Persistent Climate Model Biases in the Atlantic Ocean's Freshwater Transport

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is considered to be one of the most dangerous climate tipping elements. From idealised model studies, it is known that the tipping behaviour is caused by a positive salt-advection feedback, which is strongly connected to the Atlantic Ocean's freshwater transport. In earlier model studies, using climate models of the Coupled Model Intercomparison Projects (phase 3 and phase 5), biases in this freshwater transport have been identified. Here, we show that these biases persist in CMIP phase~6 models, as well as in a climate model with an eddying ocean, and provide a more detailed analysis of the origin of the biases. The most important model bias is in the Atlantic Surface Water properties, which arises from deficiencies in the surface freshwater flux over the Indian Ocean. The second largest bias is in the properties in the North Atlantic Deep Water and arises through deficiencies in the freshwater flux over the Atlantic Subpolar Gyre region. Due to the biases, the Atlantic Ocean's freshwater transport is not in agreement with available observations and the strength of the salt advection feedback is underestimated. To better project future AMOC behaviour, an urgent effort is needed to reduce biases in the atmospheric components of current climate models.