Impact of freshwater release in the Mediterranean Sea on the North Atlantic climate
Sediment cores from the Mediterranean Sea have evidenced several periods of Sapropel deposition, which can be explained by events of anoxic bottom conditions. An explanation for such events calls for a very stratified sea, possibly related with freshwater input through increased precipitations and runoff discharges. It has been suggested that such a stratified Mediterranean Sea may in turn weaken the Atlantic meridional overturning circulation (AMOC) through changes in the Mediterranean overflow water (MOW). Nevertheless, models used to establish this result were relatively simple and the mechanisms leading to such an impact remained elusive. To improve on those previous studies, we analyse the impact of different freshwater releases with rates of 0.2, 0.1, 0.05 and 0.02 Sv (1 Sv = 106 m3/s) in the Mediterranean Sea using the IPSL-CM5A-LR model in a few multi-centennial simulations. We focus the analysis on the impact of a decrease in the MOW on the large-scale Atlantic circulation. We find a consistent change in horizontal currents in the upper Atlantic Ocean in all simulations in the first century, with a large enhancement of the northward current west of Rockall in the northeast Atlantic. Concerning the AMOC response, we identify three different processes that impact its fate. The first is related to changes in geostrophic currents at depth induced by the disappearance of the MOW, which tends to weaken the AMOC. On the contrary, the second enhances the AMOC and is associated with the increase in northward currents in the horizontal upper circulation. The last process is due to the spread of surface freshwater anomalies out of the Mediterranean Sea that freshens the North Atlantic convection sites and weakens the AMOC. Depending on the rate of the freshwater release, the strength and balance of these three processes are different. For rates larger than 0.05 Sv, we observe a strong reduction of the AMOC, while for lower rates, we notice an enhancement in the upper cell. The climatic response follows that of the upper AMOC with a warming of the North Atlantic for rates lower than 0.05 Sv and a cooling for higher rates. Given that past estimates of freshwater release in the Mediterranean Sea indicate rates lower than 0.05 Sv, we argue that Sapropel events may have enhanced of the upper AMOC and warmed of the North Atlantic.
We thank Eleanor Georgiadis for very significant help in the language improvement of this paper, Julie Deshayes for enlightening discussion concerning the parametrization used in the NEMO model, Jonathan Beuvier for useful discussions on the present-day Mediterranean circulation and Olivier Marti for well-thought insights concerning the way to perform sensitivity experiments. The research leading to this study has received funding from the French National Research Agency HAMOC project (Grant ANR-13-BS06-0003). This work was granted access to the HPC resources of TGCC under the allocation No. 2016-017403. We thank Patrick Brockmann for help with figure design. This research also benefited from Blue-Action project (European Union’s Horizon 2020 research and innovation program, Grant Number: 727852).
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