Cobra Effect · Weather and climate
The warm water that shifts the world’s weather
How warm water off Peru turned out to steer rain and drought around the world.
7 cards, read aloud in 2:52, with a test and sources.
For centuries, fishermen off Peru noticed warm water arriving around Christmas.
In some years it was stronger, and lasted for months. The fish that normally thrived in the cold coastal water became scarce. The warm current came to be called El Nino, the Christ child.
Far away in India, Gilbert Walker was trying to predict the monsoon.
Walker was a British mathematician who became head of India’s weather observatories in 1904. When the monsoon rains failed, crops failed, and famine could follow. He searched weather records from around the world for patterns linked to the rains.
In the 1920s, Walker found a see-saw in the air across the tropical oceans.
When air pressure rose over the eastern Pacific, it tended to fall over the Indian Ocean, and the other way round. He called it the Southern Oscillation. It was linked to rainfall in places far apart. But he could not explain what drove it.
In 1969, Jacob Bjerknes joined the two puzzles together.
Bjerknes was a Norwegian meteorologist working in California. He showed that the warm water off Peru and Walker’s see-saw were two sides of one event. Normally, winds push warm surface water west across the Pacific. When the winds weaken, the warm water flows back east, and that weakens the winds further.
That shift in the Pacific changes weather around the world.
Rain follows the warm water, so some places flood while others face drought. El Nino years often bring drought to parts of Australia and Indonesia, and heavy rain to Peru. Scientists now call the whole pattern ENSO, short for El Nino Southern Oscillation.
In 1982, one of the strongest El Ninos on record arrived almost unnoticed.
Scientists did not recognise it until it was near its peak. Afterwards, countries built a network of instrument buoys across the tropical Pacific. When a powerful El Nino began in 1997, it was spotted early and tracked as it grew. Forecasts can now often warn of an El Nino months ahead.
So when events far apart seem to move together, ask what could connect them.
Walker found the pattern long before anyone could explain it. Bjerknes supplied the explanation decades later. A reliable pattern is worth recording, even before its cause is known. Understanding the cause is what made forecasting possible.
Sources
- Understanding El Niño, NOAA. What El Niño is, and how it affects weather around the world.
- The Rise of El Niño and La Niña, NOAA Climate.gov. How scientists pieced the pattern together.
- History of the Southern Oscillation Index, NOAA Physical Sciences Laboratory. Walker’s see-saw and how it came to be measured.
Nearby ideas
- The first public weather forecasts. How a storm, a telegraph and a sea captain gave us the weather forecast.
- The rounded number that changed the forecast. How a rounded number revealed why the weather can't be forecast far ahead.
- The climate model that passed the test of time. How a simple computer model predicted climate change decades before it arrived.
- The gases that trap heat. How two experiments showed that a few invisible gases keep the Earth warm.
- A year of sums about carbon dioxide. How a chemist with a pencil made the first estimate of global warming.