El Nino and the Southern Oscillation

1
El Nino and the Southern Oscillation

• Jon Schrage

2
Two Aspects of ENSO

• Atmospheric Aspect Southern Oscillation
• Ocean Aspect El Nino and La Nina
• However, this distinction is not universal and
  not realthe atmosphere and ocean are an
  interacting system!

3
Southern Oscillation
This is the normal situation in the atmosphere.
4
This is the normal situation in the atmosphere.
5
It is (part of) what drives the observed ocean
currents in the equatorial Pacific.
6
This is the normal situation in the atmosphere
and the ocean.
7
(No Transcript)
8
Notice that it depends on there being high
pressure in the southeastern Pacific and
(relatively) low pressure over Indonesia!
9
Normal Situation
El Nino Situation
El Nino/Southern Oscillation
10
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!

11
(No Transcript)
12
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.

13
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.)

14
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.

15
Either way

• The trade winds along the equator were important.
• And their strength depends on the east-west
  pressure difference!

Low
High
16
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

17
Low
High

• During La Nina conditions
• Pressure at Tahiti is high, pressure at Darwin is
  low, so SOI gt 0.

• During El Nino conditions
• Pressure at Tahiti is low, pressure at Darwin is
  high, so SOI lt 0.

High
Low
18
(No Transcript)
19
During El Nino, when the SOIlt0, the trade winds
can no longer support the higher SSHs in the
Western Pacific, and the water moves eastward.
20
The SSH anomaly moves as a Kelvin Wave!

• Here is a SSH anomaly along the equator.

High
0N
21
The SSH anomaly moves as a Kelvin Wave!

• There is a PGF acting to disperse the anomaly.

High
0N
22
The SSH anomaly moves as a Kelvin Wave!

• The ocean in geostrophic balance CF balances PGF.

PGF
PGF
PGF
High
0N
23
The SSH anomaly moves as a Kelvin Wave!

• Resulting in the following flow

PGF
PGF
PGF
High
0N
PGF
PGF
PGF
24
The SSH anomaly moves as a Kelvin Wave!

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

PGF
PGF
PGF
High
0N
PGF
PGF
PGF
25
The SSH anomaly moves as a Kelvin Wave!

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

PGF
PGF
PGF
High
0N
PGF
PGF
PGF
26
The SSH anomaly moves as a Kelvin Wave!

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

0N
27
The SSH anomaly moves as a Kelvin Wave!

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

CF
CF
CF
Low
0N
PGF
PGF
PGF
CF
CF
CF
28
The SSH anomaly moves as a Kelvin Wave!

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

0N
29
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.

30
(No Transcript)
31
Effects of El Nino
32
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.

33

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