El Nino and La Nina Explained: How One Patch of Pacific Ocean Redraws Global Disaster Risk

Direct answer: El Nino and La Nina are the two opposite phases of the El Nino-Southern Oscillation (ENSO), a natural see-saw in sea surface temperature and air pressure across the tropical Pacific. El Nino is the warm phase; La Nina the cool phase. Because the tropical Pacific is vast, the resulting shift in where thunderstorms form rearranges rainfall and temperature patterns worldwide — driving droughts on one continent and floods on another for a year or more at a time.

The mechanism

In normal conditions, trade winds blow from east to west along the equatorial Pacific, piling warm surface water up near Indonesia and allowing cold water to well up off South America. That warm western pool fuels deep convection and heavy rain over the Maritime Continent.

El Nino: the trade winds weaken or reverse. Warm water sloshes eastward across the Pacific, the upwelling off South America shuts down, and the zone of heavy convection migrates east into the central and eastern Pacific. Because that convection anchors the jet streams, weather patterns downstream across the globe shift with it.

La Nina: the trade winds strengthen, cold upwelling intensifies, the warm pool packs further west, and the pattern is broadly the mirror image.

Each phase typically lasts 9 to 12 months, sometimes years, and events recur irregularly every two to seven years. They are declared by agencies such as NOAA and the Bureau of Meteorology using ocean temperature indices in defined Pacific regions, combined with atmospheric response.

What El Nino tends to bring

• Drought in Indonesia, Malaysia, the Philippines, eastern Australia and Papua New Guinea — with a strong associated increase in wildfire and peat-fire risk.
• Drought in southern Africa, parts of India and Central America.
• Heavy rain and flooding along the Pacific coast of South America, particularly Peru and Ecuador, and in parts of East Africa.
• Fewer Atlantic hurricanes, because increased wind shear over the Atlantic tears storms apart, but more eastern and central Pacific tropical cyclones.
• Higher global average temperature, as the ocean releases heat to the atmosphere. The warmest years on record have tended to be El Nino years.
• Collapse of the Peruvian anchovy fishery, as the nutrient-rich upwelling stops — the original observation from which El Nino got its name.

What La Nina tends to bring

• Heavy rain and flooding in eastern Australia, Indonesia and southern Africa.
• Drought in the southern United States, the Horn of Africa and coastal South America. Consecutive La Nina years contributed to a devastating multi-season drought in Somalia, Ethiopia and Kenya.
• More active Atlantic hurricane seasons, because wind shear is reduced.
• Colder winters in parts of the northern United States and northern Asia.

Important caveats

ENSO shifts probabilities, not certainties. Three things are routinely got wrong:

• Not every event follows the textbook. Regional outcomes depend on where in the Pacific the warming peaks — a "central Pacific" or Modoki El Nino produces different impacts from an eastern one.
• Other oscillations compete. The Indian Ocean Dipole strongly modulates East African and Australian rainfall and can reinforce or cancel the ENSO signal. The Madden-Julian Oscillation drives shorter-term bursts on top of it.
• ENSO is not climate change. It is a natural cycle that has operated for millennia. It is superimposed on a warming baseline, so an El Nino now delivers hotter extremes than an El Nino of the same strength decades ago.

Why the forecast is genuinely useful

ENSO is one of the few sources of skilful seasonal forecasting that exists. Because ocean heat changes slowly, a developing phase can be identified months ahead — long enough to act on:

• Governments pre-position food and water in regions facing likely drought.
• Farmers adjust planting dates and crop choices.
• Water utilities manage reservoirs differently.
• Health services prepare for cholera or dengue outbreaks that follow flooding.
• Insurers and hurricane-exposed regions adjust their seasonal outlook.

This is anticipatory action rather than reaction, and it is one of the highest-value uses of climate science currently available.

Where to follow it

NOAA's Climate Prediction Center issues a monthly ENSO diagnostic discussion and alert status; the Australian Bureau of Meteorology publishes a fortnightly ENSO Wrap-Up; the WMO issues periodic global updates. All are free.

See how the resulting hazards play out on the drought, flood and tropical cyclone monitoring pages.

Frequently asked questions

How long does an El Nino or La Nina last?

Typically 9 to 12 months, though some events persist for two years or more. They recur irregularly every two to seven years.

Does El Nino mean more hurricanes?

It depends on the ocean. El Nino usually suppresses Atlantic hurricane activity by increasing wind shear, while increasing activity in the eastern and central Pacific. La Nina broadly reverses this.

Is El Nino caused by climate change?

No. ENSO is a natural cycle that has operated for thousands of years. It sits on top of a warming baseline, however, so the same phase now produces hotter extremes than it did decades ago.

Can El Nino be forecast in advance?

Yes — this is one of the few areas of genuinely skilful seasonal forecasting. Because ocean heat changes slowly, agencies can identify a developing phase months ahead, which allows governments and farmers to prepare.

What is the Indian Ocean Dipole and how does it relate?

It is a separate oscillation in Indian Ocean sea surface temperature that strongly influences rainfall in East Africa, India and Australia. It can reinforce or cancel out the ENSO signal, which is why El Nino impacts are not identical every time.

Sources

• NOAA Climate Prediction Center — ENSO Diagnostic Discussion
• NOAA Climate.gov — ENSO Blog
• Australian Bureau of Meteorology — ENSO Wrap-Up
• World Meteorological Organization — El Nino/La Nina Update

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