Over the Pacific Ocean, air pressure rises and falls, repeating every few years. When pressure shifts, the ocean warms or cools, and weather around the globe goes haywire: droughts in some places, floods in others. Scientists have long noticed these cycles but could not explain why they are so long, from two to fourteen years. Ordinary atmospheric waves, which arise from Earth's rotation, live no longer than a hundred days. So there must be another type of wave.
Air near the ground is dense, while high in the sky it is thin, like cotton. This density difference with height is called stratification. Because of it, the atmosphere can host waves that move not only horizontally but also vertically. These are three-dimensional slow Rossby waves. They resemble slow breaths of a giant air ocean. In the new study, scientists calculated the periods of such waves and found that they match the periods of the Southern Oscillation, from 2.5 to 14 years.
Now that we know slow Rossby waves set the rhythm of climate, we can better predict El Niño and its impacts. This will help farmers prepare for droughts and cities for floods. Previously, the cause of the long periods was a mystery; now it is explained by the simple physics of waves in a layered, rotating atmosphere.
The periods of slow Rossby waves are comparable to the time spent in school: from 2.5 to 14 years. The Southern Oscillation Index is measured by the pressure difference between Tahiti and Darwin, and its fluctuations are visible even after smoothing over 5 months.