The relationship between ENSO variability and the mean state: results from the paleoclimate simulations

The changes in El Niņo-Southern Oscillation (ENSO) features are analyzed from the mid-Holocene (MH) and Last Glacial Maximum (LGM) simulations of the Paleoclimate Modeling Intercomparison Project phase 3 (PMIP3). The Bjerknes stability (BJ) index (Jin et al., 2006) is used to access the ENSO stability and its changes associated with the mean state. Results show a large spread in ENSO stability as defined by BJ index, which may be related to the different representation of the mean climate and the air-sea interactions. Comparisons of the respective components in the BJ index with those derived from the observation show that all the PMIP models underestimated the positive effect of the thermocline feedback in the pre-industrial (PI) simulations. In the paleoclimate simulations, most of the PMIP3 models show a positive correlation between the changes in BJ index and in ENSO amplitude (the larger BJ index the stronger ENSO amplitude, and vice verse). The changes in thermocline feedback and zonal mean advection are found to be more important in regulating the ENSO amplitude; however, no consistent changes (increased or decreased) can be concluded among models.