Reduction in ENSO variability during the mid-Holocene: a multi-model perspective

Paleoclimatic reconstructions have suggested a reduction inthe variability of the El Niño Southern Oscillation (ENSO) during the mid-Holocene (MH). Model simulations have largely failed to capture thisreduction, potentially due to the inadequate representation of the Green Sahara.The presence of a vegetated Sahara has been shown to have significant impacts on both regional and remote climate but remains inadequately addressed in Paleoclimate Modelling Intercomparison Project / Coupled Model Intercomparison Project (PMIP/CMIP) boundary conditions. Specifically, the incorporation of a Green Sahara has been shown to impact ENSO variability through perturbations to the Walker Circulation. In this study, we evaluate the MH (6,000 years BP) ENSO signatures of simulations from four models, namely —EC-Earth 3.1, iCESM 1.2, University of Toronto version of CCSM4 and GISS Model E2.1-G. Two simulations are considered for each model—a standard PMIP simulation (MH_PMIP) with the mid-Holocene orbital parameters and greenhouse gas concentrations with vegetation prescribed to preindustrial conditions, as well as a Green Sahara simulation (MH_GS) which additionally incorporates factors such as enhanced vegetation, reduced dust, presence of lakes, and land and soil feedbacks. All models show a reduction in ENSO variability due to the incorporation of Green Sahara conditions. This variability is interpreted in the context of perturbations to the Walker Circulation, triggered by the strengthening of the West African Monsoon.