El Nino and the Southern Oscillation

• Jon Schrage

Two Aspects of ENSO

• Atmospheric Aspect : “Southern Oscillation”

• Ocean Aspect : “El Nino and La Nina”

• However, this distinction is not universal and not real—the atmosphere and ocean are an interacting system!

Southern Oscillation

This is the “normal” situation in the atmosphere.

This is the “normal” situation in the atmosphere.

It is (part of) what drives the observed ocean currents in the equatorial Pacific.

This is the “normal” situation in the atmosphere and the ocean.

Notice that it depends on there being high pressure in the southeastern Pacific and (relatively) low pressure over Indonesia!

El Nino/Southern Oscillation

Normal Situation

El Nino Situation

El Nino

• Happens (on average) every 2-7 years.

• Average: 4 years apart

• El Nino conditions last for about a year.

•KEY IMPACT: Warm water anomalies in the Western Pacific move to the eastern Pacific!

History of the term “El Nino”

• A reference to the Christ child– There is a warming event along the coast of

Peru every December, named “El Nino” in deference to the nativity.

History of the term “El Nino”

• Climate researchers have come to refer to the much stronger warming that occurs every 4-5 years as “El Nino”.

• The opposite of this condition is “La Nina”, or “anti-El Nino”. (Unusually cold water off the coast of South America.)

Why did the warm water move from the western PAC to the eastern PAC?

• Either:– The pile of water in the western PAC got so big

that the trade winds were not capable of holding it back any more,

• Or:– The trade winds weakened, and the pile of

water “sloshed” eastward.

Either way:

• The trade winds along the equator were important.

• And their strength depends on the east-west pressure difference!

Southern Oscillation Index (SOI)

• Reflects the magnitude and phase of the ATMOSPHERIC component of ENSO:

• SOI = (Normalized sea level pressure anomaly at Tahiti) – (Normalized sea level pressure at Darwin)

• SOI = Tahiti - Darwin

During La Nina conditions:

•Pressure at Tahiti is high, pressure at Darwin is low, so SOI > 0.

During El Nino conditions:

•Pressure at Tahiti is low, pressure at Darwin is high, so SOI < 0.

During El Nino, when the SOI<0, the trade winds can no longer support the higher SSHs in the Western Pacific, and the water moves eastward.

The SSH anomaly moves as a Kelvin Wave!

• Here is a SSH anomaly along the equator.

0°N

The SSH anomaly moves as a Kelvin Wave!

• There is a PGF acting to disperse the anomaly.

0°N

PGF

PGFPGFPGF

PGFPGF

The SSH anomaly moves as a Kelvin Wave!

• The ocean in geostrophic balance: CF balances PGF.

0°N

PGF

PGFPGFPGF

PGFPGF

CFCFCF

The SSH anomaly moves as a Kelvin Wave!

• Resulting in the following flow:

0°N

PGF

PGFPGFPGF

PGFPGF

CFCFCF

The SSH anomaly moves as a Kelvin Wave!

• Water is moved away from the west side--SSH falls.

0°N

PGF

PGFPGFPGF

PGFPGF

CFCFCF

The SSH anomaly moves as a Kelvin Wave!

• Water is moved towards the east side--SSH rises.

0°N

PGF

PGFPGFPGF

PGFPGF

CFCFCF

The SSH anomaly moves as a Kelvin Wave!

• Water is moved towards the east side--SSH rises.

0°N

PGF

PGFPGFPGF

PGFPGF

CFCFCF

The SSH anomaly moves as a Kelvin Wave!

• This works just as well for a negative SSH anomaly!

0°N

CF

CFCFCF

CFCF

PGFPGF

PGF

The SSH anomaly moves as a Kelvin Wave!

• This works just as well for a negative SSH anomaly!

0°N

CF

CFCFCF

CFCF

PGFPGF

PGF

Kelvin Waves:

• All move eastward

• All are equatorially trapped

• Can happen in the atmosphere or in the ocean.

• Can happen at the surface of the ocean, or on the thermocline.

Effects of El Nino

So does the Southern Oscillation cause El Nino, or does El Nino cause

the Southern Oscillation?

• The question is meaningless. They happen together as a single manifestation of the internal variability of the ocean-atmosphere system.

• It is true that sometimes the ocean either leads or lags the atmosphere, however.

• This would be a good time to watch the little video about the El Nino of 1997-98.