Circulation and transformation of Atlantic and Arctic water masses in climate models

Abstract

Ocean heat transport and associated heat loss to the atmosphere contributes significantly to the anomalously mild climate of northwestern Europe and its variability. In this thesis, the circulation and transformation of water masses in the northern North Atlantic and the Nordic Seas have been assessed and explored in state-of-the-art climate models. A most important aspect of model evaluation is to identify the degree of realism in model climatology and variability, e.g., for model improvement or in order to assess the potential for decadal-scale climate prediction. A main approach for assessing simulated ocean circulations herein is water mass analysis as routinely applied in observational oceanography. Air-sea exchange and water mass transformation at northern high latitudes are accordingly related to the Atlantic Meridional Overturning Circulation (AMOC). The variable overturning of the Bergen Climate Model (BCM) – the core model system in this thesis – is found to reflect decadal variability in dense water formation in the Labrador Sea and in the oceanic heat transport into the Nordic Seas, the overall constraint on the northernmost water mass transformation. The simulated AMOC is strongly interconnected with the horizontal Subpolar Gyre circulation. Decadal variability of BCM’s Subpolar Gyre, as its AMOC, can partly be explained as a response to distinct patterns of atmospheric variability. The intercomparison of BCM with two other climate models finds the model pathways for the North Atlantic Current and the model sea-ice covers to differ substantially, and hence their oceanic poleward transport of heat, their air-sea exchange, and consequent northern water mass transformation to be very different.