Toward Understanding the Suppressed ENSO Activity during the Mid-Holocene in PMIP2 and PMIP3 Simulations

The mechanisms of El Niño–Southern Oscillation (ENSO) variability change during the mid-Holocene (MH) were investigated through analyzing the model simulations from the Paleoclimate Modelling Intercomparison Project Phases (PMIP) phase-2 and phase-3. The majority of PMIP2 and PMIP3 model simulations show a lower level of ENSO activity in the MH simulation compared to the pre-industrial (PI) simulation, which is qualitatively consistent with that inferred from the paleoclimate proxies. Through employing the Bjerknes stability index, we quantified the dynamic and thermodynamic air-sea feedbacks in the PI and MH simulations. The quantitative analyses showed that the reduced ENSO variability in MH arose from the weakening in the thermocline (TH), zonal-advection (ZA) and Ekman (EK) feedback terms. We found that all the weakened TH, ZA, and EK terms are associated with the reduction in the response of anomalous thermocline depth to the zonal wind stress anomaly in MH compared to PI. The reduced response of anomalous thermocline depth is attributed to the flattened meridional structure of ENSO-related wind field in MH, which is linked to the enhanced surface poleward mean meridional current in MH. Among many aspects of the mean state changes under the MH orbital forcing, this study identified that the surface mean meridional current change might be a key factor behind the suppressed ENSO variability in MH. Lastly, through comparing our findings with the ENSO future projection studies, we found that the wind-thermocline feedback is susceptible in a changing climate, which implies that minimizing the uncertainty in the wind-thermocline feedback change may help constrain future ENSO response.