The El Niño Pacific weather event affects how long the day is, but two types of El Niño do this in two different ways.

Weather changes affect the planet's rotation speed, and thus day length, by changing the atmosphere's pressure over topographical features. A team led by Olivier de Viron, now at the University of La Rochelle in France, studied atmospheric behaviour between 1948 and 2013.

The researchers found that when El Niños make Pacific waters warmer in the east, they set up strong pressure gradients above big mountain ranges (such as the Andes) that increase the time it takes the planet to spin by slightly more than 0.1 millisecond. By contrast, El Niños with warmer central Pacific waters produce only about half as much Earth-changing drag.

Geophys. Res. Lett. http://doi.org/snq (2014)