El Niño/Southern Oscillation
Characteristics
SST and meteorological anomalies
Predictability
Walker Circulation
•  trade winds reinforce the
east-west SST contrast
•  SST contrast reinforces
the winds
http://forces.si.edu/elnino/exhibition_3b2.html
http://www.pmel.noaa.gov
Southern Oscillation Index
Based on Pressure Anomalies
Southern Oscillation Index
⎛
⎞
⎜
⎟
(
T
−
D
)
−
(
T
−
D
)
⎟
SOI = 10⎜
⎜
⎟
σ
⎜
⎟
⎝
⎠
El Nino
⎛
⎞
⎜
⎟
(
1011
−
1012
)
−
(
1011
−
1009
)
⎟
SOI = 10⎜
⎜
⎟
σ
⎜
⎟
⎝
⎠
La Nina
⎛
⎞
⎜
⎟
(1013 − 1009) − (1011 − 1009) ⎟
⎜
SOI = 10
⎜
⎟
σ
⎜
⎟
⎝
⎠
Source: Cane 2005. Earth
and Planetary Science
Letters 230: 227-240.
Thermocline changes
Self-sustained Oscillators
http://iri.columbia.edu/climate/ENSO/theory/coupled.html
Self-sustained Oscillators
http://iri.columbia.edu/climate/ENSO/theory/coupled.html
Self-sustained Oscillators
http://iri.columbia.edu/climate/ENSO/theory/coupled.html
PMEL loop
Delayed Oscillator
Theory
Initial westerly wind
stress
Delayed Oscillator
Theory
Initial westerly wind
stress
Delayed Oscillator
Theory
Initial westerly wind
stress
Waves generated by wind stress
Kelvin Wave
- mass surplus
- downwelling
Waves generated by wind stress
Rossby Wave
- mass deficit
Kelvin Wave
- mass surplus
- downwelling
Height anomaly associated with…………..
Rossby Wave
Kelvin Wave
Rossby Wave
~1m/s
Kelvin Wave
~3m/s
25 days
50 days
75 days
100 days
Wave “reflects”
off coast; returns
as Rossby wave
125 days
Orange: Reflected off eastern
boundary with positive height
anomaly
175 days
225 days
Blue: Reflected off western
boundary with negative height
anomaly and upwelling
275 days
Self-sustained Oscillators
http://iri.columbia.edu/climate/ENSO/theory/coupled.html
The termination of an El
Niño event
•  Common idea:
–  Rossby waves reflected off western
boundary of Pacific
–  Return as upwelling Kelvin energy causing
thermocline to shoal
•  Harrison and Vecchi (1999):
–  Southward shift in westerly wind anomaly
(observed)
–  GCM to test theory
Current Conditions
http://www.esrl.noaa.gov/psd/map/images/sst/sst.daily.anom.gif
Current Conditions
Animations:
Current:
http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml
Archives: http://www.pmel.noaa.gov/tao/jsdisplay/
Comparison of events:
http://www.cdc.noaa.gov/map/clim/sst_olr/el_nino_anim.shtml
Australian Bureau of Meteorology
Climate Prediction Center; International Research Institute
Two types
Wang et al., 2012