An Extended Empirical Orthogonal Function (EEOF) analysis of sea surface temperature anomaly (SSTA) with a 41-year moving window shows a marked interdecadal shift of El Niño onset location around 1970, that is, El Niño started its warming in eastern Pacific (EP) before 1970 but in western Pacific (WP) after 1970. Accompanied to the distinctive onset location shift are opposite zonal propagation directions of the maximum SSTA center (Fig. 1a-b). This characteristic is consistent with the occurrence of a moderate eastern Pacific (MEP) onset type prior to 1970 but a strong basin-wide warming (SBW) and a moderate central Pacific (MCP) type after 1970 (Wang et al. 2019), as seen in Fig. 1c-n.

Physical mechanisms responsible for this interdecadal shift was investigated through theoretical, modeling and observational analyses. Firstly, we constructed a simple coupled atmosphere-ocean model suitable for air-sea interaction in WP. It consists of the Gill model for the atmospheric component and ocean mixed-layer temperature (MLT) tendency and ocean mixed-layer momentum equations for the oceanic component. An eigenvalue analysis of the coupled model demonstrates that the post-1970 mean state favors the growth of an SSTA perturbation in WP whereas the pre-1970 mean state does not. The dominant process responsible for the instability is the wind-evaporation-SST (WES) feedback, while the Ekman feedback and the zonal advective feedback also play a role (Fig. 2c-d). Therefore, SSTA perturbations can be easily triggered and maintained in WP under the post-1970 mean condition.

It is further revealed distinctive preceding SSTA patterns occurred prior to El Niño onset between the pre- and post- 1970 periods. A single-pole La Niña pattern appeared in the preceding winter (DJF[0]) prior to 1970, whereas a dipole SSTA pattern with a positive (negative) SSTA in WP (EP) occurred after 1970 (Fig. 1c, g, k). The different SSTA patterns led to distinctive zonal wind responses at the equator. In the former, weak westerly anomalies appeared in EP in DJF(0), which worked together with ocean dynamic processes and triggered El Niño onset in EP a season later. In the latter, pronounced easterly anomalies appeared in EP, which suppress initial warming in situ.

While the post-1970 mean state favors the growth and maintenance of an SSTA perturbation in WP, two specific processes triggered El Niño onset in WP after 1970. One is via anomalous downward solar radiation forcing in connection with atmospheric meridional overturning circulation. Another is via anomalous zonal advection in association with thermocline-induced geostrophic currents.

The current study reveals, from both dynamical and thermodynamical air-sea coupling perspectives, the role of the interdecadal mean state change in causing the El Niño onset location shift. Through a simple theoretical model, we demonstrate the important role of the background mean state in particular mean moisture change (Fig. 2a- b) in affecting the SSTA growth in WP. The diagnosis of the mixed-layer heat budget further reveals key processes triggering El Niño onset prior to and after 1970. Idealized atmospheric GCM experiments confirm the role of the preceding dipole and single-pole SSTA patterns in causing distinctive zonal wind responses in EP. This study sheds lights in understanding the future projection of El Niño. We anticipate that more frequent El Niño onset will happen in WP as long as the greater WP warming trend persists.

Reference:

Pan, X., Li, T. & Yu, J. Change of El Niño onset location around 1970. npj Clim Atmos Sci 7, 163 (2024). https://doi.org/10.1038/s41612-024-00709-y