SSH anomalies from satellite
Observed annual mean state
Circulation creates equatorial cold tongues eastern Pacific
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Trades -> Ocean upwelling along Equator -> Thermocline Tilt ->
Mean SST
East-West SST Gradient -> Walker Circulation
Positive (Bjerknes) feedback amplifies SST gradient and Walker Circulation
Normal conditions
El Nino onset conditions
El Nino conditions
ENSO Wind and SST
ENSO Ocean Temperatures
Sea surface temperature (SST) and zonal wind anomalies vary in a quasi-stationary fashion.
Thermocline anomalies along the equator show a systematic space and time evolution relative to SST anomalies.
1996
1999
1998
1997
Mean P Tahiti>P Darwin. Negative SOI is weakening of trades
SOI =10Pdiff −Pdiffav( )SD(Pdiff )
Pdiff =PTahiti −PDarwin
Precipitation Anomalies
Small changes in the distribution of sea surface temperature are coordinated with changes in atmospheric circulation and rainfall patterns;
Temperature Anomalies
Nino3.4 or CT
There is a tight coupling between the atmosphere & ocean.
Sea Surface Temperature and Sea Level Pressure
CT
SOI
r = 0.93
The spectrum of ENSO features a broad interannual (3-7 year) peak
ENSO has most variance at the end of the calendar year
Thompson and Battisti 2001
ENSO affects the global climate through atmosphere teleconnections
Upper level circulation
Anomalies during El Nino: the “warm phase” of ENSO
Zhang Battisi Wallace 1997
ENSO affects the global climate through atmosphere teleconnections
Climate Impacts of thePacific-North American Pattern (Winter)
Air Temperature Precipitation
+2°C- 2°C 0°C +1- 1 cm per month
The impacts of ENSO (cont).
• ENSO alters the Pacific storm tracks, and the probabilities of extreme weather events on a global scale.
State of the tropical Pacific today
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State of the tropical Pacific today
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State of the tropical Pacific today
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1. Kelvin wave: travels as a first baroclinic mode gravity wave with speed sqrt( g’h) to the east, about 3 m/s to the EAST. Crosses the Pacific in about 2 months.
2. Rossby wave (first meridional mode) travels as a first baroclinic mode with phase speed equal to sqrt(g’h)/3, about 1 m/s to the WEST Cross the Pacific in 6 months. In oceanic Rossby waves, meridional advection of planetary vorticity is balanced by stretching not relative vorticity
Delayed Oscillator Theory: introduction to KW and RW
Zonal wind anomaly on the Equator (westerly)
Wind stress at 175W Wind stress curl at 175W
Delayed Oscillator Theory: a model of SSH evolution
Ocean sea surface Height anomaly(remember thermocline is opposite)
Ocean sea surface height anomaly along 140WDownwelling Kelvin Wave
Ocean sea surface height anomaly along 180E, upwellingRossby wave
Delayed Oscillator Theory: forcing of RW and KW
25 days
50 days
275 days
175 days
125 days
75 days
100 days
225 days
Delayed Oscillator Theory: evolution of RW and KW
ENSO Theories
1. El Nino is one phase of a self-sustained unstable and naturally oscillatory mode
2. El Nino is a stable (or damped) mode triggered by atmospheric random noise forcing
3. It is marginally stable, and that weather noise is needed to kick it off
ENSO Theories
1. Delayed-oscillator model2. Discharge Oscillator-pior to El Nino, warm water
volume builds up, then during El Nino is discharged to higher latitudes
3. There is a lagged feedback from the western boundary in the ocean
ENSO Theories
1. Some agreement that it is a marginally stable mode that needs noise to kick it off, most likely the Madden Julian Oscillation, an oscillatory pattern that travels the atmosphere around the globe, but can only be seen at the surface in the western Pacific. 40-50 day period and propagate to the east.
2. MJOs are not symmetric, with westerly winds bursts only occuring when the water is above 27 to 28C.
3. ENSO is not symmetric, with El Nino being understood as an event, and La Nina as “normal”, it is not sinuisoidal, although it is in simple models and in many coupled GCMs
Challenges
1. El Nino cannot be predicted before it starts, once it starts, then useful predictions can be made (9 months)
2. El Nino starting up seems to rely on atmospheric phenomenon like MJOs that are difficult to model in GCMs
Decadal ENSO-like variability (aka PDO)• After filtering to remove the interannual variability, the leading pattern of
variability in the Pacific has an ENSO-like pattern in SST and atmosphere circulation.
Decadal ENSO-like variability (aka PDO)
• The PDO spectrum is red• Due to stochastic forcing (a) the Aleutian Low (white) and (b) ENSO
teleconnections• Time scale is determined by surface ocean ocean heat capacity
(including re-emergence).