What changes when La Nina ends?
d ~ 388'492'800 km: day 151
Image © NASA JPL Ocean Surface Topography Team. (La Nina is shown in this Ocean Surface Topography Mission (OSTM)/Jason-2 satellite image of the Pacific Ocean, based on the average of 10 days of data centered on Dec. 26, 2010. The image depicts places where the Pacific sea-surface height is higher (warmer) than normal as yellow and red, while places where the sea surface is lower (cooler) than normal are shown in blue and purple. Green indicates near normal conditions)
It's easy to forget how large our planet is. If you were to start anywhere on the Equator and follow it around the planet, you'd pass 25,000 miles of mountains, forests, deserts and ocean (mostly ocean) before you arrived back at your starting point. And when you got back, covered in sand, salt, tropical insects and the odd bird dropping, it would be hard to imagine that the weather over a full third of that enormous distance is all connected. But that's what El Niño and La Niña represent - two extreme states of one huge system that covers a third of the Equator.
Digesting that fact might stretch the brain a bit, but it doesn't stop there. The atmosphere and the ocean are equal partners in all this. When the airflow across all those 10,000 miles of the Equator changes, the flow of millions of tonnes of ocean water changes too. Today, we can see that a huge change in the flow of all this air and water is about to happen - a period of La Niña conditions is giving way to "neutral" conditions. But what does that mean?
Let's start in the atmosphere near South America. High up, there's cold dry air travelling eastwards from Australia. It hits the Andes, and it sinks. That pushes down on the air that was already there and this lower air is pushed back westwards along the Equator to Australia. So we have a giant conveyer belt of air that is travelling west along the Equator at the ocean surface. If you blow sideways on a hot cup of tea, you'll see that you push the surface tea away from you, and this is what happens in the ocean. The water at the surface of the ocean is pushed along to the west. But what takes its place? The answer is cold water from deeper down in the ocean, so in normal conditions, the water right at the South American coast is cold, because all the warm water is being pushed along towards Australia. This is a "neutral" state.
El Niño happens when that westwards flow of air weakens, and so the warm water can slosh back towards South America. This is where the name "El Niño" came from - it's what Peruvian fishermen called the warm water that sporadically appeared off their coast. When this happens, the cold water never reaches the surface. La Niña conditions are the opposite - the westwards winds get even stronger, and push the warm water even further west. It's a bit like a giant water seesaw. This enormous system of air and water switches between these two states every few years, but we can't really predict when it's going to happen.
During El Niño years, there can be droughts in Australia and Indonesia, and whole ecosystems can be disrupted because the nutrient-rich cold water never reaches the ocean surface. In La Niña years, there is really strong rainfall in Australia and much less rain in South America.
Image © NASA JPL