European seasonal temperature change since the Last Glacial Maximum from climate models

Abstract. The Last Glacial Maximum (LGM, ~21 ka BP) is a key period for evaluating General Circulation Models (GCMs) used for future climate projections. Determining whether regional cooling reflects a uniform decrease in monthly temperatures throughout the year (i.e., no change in temperature seasonality) or a change in temperature seasonality, would have significant implications for many paleotemperature reconstructions, including those based on paleo-glacier equilibrium lines. In this study, we provide an assessment of GCMs as well as iLOVECLIM Earth system model of intermediate complexity downscaled over Europe in terms of temperature seasonality and of its changes at the LGM, compared to the pre-industrial era. Although models show a large dispersion, some common features emerge. The average temperature seasonality decreases from PI to LGM over southern Europe, while it increases further north. Models simulate a slight and consistent increase in seasonality changes along the Atlantic coast, but a larger and more variable increase over eastern Europe. We identify variations in LGM MTCO (mean temperature of the coldest month) as the primary drivers of both temperature seasonality changes and inter-model dispersion in across the GCM and iLOVECLIM outputs, with the largest disagreements occurring in northeastern Europe, over and near the Fennoscandian ice sheet. Motivated by the fact that the iLOVECLIM model produces some features largely different from the GCMs, we perform a series of sensitivity experiments with iLOVECLIM, to understand its different sensitivity to LGM boundary conditions and forcings. These experiments study the sensitivity of the simulated temperature seasonality amplitude to changes in greenhouse gas concentration, thermohaline circulation, albedo and topography of the Fennoscandian ice sheet and vegetation cover. We show that none of these processes, including glacial forcings (inter-model ice sheet extents), explain the observed seasonality change mismatch between iLOVECLM and GCMs. A significant reduction in this mismatch is achieved only by modifying the vertical parametrisation of radiative profiles in iLOVECLIM, suggesting that the lack of an explicit vertical representation of radiative profiles in iLOVECLIM may bias the simulated temperature seasonality changes at the LGM relative to PI. Pollen-based reconstructions are generally consistent with model results. However, the regions that display the largest inter-model differences are also not covered by this type of reconstruction, which is restricted to areas outside the ice sheets. European temperature seasonality since the LGM therefore remains a key yet poorly constrained characteristic of LGM climates, calling for more single-model sensitivity experiments to improve our understanding of past and future temperature seasonality changes.