Taking a breath in the Southern Ocean

The Southern Ocean has a strong impact on the global climate: it acts as a buffer for global warming, by pumping heat and carbon out of the atmosphere and injecting them in the deep seas for centuries to millennia.

The cold waters lying at the surface of the Southern Ocean allow for the drawing down of large amounts of heat from the atmosphere, and enhance the dilution of gases such as carbon dioxide. But in order to buffer climate change efficiently, the carbon dioxide and heat of the atmosphere, which enter the Southern Ocean surface layer, need to be transported away from the ocean surface into the deep-seas.

In the vast majority of the world’s oceans, the ocean is layered in a similar way as an onion, with very little connections between the surface layer and the deeper layers. However, in the Southern Ocean, the presence of the most energetic currents on the planet, associated with very harsh climate conditions, allows the wind-stirred surface layer of the ocean to reach great depth, up to 600m in places. These deep surface layers and their associated intense vertical circulations create funnels injecting carbon and heat (and other atmospheric gases such as oxygen) into the deep seas, away from the atmosphere (the location of these funnels for the Atlantic region of the Southern Ocean are shown in orange shadings in the side figure; From Sallée et al., 2012 - Nature Geoscience). The carbon then fills large basins at depth on the northern side of the Southern Ocean (see the green/cyan color on the figure below).

From Sallée et al., 2012 - Nature Geoscience

Despite their crucial role for climate, the turbulent processes and the vertical velocities of the ocean surface are not well understood. The main reason for this lack of understanding is that some of the most important processes of the ocean surface are very small, which make them challenging to represent numerically in ocean models, and even more challenging to observe in the real ocean.

In this project we tackle both of these challenges, using state-of-the-art numerical models to best inform us about the processes based on physics law grounds, which we then use to go chasing the small-scale processes in the real Southern Ocean. We are now on the RRS James Clark Ross trying to find these small-scale processes and observe them using a range of appropriate tools. For instance, we use small sensors towed behind the ship, which allow us to resolve maps, and vertical slices of the ocean at high resolution. Below is a vertical slice of the ocean we made a few days ago, in which we represented in color oxygen measurements. These measurements show highly oxygenated water plunging from the ocean surface to the deep seas, on the southern side of our survey: a region of strong current and very cold water. We were excited to catch live, a clear event of deep-sea breathing, drawing down, at the same time as oxygen, carbon and heat, which are then locked away from the atmosphere for centuries…