El Niño is coming for your gravity!
Above the surface, there are fluids that can move independently of the Earth, like the atmosphere. That motion can actually apply a torque that acts to speed up or slow down the Earth’s rotation. The El Niño Southern Oscillation is a major source of year-to-year variability in Earth’s average surface temperature and, it turns out, its rotational velocity. During La Niña conditions, the winds conspire to push warm surface water in the Eastern Pacific westward, bringing cooler water up to the surface. Conversely, during an El Niño, the warm surface water extends to the eastern side of the Pacific, keeping a lid on the cool water beneath. This difference has a large effect on atmospheric circulation patterns.
It has been known for a while that this manages to slightly alter the Earth’s rotation, but University of La Rochelle researcher Olivier de Viron and Jean Dickey of NASA’s Jet Propulsion Laboratory set out to study how two slightly different flavors of El Niño compare. In the canonical El Niño, the warmest surface water (relative to the local average) is found in the Eastern Pacific. In what’s sometimes referred to as an El Niño “Modoki,” however, that peak anomaly is located in the Central Pacific. That means that the atmospheric circulation patterns are a little different, as are the regional weather impacts.
The researchers worked with a global “reanalysis”—a sort of atmospheric model that produces global conditions consistent with all the available data—going back to 1948 to compare the effects of Eastern Pacific and Central Pacific El Niños. They found that broad areas of higher and lower atmospheric pressure set up in different locations. And that meant that the Eastern Pacific El Niños would have a larger impact on Earth’s rotation, lengthening the day by a little over 0.1 milliseconds, as compared to about 0.05 milliseconds for the Central Pacific El Niños.