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Mediterranean overturning circulation

Final Report Summary - MEDOC (Mediterranean overturning circulation)

The overturning circulation, commonly referred to as the great conveyor belt, is driven by large scale temperature and salinity (and thus density) gradients leading to oceanic currents (just like winds are driven by atmospheric temperature (pressure) gradients). In specific oceanic regions, intense air-sea fluxes modify the temperature and salinity of the surface water leading to a buoyancy loss and the formation of deep water which will subsequently flow at depth, slowly mixing with surrounding waters to eventually return to the surface after many decades. This global circulation is a key component of the climate system, transporting heat, freshwater, carbon and nutrients across the world; and acting as a buffer of anthropogenic perturbations by storing excess heat and carbon dioxide in the abyssal layers of the ocean for several decades.

Despite the fundamental importance of understanding this circulation, the time and spatial scales of the overturning (1000s of kms, 100s of years) have limited our understanding of the underpinning physical processes. The aims of the MedOC project are twofold:

1) Investigate how oceanic and air-sea processes shape the water mass structure (the temperature and salinity distribution) of the oceans and how they impact on the thermohaline circulation. This project focuses on the Mediterranean Sea, a miniature ocean with a large-scale circulation very similar to the global ocean but with reduced time and spatial scales, thus offering a unique possibility to look at the variability of this circulation.

2) Improve our understanding of how the overturning circulation shapes the distribution of biogeochemical elements in order to better isolate the impact of biogeochemical processes. The project focused on the distribution of dissolved barium, a trace metal thought to be a good proxy of the remineralisation of particulate matter, and thus of the biological carbon pump.

Concerning the first objective, the project has mainly focused on the analysis of outputs from a State-of-the-art numerical ocean circulation model of the Mediterranean Sea. The model is a setup of the NEMO (Nucleus for European Modelling of the Ocean) model, adapted to the Mediterranean Sea. It is high-resolution (1/12th of a degree of horizontal resolution equivalent to 7-8kms, 75 vertical levels) and has been shown to reproduce some of the important changes in the Mediterranean circulation that have been observed in situ. Following recent methodology applied to the Arctic Ocean, the model outputs are projected in potential temperature-salinity (Θ-S) coordinates. This framework offers unique insights to isolate the relative contribution of air-sea fluxes and interior mixing to the changes in temperature and salinity.

Thanks to this approach, the project MedOC has revealed several key aspects of the Mediterranean Overturning Circulation:
1) The outflow of Mediterranean water in the Atlantic via Gibraltar is mostly controlled by salinity changes due to interior mixing.
2) The volume of outflowing water and its salinity are insensitive to transient water mass changes observed in the Mediterranean Sea.
3) Air-sea fluxes provide the initial buoyancy loss required to start the overturning circulation. Interior mixing dominates in terms of volume of water transformed
4) Interior mixing, previously thought to be small in the Mediterranean Sea due to the absence of significant mechanical forcing, is in fact critical and seems to occur mostly in specific regions of the basin.

Concerning objective 2, in situ data collected during a quasi-zonal section across the Mediterranean Sea from Gibraltar to Lebanon have constituted the core of the study. A simple water mass framework (called Parametric Optimum Multi-Parametric, or POMP, analysis) water employed in order to reconstruct the background distribution of dissolved barium, driven by the oceanic circulation. Comparing this distribution with the observations allowed to identify the presence of several horizons where biogeochemical processes (remineralisation of particulate organic matter or sediment resuspension) played a significant role in setting the distribution of dissolved barium. The main results of this work are:
1) Background D_Ba distribution reflects the balance between the overturning circulation and biogeochemical processes over long timescales.
2) Significant depletion compared with the D_Ba background in the mesopelagic interior indicate organic carbon remineralisation horizons.
3) Water mass framework is a powerful tool to better understand biogeochemical elements distribution and build robust geochemical tracers

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