What are the key processes that control the size and shape of the global ocean overturning circulation, and how will they combine to influence its future evolution?

The meridional overturning circulation (MOC) of the global ocean has a governing role over the Earth’s climate, a consequence of the enormous capacity of the ocean to store and redistribute heat, freshwater, carbon dioxide and other climatically-important tracers. This results in the MOC being arguably the most prominent feature of the climate system that transcends physical, chemical and biological processes, linking atmosphere-ocean-ice interactions with the carbon cycle and other biogeochemical processes, including ocean acidification. The MOC is known to exhibit variability on timescales from months to millennia, and has the potential to induce abrupt change in climate, most pertinently in the region of the North Atlantic.

The mechanisms governing the strength, shape and variability of the overturning circulation are not fully understood, but include wind forcing, buoyancy forcing and mixing. It has been postulated that the polar regions, where global climate change impacts are predicted to occur earliest and with largest amplitude, play a disproportionately important role in these. A combination of theoretical studies, model simulations and observational analysis (including monitoring of the present state and assessment of paleo conditions) is essential to better understand these mechanisms and to predict future changes.

Can we quantify the meridional overturning circulation (MOC) as a function of space and time?

The MOC moves 90% of the heat and dissolved gases around the planet yet it is very poorly quantified in space and time.

The MOC is changing markedly as the drivers in the polar regions change ever faster (sea ice formation, ice-shelf distribution, temperature and salinity changes etc.).

Understanding and quantification is very low at present, partly because it has been very difficult and expensive to measure but new technology provides important new opportunities to establish an appropriate long term monitoring network. Data from such a global network would offer fundamental improvements to models and hence increase the accuracy of projections.