A new metric defining how the winds and ocean eddies in the Southern Ocean alter atmospheric CO2

What is it about?

The Southern Ocean is a unique region where carbon dioxide is both taken up by the upper ocean and returned from the deep ocean to the atmosphere. This contrasting response is partly due to the action of the winds, which drive surface waters northward across the Antarctic Circumpolar Current and leads to upwelling of carbon-rich deep waters to the south of the Antarctic Circumpolar Current. The effect of the winds are partly offset by the action of ocean eddies that act to flatten density surfaces. We develop a dynamically-motivated metric, the residual upwelling, that measures the primary effect of Southern Ocean dynamics on atmospheric CO2 on centennial to millennial timescales by determining the communication with the deep ocean. We tested the metric by conducted a suite a suite of coarse-resolution and eddy-resolving, global coupled ocean circulation and biogeochemistry experiments, each experiment integrated for 10,000 years. We found a positive correlation is obtained between the overturning in the Southern Ocean and atmospheric CO2: stronger or northward-shifted westerly winds in the Southern Hemisphere result in greater upwelling of carbon-rich deep waters and oceanic outgassing. Increased residual upwelling drives elevated atmospheric CO2 by enhancing the communication between the atmosphere and deep ocean at a rate of typically 1 parts per million in mixing ratio per Sverdrup (1 million cubic metres per second).

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Why is it important?

Understanding the extent that changes in winds in the Southern Ocean alter atmospheric CO2 is important in providing a context for past glacial changes in atmospheric CO2 and the ongoing anthropogenic increase in atmospheric CO2. Our new metric provides a way to understand the effect of the winds and ocean eddies in controlling atmospheric CO2. We tested our metric by conducted a suite a suite of coarse-resolution and eddy-resolving, global coupled ocean circulation and biogeochemistry experiments, each experiment integrated for 10,000 years. We found a positive correlation is obtained between the overturning in the Southern Ocean and atmospheric CO2: stronger or northward-shifted westerly winds in the Southern Hemisphere result in greater upwelling of carbon-rich deep waters and oceanic outgassing.

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