Abstract
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 CO2 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 CO2 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 CO2 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 CO2 at a rate of typically 1–1.5 parts per million/106 m3 s−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 CO2, alongside other metrics, such as involving the proportion of preformed nutrients and the extent of sea ice cover.
Highlights
The Southern Ocean plays a fundamental role in the climate system, ventilating much of the global ocean by forming subtropical mode, intermediate and bottom waters, as well as uniquely returning deep waters to the surface (Marshall and Speer 2012)
Motivated by the importance of the Southern Ocean for climate change in the present day and the past, here we propose a new metric—the strength of Southern Ocean residual upwelling—to represent the dominant effect of Southern Ocean dynamics on atmospheric CO2 on centennial to millennial timescales
The detailed response of the individual ocean carbon pools may differ with model resolution and complexity, even though their integrated response may be similar. This modelling study suggests that residual upwelling provides a robust metric of how ocean dynamics in the Southern Ocean affect atmospheric CO2, giving an increase in atmospheric CO2 of typically 1.5 ppm for every 1 Sv increase of residual upwelling in the Southern Ocean
Summary
The Southern Ocean plays a fundamental role in the climate system, ventilating much of the global ocean by forming subtropical mode, intermediate and bottom waters, as well as uniquely returning deep waters to the surface (Marshall and Speer 2012). In response, ocean eddies act to flatten density surfaces, inhibiting this steepening, and provide an opposing southward upper ocean transport (Marshall 1997; Marshall and Radko 2006; Viebahn and Eden 2010) This partial compensation leads to the residual circulation, defined by the sum of the Eulerian-mean and eddy circulations, only weakly increasing with strengthening winds (Munday et al 2013). This residual circulation provides the overturning transport of heat and carbon across the ACC, connecting the Southern Ocean with the northern ocean basins. These experiments, employing different domains and resolutions, are designed to reveal the extent that Southern Ocean dynamics controls atmospheric CO2 for a long-term equilibrium
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
