Transcript Lecture 5: The El Nino Southern Oscillation (ENSO)

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
2.1 Introduction: What is Interannual and Decadal Variability?
Time series of SSTs in the East Pacific (region Nino3.4)
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).
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.
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).
2.1 Introduction: Can we predict SI Variability?
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)
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.html
http://www.gfdl.noaa.gov/~atw/enso
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
The tropical Pacific thermocline
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 =>0)
Low
slp
High
slp
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
El Nino
Mean climate
El Niño ’82/83
SST anomalies during El Nino
Dec 1982
Sept 1987
The 1997/98 El Nino
Jan 1997
The 1997/98 El Nino
Nov 1997
Jun
1997
The 1997/98 El Nino
Nov 1997
The 1997/98 El Nino
Mar 1998
The 1997/98 El Nino
Mar 1997
Jan
1998
The 1997/98 El Nino
Jun 1997
The 1997/98 El Nino
Nov 1997
The 1997/98 El Nino
Mar 1998
What about La Nina?
Dec 1982
In La Nina conditions
SST in the central and
eastern equatorial Pacific
is unusually cold &
easterly trade winds are
unusually strong
Nov 1988
La Nina conditions
sometimes occur in the
year following an El
Nino event (e.g. 1988
followed 1987 El Nino)
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
The Southern Oscillation Index
Darwin
x
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
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)
The “Nino3” SST index
Note that El Nino events
do not occur regularly
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)
Impacts on global climate, ecosystems and society
Observing El Nino
TAO Data
TAO on Youtube:
http://www.youtube.com/noaapmel#p/u/0/nzBAWirHMvA
Live TAO data:
http://www.pmel.noaa.gov/tao/jsdisplay/
Satellite
Observations
of Sea
Surface
Height
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.
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.
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.
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 OceanGlobal Atmosphere (TOGA) program, a 10year 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.
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.