Echoes of El Niño and La Niña affect the Atlantic a year later

The conditions of the tropical Pacific Ocean have an outsize effect on the world’s weather and climate. Indian monsoons and Atlantic hurricanes are among the meteorological phenomena that are heavily influenced by the natural cycle of Pacific sea surface warming (designated by the term El Niño) and cooling (La Niña), known as the El Niño–Southern Oscillation (see the article by David Neelin and Mojib Latif, Physics Today, December 1998, page 32 ). Whereas those linkages depend on the current ENSO phase, researchers have now identified a delayed connection that is tied to the Pacific conditions the previous year.

Researchers have long known that there is a relationship between the ENSO phase and that of the North Atlantic Oscillation (NAO), a cyclical pattern of air pressure differences that affects wintertime conditions in Europe and eastern North America (see the figure). During El Niño winters, the NAO phase tends to be negative: Both a recurring low-pressure system near Iceland and a high-pressure system to the south weaken, and the typically strong west-to-east winds in the North Atlantic slacken. (Conversely, the pressure difference and wind speeds increase during La Niña.)

Adam Scaife, of the University of Exeter and the UK government’s Met Office, and colleagues flagged the delayed connection in an analysis of a century and a half of sea surface temperature and air pressure data for the tropical Pacific, North Atlantic, and Arctic. The newly discovered correlation between ENSO and the NAO is about as strong as the known one, but the effect is reversed: In the winter following El Niño, a positive NAO phase tends to develop. That trend holds regardless of whether El Niño persists. The researchers ran a global climate model fed with historical data and found that it reproduced the one-year lag effect.

Scaife, who heads the Met Office’s long-term predictions, and his team attribute the effect to the atmospheric imprints of an El Niño or La Niña event. The warming or cooling of the tropical Pacific waters alters wind patterns above the equator, changing the rotation of the atmosphere with respect to that of the solid Earth. Previous research has shown that those ENSO-triggered angular momentum anomalies gradually migrate poleward and persist even after El Niño or La Niña wanes. That atmospheric mechanism is distinct from better-studied atmospheric waves that facilitate the concurrent ENSO–NAO connection.

The new finding could help forecasters like Scaife get a leg up on improving long-range predictions of the NAO. Emily Becker of the University of Miami says, “Given the large effect that the NAO has on North American and European winter climate, getting a preview of its potential state a year ahead is valuable.” (A. A. Scaife et al., Science 386, 82, 2024 .)