Southern Ocean uptake of anthropogenic CO2 is currently estimated to be responsible for about 40% of the global oceanic CO2 uptake. Fluxes of CO2 into the ocean are driven by the differences in the partial pressure (pCO2) between the atmosphere and the surface ocean layer. As the surface ocean layers increase their pCO2 values, the uptake rate of the Southern Ocean should slow, unless other process (e.g. deep advection, surface cooling?) counteract this. Measurements of Southern Ocean pCO2 values over the past several decades have indicated a warming of the massive Antarctic Circumpolar Current (ACC) system, also leading to increasing surface pCO2. Upwelled water south of the various ACC fronts further brings increased (respired) pCO2 levels to the surface, but may be offset by poleward shifts in the strong westerly wind systems south of the fronts which encircle the Antarctic. Such shifts in the westerlies serve to reduce the area of the ocean over which pCO2 exchange can take place. The interplay of these different yet interacting physical factors is a useful diagnostic for the success of the next generation of climate-earth systems models in predicting future atmospheric CO2 concentrations, and the skill brought to future climate projections.

RAPID funding support will allow the examination of the variations, over decadal time sales, of the Southern Ocean pCO2 fields in the next variants of the Intergovernmental Panel on Climate Change (IPCC) coupled carbon/ climate models (earth system models) being produced as part of the next IPCC Assessment Report (IPCC AR5). Intercomparisons of Southern Ocean pCO2 trends over time, as revealed by in IPCC AR5 EaSMs, provides a way of assessing the skill of best available climate model predictions.

RAPID support is appropriate due to the short time window between the arrival of the model output in a publicly accessible archive and the deadline for papers analyzing the model output to be accepted for publication. AR5 chapter authors are prohibited from citing research which has not been accepted for publication in a peer-reviewed journal, and papers must be accepted by August 2012 in order to be referenced in the AR5.

Project Report

In this project we intercompare the spatial variations in Southern Ocean surface water partial pressure of CO2 (pCO2) in Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (ESMs) from 2002 to 2012, and assess the skill of the climate projections by comparing with underway decade-long shipboard measurements in the Drake Passage. CMIP5 ESMs for the first time abandon the pre-scribed CO2 concentration scenarios and couple the ocean carbon cycle with the climate model. The skill of the models to represent Southern oceanic physical processes holds the key to predicting atmospheric CO2 concentrations and projecting future climate. We found that all eight available CMIP5 ESMs show comparable latitudinal variations of surface pCO2 with the shipboard measurements in the Drake Passage. However, four of these indicated excessive pCO2 values at the Antarctic Circumpolar Circulation fronts. In addition, these models simulate strong upwelling and equator-ward Ekman transports by the erroneous-intensified westerlies, and parameterize too weak pole-ward eddy transports to compensate the equator-ward Ekman transports. This net equator-ward meridional transport moves the upwelled rich dissolved inorganic carbon out of the Drake Passage region. And the strong stratification in some ESMs also prohibit the pCO2 being ventilated into the deep ocean. These physical processes reduce the capacity and inventory in the Southern Ocean and pose concerns for the future atmospheric CO2. Our study indicates several shortfalls of some ESMs: 1) strong upwelling and Ekman transport by the erroneous intensified westerlies; 2) weak polarward transport by the mesoscale eddies; and 3) less ventilation to deep ocean owing to the strong stratification. Some emphasis should be put on improving the representation of the surface meridional ocean transports near the Polar Front, and the mesoscale eddies in the meridional transports of water properties appear to be one of the greatest challenges facing CMIP5 ESMs. Dr. Jiang is officially certified by the Pacific Science Center as a Science Communication Fellow. She completed a four month NASA funded, Science Communication Short Course and is volunteering at the Pacific Science Center at least three shifts a year. This project and the principle investigator will be featured in July issue of the International Innovation by Research Media Ltd. in London. The manuscript titled "Evaluation of the Southern Ocean sea water pCO2 in CMIP5 ESMs using in-situ observations" by the princile investigator and coauthors will be submitted to J. Geophys. Res. and be included in the IPCC AR5 report.

National Science Foundation (NSF)
Division of Polar Programs (PLR)
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Peter J. Milne
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Earth and Space Research
United States
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