The influence of climate regime shift on ENSO

Observations indicated that for the El Niño/Southern Oscillation (ENSO) there have been eastward displacements of the zonal wind stress anomalies and surface heat flux (short wave heat flux and latent heat flux) anomalies during El Niño episodes in the 1981-1995 regime relative to the earlier regime of 1961-1975 (without corresponding displacements during La Niña episodes). Our numerical experiments with the Zebiak-Cane coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed climatologies (of sea surface temperature, surface wind and associated atmospheric divergence) and simulated climatologies (of oceanic surface layer currents and associated upwelling) for the 1981-1995 regime. Sensitivity tests indicated that the background atmospheric state played a much more important role than the background ocean state in producing the displacements, which enhanced the asymmetry between El Niño and La Niña in the later regime. The later regime climatology state resulted in the eastward shifts in the ENSO system (wind stress and sea surface temperature) only during El Niño, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. That the change in the mean state of the tropical Pacific atmosphere and ocean after the mid 1970s could have produced the observed changes in ENSO properties is consistent with our findings.