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
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 the Pacific-North American Pattern (Winter) Air TemperaturePrecipitation +2°C - 2°C0°C0°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.
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 175WWind 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 140W Downwelling Kelvin Wave Ocean sea surface height anomaly along 180E, upwelling Rossby 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 model 2.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).