Asymmetry in zonal phase propagation of ENSO sea surface temperature anomalies

It is often emphasized in the literature that the phase propagation of El Nino sea surface temperature (SST) anomalies along the equator changed from westward to eastward after a mid-to late-1970s climate regime shift in the Pacific. Theories have been developed to explain this change of phase propagation in terms of changes in background state on which El Nino events develop. Those theories also suggest that the direction of La Nina anomaly phase propagation should have changed from westward to eastward as well. However, the direction of La Nina SST anomaly phase propagation did not change after the mid- to late-1970s. Instead, La Nina SSTs continued to exhibit westward phase propagation along the equator, a feature overlooked in both observational analyses and theories. This paper highlights the asymmetry in zonal phase propagation between El Nino and La Nina sea surface temperature anomalies since 1980 and discusses the implications of that asymmetry for understanding El Nino/Southern Oscillation (ENSO) dynamics. Citation: McPhaden, M. J., and X. Zhang (2009), Asymmetry in zonal phase propagation of ENSO sea surface temperature anomalies, Geophys. Res. Lett., 36, L13703, doi: 10.1029/2009GL038774.