Diagnosing and Understanding the AMOC Biases in NorESM

Abstract

We investigate the Atlantic Meridional Overturning Circulation (AMOC) in the Norwegian Earth System Model (NorESM) featuring isopycnal ocean component (MICOM). The Coupled Model Intercomparison Phase 5 (CMIP5) NorESM historical simulations showed a decline of the AMOC after 1980 concurring with the recent observations from RAPID-MOCHA program. The NorESM future projections predict reduction of the AMOC with 12 to 30% under different warming (RCP2.6-RCP8.5) scenarios. In the CMIP5 model intercomparison project, the NorESM ocean component demonstrated an intense AMOC and took place in the upper end of the AMOC magnitudes model range. The NorESM AMOC strength was found to be sensitive to oceanic grid resolution and whether coupled or uncoupled configuration is used. However, the AMOC tends to be on the strong side in all configurations. In order to find the causes of this vigorous AMOC we carried out a careful diagnostics of the AMOC and explored possible relationship to the model biases found in the Atlantic thermohaline structure, and water mass formation. Several processes has been investigated to understand further their connection and significance to the AMOC strength and variability: 1) The North Atlantic Mode Waters Formation (STMW and SPMW); 2) The Labrador Sea Water Mass formation and variability. Furthermore, the AMOC sensitivity to sub-grid scale physical parameterizations such as isopycnal eddy mixing and the impact of model resolution on the representation of overflows is examined.