17 Jul 2023
 | 17 Jul 2023

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

René M. van Westen and Henk A. Dijkstra

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.

René M. van Westen and Henk A. Dijkstra

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1502', Anonymous Referee #1, 23 Aug 2023
    • AC1: 'Reply on RC1', René van Westen, 25 Oct 2023
    • RC2: 'Reply on RC1', Anonymous Referee #2, 31 Oct 2023
  • EC1: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 19 Oct 2023
  • EC2: 'Comment on egusphere-2023-1502', Bernadette Sloyan, 01 Nov 2023
René M. van Westen and Henk A. Dijkstra
René M. van Westen and Henk A. Dijkstra


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Short summary
The Atlantic Meridional Overturning Circulation (AMOC) is an important component in the global climate system. Observations of the present-day AMOC indicate that it may weaken or collapse under global warming, with profound disruptive effects on future climate. However, AMOC weakening is not correctly represented because an important feedback is underestimated due to biases in the Atlantic's freshwater budget. Here we address these biases in several state-of-the-art climate model simulations.