Easterly anomalies during El Niño in January 2024: What caused the “decoupling”?

Easterly anomalies occurred in the equatorial central Pacific “unexpectedly” in January 2024, when an El Niño was in its peak. The cause of this “abnormal” zonal wind condition is investigated through the decomposition of multi-timescale signals. The result indicates that the easterly anomalies arose from the combined effects of the Madden-Julian Oscillation (MJO), the interdecadal variability-global warming trend (IDGT) signal and the “pure” interannual signal. It is found that a slowly moving active-phase MJO appeared over the Indo-Pacific warm pool during January 2024, and as a result, there is a net positive precipitation anomaly over the warm pool, leading to easterly anomalies in the western-central equatorial Pacific. The IDGT signal since 1979 exhibits an enhanced zonal SST gradient across the equatorial Pacific, which strengthens the trade wind at the equator. The summation of the MJO and IDGT signals surpassed the El Niño induced westerly anomaly, leading to an unexpected equatorial easterly anomaly in January 2024. An assessment of observational data since 1979 shows a 10% chance of the occurrence of such an “uncoupling” during El Niño, during which the MJO and/or IDGT modes did play a role.