The Deep Arctic Ocean and Fram Strait in CMIP6 Models

Abstract

Abstract 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 that of 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 phase 6 of the Climate Model Intercomparison Project. Compared to observational climatologies and hydrographic profiles, the modeled Atlantic layer core is on average too cold by −0.4°C and too deep by 400 m in the Nansen Basin. The Atlantic layer is too thick, extending to the seafloor in some models. Deep and bottom waters are in contrast too warm by 1.1° and 1.2°C. Furthermore, the modeled 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 volume fluxes through Fram Strait, biased low by up to 6 Sv (1 Sv ≡ 106 m3 s−1), 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. Significance Statement Coupled climate models are routinely used for climate change projection and adaptation, but they are only as good as the data used to create them. And in the deep Arctic, those data are scarce. We determine how biased 14 of the most recent models are regarding the deep Arctic Ocean and the Arctic’s only deep gateway, Fram Strait (between Greenland and Svalbard). These models are very biased: too cold where they should be warm, too warm where they should be cold, not stratified enough, not in contact with the surface as they should, moving the wrong way around the Arctic, etc. Some problems are induced by biases in regions outside of the Arctic and/or from the sea ice models.