Impact of a decrease in the intensity of deep ocean circulation in the North Atlantic on the climate of Western Europe

Details

Supervisors

Goosse, Hugues

Faculty

Faculté des sciences

Degree label

Master [60] en sciences physiques

Abstract

enThe Atlantic Meridional Overturning Circulation (AMOC) plays a fundamental role in shaping European climate through northward heat transport, while European atmospheric circulation is strongly regulated by the North Atlantic Oscillation (NAO). Using the Canadian Earth System Model version 5 (CanESM5) under the NAHosMIP framework, this thesis examines the influence of the NAO on European temperatures under AMOC weakening and the physical mechanisms governing the changes in European temperature standard deviation. The findings reveal that an AMOC decline causes widespread continental cooling (from -1.17°C to -2.62°C on average), which is most severe across Northern Europe and during winter. Under this low-AMOC state, the relative contribution of the NAO to European temperatures increases substantially, with its influence becoming especially dominant during winter over Northern Europe. Negative NAO phases amplify this cooling via expanded sea ice and displaced thermal gradients, while positive phases dampen it. The NAO contribution to European temperatures becomes more important under AMOC weakening. Furthermore, a weakened AMOC enhances North Atlantic temperature standard deviation in regions similar to those of sea ice expansion and to the southward shift of thermal gradients, affecting the Northern European region and the winter season as well. An advection analysis shows this increase to be thermodynamically driven by increased horizontal temperature gradient fluctuations rather than wind volatility. These results demonstrate that European temperatures under low-AMOC state are governed by coupled interactions between oceanic heat transport, atmospheric circulation, and sea ice feedback.