Atmosphere slows Earth’s rotation during El Niño

Ars Technica : El Niño and La Niña events are known to slow and speed Earth’s rotation, respectively, by a fraction of a second. Now Olivier de Viron of the University of La Rochelle in France and Jean Dickey of NASA’s Jet Propulsion Laboratory believe they’ve figured out why. They used historical atmospheric data dating back to 1948 to model global conditions during El Niño and La Niña periods. They then compared traditional El Niño effects, when the area of warmest surface water is located in the Eastern Pacific, to a variant El Niño, when the warmest water is in the Central Pacific. During traditional El Niños, atmospheric low-pressure systems commonly formed on the western sides of three major mountain systems—the Andes, the Rockies, and the Himalayas. The higher pressure on the other side of the mountains resulted in a push westward, counter to Earth’s rotation, which temporarily slowed it and thus lengthened the day by 0.1 ms. During the variant El Niños, the low-pressure systems did not line up with the mountain ranges, which meant that less force was exerted and thus only 0.05 ms was added to the length of the day. During La Niña periods the process is reversed, which shortens the day.