Benoit Thibodeau, Erwin Lambert

The Arctic Mediterranean can be described as a double estuarine circulation regime. This observed circulation feature, which connects the North Atlantic to the Arctic Ocean, is composed of two interconnected branches of circulation: an overturning circulation, where dense water formed in the Nordic Seas returns toward the Atlantic and an estuarine circulation, where the East Greenland Current exits the Arctic Mediterranean. A conceptual box model has previously built upon Henry Stommel’s original version, concluding that a double estuarine circulation is less sensitive to perturbations in northern freshwater input than an overturning circulation in isolation. This extended model exhibits a similar freshwater sensitivity to several coupled atmosphere-ocean general circulation models (AOGCMs), which require about 1 Sv of freshwater input to induce a transition to a qualitatively weakened overturning in the Atlantic Ocean. Besides the amount of freshwater that would be required to abruptly weaken the Atlantic overturning circulation, it is essential to determine over what time scale such a transition could occur. To address this temporal aspect of potential abrupt transitions in a double estuarine circulation, we built a numerical version of the box-model in Stella®. The Stella® version of the Arctic Mediterranean Double Estuarine Circulation model (SAMDEC) is thus a new and widely-available numerical box-model representing the Arctic Mediterranean Double Estuarine Circulation and is intended to provide a numerical tool to easily solve this double estuarine theoretical framework. In addition to its simplicity of use, one of the most important added values of SAMDEC is the ability to easily determine and visualise transition times between two circulation regimes at very low computing cost, making it a valuable tool for research and education. This allows for a quantitative assessment of the response of the Arctic Mediterranean circulation to variable freshwater fluxes and temperature changes over short and long time scales. To highlight the features of SAMDEC, we showcase here two freshwater flux scenarios; 1) increased freshwater input in the Nordic Seas, which is most comparable to common ‘hosing’ experiments; and 2) increased freshwater input in the Nordic Seas and the Arctic. Similar to previous experiments performed with AOGCMs, the weakening caused by realistic freshwater addition is relatively slow during the first 100 years of simulation and increases thereafter. Finally, under a realistic freshwater increase, SAMDEC indicates that 88 to 176 years are needed to achieve a 15% weakening of the overturning. Overall, SAMDEC can provide insights into the dynamic of transition between circulation regimes under changes in freshwater input for both near-future and geological timescale investigations. A light version of the model can be accessed online with any internet browser and the full model can be downloaded, modified and used on a personal computer.