2.1.1 ENSO

What does ENSO stand for?

El Niño Southern Oscillation.

What is ENSO?

Irregular fluctuations in SST, air pressure and winds across the tropical Pacific.

How frequently does ENSO occur and how long does it last?

• Occurs every 3–7 years

• Lasts about 1–2 years

What are the 3 ENSO states?

• Normal/Neutral

• El Niño = warm phase

• La Niña = cool phase

What is the Southern Oscillation?

The atmospheric component of ENSO, involving pressure differences between Tahiti and Darwin.

What measures the Southern Oscillation?

SOI = Southern Oscillation Index.

🔴 El Niño = Negative SOI

🔵 La Niña = Positive SOI

What is the Walker Circulation?

An east-west ocean-atmosphere circulation particularly prominent in the equatorial region.

Explain normal Walker Circulation.

Easterly trade winds
→ push warm water west
→ warm water accumulates near Australia/Indonesia
→ evaporation + uplift
→ low pressure + heavy rainfall in west

Meanwhile:
→ Eastern Pacific surface water moving west is replaced by cold, nutrient-rich upwelling off Peru/Ecuador
→ high pressure + dry conditions in east

What happens to the thermocline under normal conditions?

• West = deep

• East = shallow

What happens to trade winds during El Niño?

They weaken or reverse.

Explain the El Niño process.

Trades weaken
→ Warm water moves east
→ Thermocline flattens
→ Upwelling decreases
→ eastern Pacific SST rises
→ Rainfall/convection shifts east

Why does the eastern Pacific warm during El Niño?

Warm surface water moves east + cold upwelling decreases.

How does El Niño change tropical Pacific rainfall?

Convection and rainfall shift eastwards.

What happens to Australia/Indonesia during El Niño?

Generally drier because warm water and convection shift east.

What happens to Peru/Ecuador during El Niño?

Generally wetter because warmer SSTs increase evaporation and convection.

What is the Bjerknes positive feedback during El Niño?

Trade winds weaken
→ warm water shifts east
→ thermocline flattens
→ upwelling ↓
→ eastern Pacific warms
→ east-west temperature/pressure gradient ↓
→ trade winds weaken further

What is La Niña essentially?

A strengthening of normal Walker Circulation.

Explain the La Niña process.

Trade winds strengthen
→ Warm water pushed further west
→ Thermocline steepens
→ Upwelling increases
→ eastern Pacific Cools
→ western Pacific becomes Wetter

What happens to western Pacific rainfall during La Niña?

Increases because more warm water accumulates there → evaporation and convection increase.

What happens to eastern Pacific rainfall during La Niña?

Generally decreases due to stronger cold upwelling and lower SSTs.

Compare Normal, El Niño and La Niña.

	Normal	🔴 El Niño	🔵 La Niña
Trade winds	Normal	Weaken/reverse	Strengthen
Warm water	Western Pacific	Moves east	Piles further west
Thermocline	Sloping	Flatter	Steeper
Eastern upwelling	Normal	Weaker	Stronger
Eastern Pacific SST	Cool	Warmer	Cooler
Western Pacific (Australia/Indonesia)	Wet	DRIER	WETTER
Eastern Pacific (Peru/Ecuador)	Dry	WETTER	DRIER
Walker circulation	Normal	Weakens	Strengthens

How does El Niño affect India's South Asian Monsoon?

It tends to make the monsoon weaker and less reliable → reduced rainfall.

Give one India El Niño example.

2009: rainfall fell to 78.2% of normal, the lowest in 37 years.

Give one Peru example.

1997–98 El Niño: heavy rainfall formed a giant lake in the Sechura Desert.

How does El Niño affect fishing off Peru?

Upwelling ↓ → fewer nutrient-rich waters → reduced fishing productivity.

Give one useful Singapore El Niño statistic.

During the 2015–16 El Niño, May–September 2015 rainfall was about 35% below the long-term average.

Why doesn't El Niño strengthen forever?

Delayed negative oceanic feedbacks eventually:

• redistribute warm water,

• restore the thermocline,

• resume upwelling,

• restore trade winds,

→ returning conditions towards neutral.