The El-Nino Southern Oscillation

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The El-Nino Southern Oscillation (ENSO)
La Nina Impacts
El Nino Impacts
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Outline
History Observed Structure and Evolution What
Causes an El-Nino? ENSO Forecasting Global
Impacts
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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.
• 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

Sir Gilbert Walker
Jacob Bjerknes
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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

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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

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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 Most highly correlated with eastward
shift of convection
Nino 3 Largest variability in SSTs over
an average ENSO cycle
Nino 4 Most highly correlated with
global weather patterns
Nino 12 Region that often first warms during
the onset of an El Nino
SOI
Darwin
Tahiti
SOI Most highly correlated with Nino 3.4 SST
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ENSO Observed Structure

• NOAAs 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
  gt 1.0
• La Nina Standardized Departures lt -1.0

82-83
97-98
El Nino
La Nina
10-11
88
98-99
Source http//www.cdc.noaa.gov/ENSO/enso.mei_inde
x.html
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ENSO Observed Structure
The 1995-1996 La-Nina Event
January 1995 December 1996
Animation
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ENSO Observed Structure
The 1997-1998 El-Nino Event
January 1997 December 1998
Animation
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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 TCs 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)
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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)
Mean Zonal Wind (m/s)
SST Anomalies (ºC)
Strong Easterlies
WWB
WWB
Weaker Easterlies
El Nino
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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 Nino12 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)

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What Causes El Nino?
The Delayed Oscillator in a Simple Ocean Model
Initial Time
Thermocline Depth
25 days
100 days
Forcing from single WWB
Kelvin Wave reflects and becomes a Rossby wave
175 days
50 days
Thermocline Depth
Rossby Wave reflects becomes a Kelvin wave
225 days
75 days
Upwelling Rossby Waves
Downwelling Kelvin Wave
Multiple reflections can lead to La Nina onset
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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
Strong Easterlies
La Nina
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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

La Nina Impacts (Winter)
El Nino Impacts (Winter)
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ENSO U.S. Impacts
El Nino Summer Temperatures
El Nino Winter Temperatures
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ENSO U.S. Impacts
El Nino Summer Rainfall
El Nino Winter Rainfall
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ENSO U.S. Impacts
La Nina Summer Temperatures
La Nina Winter Temperatures
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ENSO U.S. Impacts
La Nina Summer Rainfall
La Nina Winter Rainfall
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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)

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ENSO Forecasting
Source http//iri.columbia.edu/climate/ENSO/curre
ntinfo/SST_table.html
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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)

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References
Climate Diagnostic Centers (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.