The deep Arctic Ocean and Fram Strait in CMIP6 models

: Arctic sea ice loss has become a symbol of ongoing climate change, yet climate models still struggle to reproduce it accurately, let alone predict it. A reason for this is the increasingly clear role of the ocean, especially the "Atlantic layer", on sea ice processes. We here quantify biases in that Atlantic layer and the Arctic Ocean deeper layers in 14 representative models that participated in the Climate Model Intercomparison Project phase 6. Compared to observational climatologies and a database of hydrographic profiles, the modelled Atlantic layer core is too cold by on average -0.4 ◦ C and too deep by 400 m in the Nansen basin, in too thick a layer that, in some models, extends to the seafloor. Deep and bottom waters are in contrast too warm by 1.1 and 1.2 ◦ C. Furthermore, the properties hardly change throughout the Arctic. We attribute these biases to an inaccurate representation of shelf processes: only three models seem to produce dense water overflows, at too few locations, and these do not sink deep enough. No model compensates with open ocean deep convection. Therefore, the properties are set by the inaccurate fluxes through Fram Strait, biased low by up to 6 Sv, but coupled to a too-warm Fram Strait, resulting in a somewhat accurate heat inflow. These fluxes are related to biases in the Nordic Seas, themselves previously attributed to inaccurate sea ice extent and atmospheric modes of variability, thus highlighting the need for overall improvements in the different model components and their coupling.