Lecture 5: The El Nino Southern Oscillation (ENSO)

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2. Natural Climate Variability    
2.1 Introduction 2.2 Interannual Variability 2.3
Decadal Variability 2.4 Climate Prediction 2.5
Variability of High Impact Weather
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2.1 Introduction What is Interannual and
Decadal Variability?
Time series of SSTs in the East Pacific (region
Nino3.4)
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2.1 Introduction What is Interannual and
Decadal Variability?
Time series of SSTs in the North Atlantic
highlighting the Atlantic Multidecadal
Oscillation (AMO) source Knight et al (2005).
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2.1 Introduction What is Interannual and
Decadal Variability?
The NAO index is defined as the anomalous
difference between the polar low and the
subtropical high during the winter season
(December through March). The positive phase is
associated with more storminess in the Atlantic
storm track and the negative phase with reduced
storminess.
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2.1 Introduction Why is Climate Variability
important?

• Interannual-to-Decadal Variability impacts
  Societies
• Food resources
• Water security
• Health
• Demographics
• Energy

Time series (1941-2001) of average normalized
April-October rainfall departure for 20 stations
in the West African Soudano-Sahel zone (11-18N
and West of 10E) following methodology of Lamb
and Peppler, 1992).
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2.1 Introduction Can we predict SI Variability?
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Section 2.2 Interannual Variability  2.2.1 El
Nino Southern Oscillation (ENSO)   (i)
Observations (ii) Theory for ENSO (iii)
Impacts 2.2.2 Interannual variability in
Atlantic SSTs 2.2.3 The North Atlantic
Oscillation (NAO)  
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2.2.1 ENSO Observations

• Philander, S.G.H, 1990 El Nino, La Nina and the
  Southern Oscillation
• Useful El Nino pages
• http//www.pmel.noaa.gov/tao/elnino/nino-home.htm
  l
• http//www.gfdl.noaa.gov/atw/enso

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Mean State of the tropical Pacific coupled
Ocean-Atmosphere System

• Warmest SSTs in west, cold tongue in east
• Precipitation associated with warmest SSTs
• Easterly trade winds advect equatorial surface
  waters westward

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The tropical Pacific thermocline
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The Walker Circulation

• Mean ascent, and low surface pressure, over
  warmest SST associated with deep convection
• Subsidence, and high surface pressure, in
  non-convecting regions
• Equatorial trades blow from high to low pressure
  (balanced by friction since Coriolis force gt0)

Low slp
High slp
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El Nino

• During El Nino trade winds slacken
• E-W tilt of thermocline upwelling of cold water
  are reduced.
• SST rises in central/eastern equatorial Pacific
• Changes Walker Circulation

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El Nino
El Niño 82/83
Mean climate
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SST anomalies during El Nino
Dec 1982 Sept 1987
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The 1997/98 El Nino
Jan 1997
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The 1997/98 El Nino
Nov 1997
Jun 1997
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The 1997/98 El Nino
Nov 1997
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The 1997/98 El Nino
Mar 1998
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The 1997/98 El Nino
Mar 1998
Jan 1997
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The 1997/98 El Nino
Jun 1997
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The 1997/98 El Nino
Nov 1997
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The 1997/98 El Nino
Mar 1998
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What about La Nina?
In La Nina conditions SST in the central and
eastern equatorial Pacific is unusually cold
easterly trade winds are unusually strong
Dec 1982
Nov 1988
La Nina conditions sometimes occur in the year
following an El Nino event (e.g. 1988 followed
1987 El Nino)
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Walker Circulation

• British mathematician, director general of
  observations for India (formed after monsoon
  failure of 1877- worst famine in Indian history)
• Arrived in 1904, shortly after huge famine caused
  by drought
• Goal to predict Indian Monsoon
• Found that many global climate variations,
  including Monsoon rains in India, were correlated
  with the Southern Oscillation

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The Southern Oscillation Index
Darwin
Tahiti

• Mean pressure is lower at Darwin than Tahiti
• The term Southern Oscillation was also coined
  by Gilbert Walker
• The SOI measures the strength of the Pacific
  Walker circulation

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ENSO

• Bjerknes recognised that the El Nino warming of
  the ocean was related to variations in the SOI.
• During El Nino
• SOI is low
• Trades are weak
• Precipitation is enhanced over central
  equatorial Pacific (indicated by low OLR)

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The Nino3 SST index
Note that El Nino events do not occur regularly
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Why is El Nino important?

• Major climate and economic impacts on countries
  around the tropical Pacific, and further afield.
• Droughts in some regions, floods in others
• Collapse of coastal fishery in
  Peru (largest average annual catch of
  marine fish in the world)

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• Impacts on global climate, ecosystems and society
  

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Observing El Nino
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TAO Data
TAO on Youtube http//www.youtube.com/noaapmel
p/u/0/nzBAWirHMvA Live TAO data
http//www.pmel.noaa.gov/tao/jsdisplay/
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Satellite Observations of Sea Surface Height
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• Chronology of Events in the History of
  Understanding El Niño and La Niña
• late 1800s Fishermen coin the name El Niño to
  refer to the periodic warm waters that appear off
  the coasts of Peru and Ecuador around Christmas.
• 1928 Gilbert Walker describes the Southern
  Oscillation, the seesaw pattern of atmospheric
  pressure readings on the eastern and western
  sides of the Pacific Ocean.

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• 1957 Large El Niño occurs and is tracked by
  scientists participating in the International
  Geophysical Year. Results reveal that El Niño
  affects not just the coasts of Peru and Ecuador
  but the entire Pacific Ocean.
• 1969 Jacob Bjerknes, of the University of
  California, Los Angeles, publishes a seminal
  paper that links the Southern Oscillation to El
  Niño.

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• 1975 Klaus Wyrtki, of the University of Hawaii,
  tracks sea levels across the Pacific and
  establishes that an eastward flow of warm surface
  waters from the western Pacific causes sea
  surface temperatures to rise in the eastern
  Pacific.
• 1976 Researchers use an idealized computer model
  of the ocean to demonstrate that winds over the
  far western equatorial Pacific can cause sea
  surface temperature changes off Peru.

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• 1982 A severe El Niño develops in an unexpected
  manner, but its evolution is recorded in detail
  with newly developed ocean buoys.
• 1985 Several nations launch the Tropical
  Ocean-Global Atmosphere (TOGA) program, a 10-year
  study of tropical oceans and the global
  atmosphere.
• 1986 Researchers design the first coupled model
  of ocean and atmosphere that accurately predicts
  an El Niño event in 1986.

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• 1988 Researchers explain how the "memory" of the
  ocean--the lag between a change in the winds and
  the response of the ocean--influences
  terminations of El Niño and the onset of La Niña.
• 1996-1997 The array of instruments monitoring
  the Pacific, plus coupled ocean-atmosphere
  models, enable scientists to warn the public of
  an impending El Niño event.