs) over a period of several days may be one trigger
• Most often associated with the Madden-Julian Oscillation (MJO)
• Atmospheric Kelvin waves also generate sustained westerly winds
• Twin TC’s straddling the equator can also generate sustained westerly winds
• Multiple sustained WWBs decrease the equatorial easterlies that induced cold upwelling
• Less upwelling combined with a west-east ocean current (forced by the WWBs)
increases the central and eastern Pacific SSTs and lowers the thermocline depth
and initiates an El Nino event
Anomalous surface winds
(i.e. a WWB) associated
with MJO convection
(centered in the box)
Tropical
M. D. Eastin
What Causes El Nino?
Onset of the 1997-98 El Nino
Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998
Zonal Wind Anomalies (m/s)
WWB
Mean Zonal Wind (m/s)
Strong
Easterlies
Weaker
Easterlies
Tropical
SST Anomalies (ºC)
WWB
El Nino
M. D. Eastin
What Causes El Nino?
An Oceanic Component
• The WWBs acting alone would lead to a gradual eastward progression of SST anomalies
(which is observed but the signal is weak)
• In contrast, observations show a pronounced rapid “emergence” of warm SST anomalies
in the equatorial east Pacific (along the Peruvian coast in the Nino1+2 region)
• What causes this rapid emergence?
• Delayed Oscillator Theory is one explanation for this rapid emergence
• Atmospheric WWBs generate equatorial Rossby and Kelvin waves in the ocean
• Oceanic waves propagate along the density contrast of the thermocline
Oceanic Rossby waves: Move westward at slow speeds
Induce upwelling (decreases the thermocline depth)
Effectively cool the ocean mixed layer and SSTs
Oceanic Kelvin waves: Move eastward very rapidly (much faster than Rossby waves)
Induce downwelling (increases the thermocline depth)
Effectively warm the ocean mixed layer and SSTs
• This theory also provides an explanation for the ENSO oscillation every 4-5 years
Tropical
M. D. Eastin
What Causes El Nino?
The Delayed Oscillator in a Simple Ocean Model
Thermocline Depth
Initial Time
Forcing from
single WWB
25 days
100 days
Kelvin Wave
reflects and becomes
a Rossby wave
Thermocline Depth
50 days
175 days
Rossby Wave reflects
becomes a Kelvin wave
Upwelling
Rossby
Waves
Downwelling
Kelvin
Wave
75 days
225 days
Multiple reflections can
lead to La Nina onset
Tropical
M. D. Eastin
What Causes La Nina?
Onset of the 1998-99 La Nina
Daily Mean Surface Values in the equatorial Pacific: 1 January 1997 thru 31 December 1998
Zonal Wind Anomalies (m/s)
Mean Zonal Wind (m/s)
SST Anomalies (ºC)
Weaker
Easterlies
Lack of Strong
WWB
Lack of Strong
WWB
Tropical
Strong
Easterlies
La Nina
M. D. Eastin
ENSO: Global Impacts
Global Impacts
• ENSO variability alters convection in tropical Pacific
• This convective variability produces zonal anomalies in the Walker and Hadley Circulations,
which, in turn, influences mid-latitude synoptic-wave patterns and alters the global weather
• Anomalous synoptic wave patterns lead to warmer/colder and wetter/drier conditions
El Nino Impacts (Winter)
Tropical
La Nina Impacts (Winter)
M. D. Eastin
ENSO: U.S. Impacts
El Nino: Winter Temperatures
Tropical
El Nino: Summer Temperatures
M. D. Eastin
ENSO: U.S. Impacts
El Nino: Winter Rainfall
Tropical
El Nino: Summer Rainfall
M. D. Eastin
ENSO: U.S. Impacts
La Nina: Winter Temperatures
Tropical
La Nina: Summer Temperatures
M. D. Eastin
ENSO: U.S. Impacts
La Nina: Winter Rainfall
Tropical
La Nina: Summer Rainfall
M. D. Eastin
ENSO Forecasting
Forecast Models
• All models forecast SSTs in the equatorial Pacific (most often for the Nino3.4 region)
Statistical models
• Employ simple multiple regression techniques based on ENSO indices
• Based on evolution of previous ENSO events (and historical records)
• Quality of forecasts reliant on quality of historical data
• Cannot forecast “record” events
• No physical interpretation possible
Dynamical Models
• Most are complex coupled atmosphere-ocean models
• Initialization requires 3-D observations of ocean and atmosphere (data sparse region)
• Small scale features are parameterized
• Can forecast record events (not bound by past events)
Tropical
M. D. Eastin
ENSO Forecasting
Source: http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/
Tropical
M. D. Eastin
The El-Nino Southern Oscillation
(ENSO)
Summary:
• History (basic timeline and rise to prominence)
• Oceanic and atmospheric structure/flows during El Nino and La Nina
• ENSO Indices (defining parameter, differences, and uses)
• Causes of El Nino
• Westerly Winds Bursts (definition, origin, impact/forcing)
• Delayed Oscillator Theory (role of waves, explain rapid onset)
• Global impacts of ENSO
• Impact of ENSO in the U.S.
• ENSO Forecasting (difference in model types)
Tropical
M. D. Eastin
References
Climate Diagnostic Center’s (CDCs) Interactive Plotting and Analysis Webage
( http://www.cdc.noaa.gov/cgi-bin/PublicData/getpage.pl )
Kindle, J. C. , and P. A. Phoebus, 1995: The ocean response to perational westerly wind bursts during the 1991-1992
El Nino. J. Geophysical. Res., 100, 4893-4920.
Knaff, J. A., and C. W. Landsea, 1997: An El Nino-Southern Oscillation Climatology and Persistence (CLIPER) Forecasting
Scheme. Wea. Forecasting, 12, 633-652.
McPhaden, M. J., 2004: Evolution of the 2002/3 El Nino. Bull. Amer. Meteor. Soc., 85, 677-695.
Tropical
M. D. Eastin