Mixed layer depth in the PMIP4 midHolocene simulations: comparison to proxy data in North Atlantic deep convection regions

Abstract

Abstract. The ocean mixed layer plays an essential role in the climate system, regulating energy fluxes at the ocean-atmosphere interface. Its representation in climate models is thus critical. Here, we evaluate the mixed layer depth (MLD) in 15 models from the Paleoclimate Modelling Intercomparison Project 4 (PMIP4) against dinocyst-based MLD reconstructions from the subpolar North Atlantic for the mid-Holocene (MH, 6 ka BP). We find a large spread in MLD responses to MH forcings across the models in the present-day deep-water formation areas, underscoring the importance of model uncertainty. Most models fail to capture the direction of MLD change, and the ensemble mean does not necessarily outperform individual models. While the ensemble mean aligns broadly with proxy data in the Nordic Seas, pronounced proxy-model discrepancy in the Labrador Sea suggests that meltwater forcing is a missing parameter, and that deep-water formation in the Labrador Sea may be particularly vulnerable under a future scenario of global warming and ice-sheet melting.