CliMOC · Climate Impacts of the Atlantic Meridional Overturning Circulation

Climate Impacts of the Atlantic Meridional Overturning Circulation

CliMOC investigated the role of AMOC in the global climate, specifically for the European continent. AMOC stands for Atlantic Meridional Overturning Circulation, which is a system of global-scale ocean currents that redistribute heat, salinity and carbon around the world. The AMOC has important consequences for the global climate. Energetically, the AMOC transports more heat into the North Atlantic than in the Southern Hemisphere oceans, which causes an inter-hemispheric imbalance of heat transport. This imbalance is believed to make the Northern Hemisphere warmer by about 1°C than the Southern Hemisphere and to shift the tropical peak of annual mean precipitation (the so called Intertropical Convergence Zone or ITCZ) to be in the Northern Hemisphere at about 5°C. Hence, the ocean heat transport by the AMOC has important consequences on temperature and precipitation, including important rainfall systems such as the tropical monsoons and the extra-tropical cyclones. However, some studies have challenged the notion that the AMOC (and specifically the Gulf Stream) plays a role in specifically making European winters milder than they would be in the absence of the AMOC. By using older modeling setups in which the AMOC was artificially removed by coupling an atmospheric model with a motionless ocean model, these studies found that it’s the Rocky Mountains that contribute to milder European winters by modifying the westerly wind flow at the mid-latitudes. In CliMOC we challenged this notion by proposing a series of novel modelling experiments and setup to investigate the climate response to changes in AMOC strength, with a particular focus on the European continent. We concluded the CliMOC project by finding that our hypothesis was correct. The earlier modeling setups masked the real role of the AMOC in the current climate because the missing ocean dynamics in the motionless ocean model that was used in those studies strongly limited the coupling between the atmosphere and the ocean, plus the associated feedbacks. We designed and made available a modeling experiment setup with state-of-the-art global climate model EC-Earth3 in which we are able to artificially tune the strength of the AMOC. Initial experiments with this setup conducted within CliMOC show an important role of the AMOC for the European hydroclimate, which has of course important implications for future climate change. In fact, because all models within the Coupled Model Intercomparison Project (CMIP) robustly project an AMOC decline in the 21st century, it is of utmost importance to assess the impacts of an AMOC decline relative to those of greenhouse gas forcing.

Data: CORDIS, © European Union