Properties of El Niño–Southern Oscillation in Different Equilibrium Climates with HadCM3

Abstract

The ENSO variability in three long, stable, steady-state integrations of the Third Hadley Centre Coupled Ocean?Atmosphere GCM (HadCM3) is analyzed, relevant to climatic conditions of the Last Glacial Maximum (LGM), of the preindustrial period [control (CTL)], and of a greenhouse stabilization scenario (GHS) at 4 times the preindustrial CO2 concentration. It is found that progressively from LGM to CTL and GHS, the SST variability pattern associated with ENSO is centered farther west, and the oscillation acquires a shorter dominant period. While there are no large changes in total SST variability, very strong events become less frequent, and El Niño events develop over a narrower period within the seasonal cycle. The westward ENSO pattern shift is concurrent with a similar shift in the climatological wind stress distribution and with increased convective activity over the west equatorial Pacific. The wind response to anomalous SSTs follows this shift and increases in strength. The thermocline feedback becomes stronger in the 4 ? CO2 integration, but the largest SST anomalies are associated with surface processes. The role of surface flux damping for the decay of anomalous SSTs is reduced in LGM and increased in GHS. From the analysis, the principal changes in mean climate that appear to affect the evolution of ENSO-related SST anomalies in HadCM3 are thus the changes in zonal wind stress over the equator, the depth of the equatorial thermocline, and the sensitivity of atmospheric convection to equatorial SST anomalies.