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A long-standing challenge for ocean carbon cycle research is identifying the pathways and mechanisms regulating the entry of carbon deriving from human activities into the ocean.

A new study, using a water mass/density framework, has identified the key role of the subtropical overturning circulation in forming the important subpolar mode water reservoirs for the storage of anthropogenic carbon in the ocean; about 20% of the oceanic carbon is redistributed by this overturning. The paper was published in Scientific Reports on November 3rd, 2016.

A team of researchers with members from Italy, the U.S., France, the UK, and Japan has found that the oceanic circulation structures play a dominant role in determining the pathways by which anthropogenic carbon enters the layered structure of the ocean interior, with implications for both the climate system and ocean acidification. In their paper published in the journal Scientific Reports (www.nature.com/articles/srep35473), the researchers present a new framework and method for analysing observational assets and ocean carbon models that sheds new light on ocean chemical cycles.

Greenhouse gas emissions into the atmosphere from human activities are changing the Earth's climate, altering temperature and weather patterns at global scale. Over the decade 2005-2014, human activity released to the atmosphere nearly 10 gigatons of carbon per year on average. In fact while 4.4 gigatons per year have gone into the atmosphere, a net 2.6 gigatons went into the oceans. In total present-day oceans store more than 150 gigatons of carbon from past human activities. The level of acidification and the residence time away from the atmosphere of this enormous reserve grossly depends on the depth of the layers where the carbon is stocked. Recently a significant threshold was crossed, with atmospheric CO2 concentration levels now being in excess of 400 parts per million, with this being significantly larger than pre-industrial levels of 286 parts per million. This sets the stage for the question of whether the mechanisms by which the ocean has thus far absorbed approximately 30% of CO2 emissions will be perturbed under climate change.

The new study, led by D. Iudicone (Stazione Zoologica Anton Dohrn, Italy), suggests that 20% of the oceanic carbon sink (over 0.52 gigatons per year when considering the last decade) is redistributed across the internal layers by the oceanic currents. How does this work? The ocean is continuously on the move, and thus it does not act like as a sponge, absorbing the carbon dioxide gas and then diffusing it slowly downward. The global ocean is made of a complex web of 3D conveyors that continuously redistribute water horizontally and vertically between the layers (overturning). This machinery can expose formerly deep, carbon-rich layers to the atmosphere or sequester carbon to depth for long times. Until now much of the interest was on polar regions despite the fact that tropical and subtropical oceanic regions make about 70% of the interface between the ocean and the atmosphere. In fact, almost 2/3 of the ocean carbon enters in these regions. However, almost ⅔ of the ocean carbon reservoir is in deep waters that are colder and less salty than the subtropical surface waters.

At low- to mid-latitudes the so called shallow overturning circulation transports a large amount of heat from the tropics to the mid-latitudes, with broad impact on local climate patterns. The study began with the analysis of the principal thermodynamical mechanisms that promote this flow toward the mid-latitudes in all the ocean basins in a simulated ocean. Water is cooled along the journey up to a point to become much denser. There it mixes with subpolar waters. Densification and wind pushing make the water rolling vertically until it reaches the layers at 400-1000m depths (so called subpolar mode waters and intermediate waters) where it then moves back to the tropics. About 20 millions of liters of water are experiencing this dive into the dark ocean each second, a hundred times the Amazon river discharge, taking with them everything that was dissolved in it.

The researchers then identified the amount of carbon that is injected into the interior by this set of conveyors, concluding that the uptake and storage of carbon from human activities is highly impacted by these processes. The subtropics and subpolar regions thus provide a central role in deciding how much carbon is stored in denser waters. By extending existing theoretical tools for the diagnosis of ocean dynamics, they further quantified the relative importance of ocean thermodynamics (that is, the role of processes like evaporation, heat exchanges and mixing), circulation and biogeochemistry in setting the ocean carbon reservoir. Since climate change will impact more and more the ocean circulation and its thermodynamics, the study thus provides new tools and information to better predict how these processes will affect the important role of the oceans in reducing the impact of the human carbon emission on the atmosphere.


Credit: NASA/YouTube

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