Mid-Holocene ENSO constraints point to future weakening of Walker Circulation

Climate models project a future weakening of the Walker circulation in the tropical Pacific in response to anthropogenic forcing, while a cooling of the eastern equatorial Pacific and a strengthening of the Walker circulation has been observed in the past decades. This discrepancy may arise from models biases in the representation of Pacific dynamics, from a transient response of the ocean atmosphere system, or from unforced decadal climate variability. Here, we propose to use paleoclimate reconstruction of ENSO variance in the mid Holocene to evaluate the skills of CMIP5 and CMIP6 models and constrain climate change projections. The 20 model ensemble shows a slight mean reduction in ENSO variance that underestimates the 50-80% reduction in reconstructions, while exhibiting a large diversity of responses ranging from a 60% decrease to a 40% increase. We show that models that best represent mid Holocene ENSO changes display a weaker modern cold tongue bias, stronger mid Holocene cooling, and a more realistic representation of the ENSO seasonality and wind response to SST. Those models also yield a stronger eastern Pacific warming and zonal gradient reduction by the end of the 21st century in global warming scenarios (SSP585 and rcp85). Although mid-Holocene climate change is driven by orbital forcing rather than GHG, the robustness of this constraint is supported by the fact that ENSO integrates large scale ocean atmosphere feedbacks, which are key to the future response of the Pacific ocean.