| Summary: | The Southern Ocean plays a pivotal role in climate change by exchanging heat and carbon, and provides the primary window for the global deep ocean to communicate with the atmosphere. There has been a widespread focus on explaining atmospheric CO[subscript 2] changes in terms of changes in wind forcing in the Southern Ocean. Here, we develop a dynamically-motivated metric, the residual upwelling, that measures the primary effect of Southern Ocean dynamics on atmospheric CO[subscript 2] on centennial to millennial timescales by determining the communication with the deep ocean. The metric encapsulates the combined, net effect of winds and air–sea buoyancy forcing on both the upper and lower overturning cells, which have been invoked as explaining atmospheric CO[subscript 2] changes for the present day and glacial-interglacial changes. The skill of the metric is assessed by employing suites of idealized ocean model experiments, including parameterized and explicitly simulated eddies, with online biogeochemistry and integrated for 10,000 years to equilibrium. Increased residual upwelling drives elevated atmospheric CO[subscript 2] at a rate of typically 1–1.5 parts per million/10[superscript 6] m[superscript 3] s[superscript −1] by enhancing the communication between the atmosphere and deep ocean. This metric can be used to interpret the long-term effect of Southern Ocean dynamics on the natural carbon cycle and atmospheric CO[subscript 2], alongside other metrics, such as involving the proportion of preformed nutrients and the extent of sea ice cover. National Science Foundation (U.S.) (Chemical Oceanography Grant 1259388) Natural Environment Research Council (Great Britain) (Grants NE/K012789/10 and NE/G018782/1)
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