ISSN 1006-9895

CN 11-1768/O4

Sensitivity of CFC-11 Uptake in a Global Ocean Model to Air-Sea Gas Transfer Velocity
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    Abstract:

    Using five different formulas for the air-sea gas transfer velocity, the oceanic uptake, distribution, and storage of CFC-11 are simulated in the global ocean general circulation model (OGCM) LICOM (LASG/IAP Climate System Ocean Model), which was developed by the Institute of Atmospheric Physics of the Chinese Academy of Sciences. The difference in the transfer velocities calculated using each formula is discussed. The largest difference in global annual mean transfer velocity among these formulas is 81%. The sea surface CFC-11 concentrations, air-sea CFC-11 fluxes, water column inventories, cumulative uptake, and vertical distributions are analyzed. The results show that the air-sea CFC-11 flux and the total uptake obtained in an experiment using the formula of Liss and Merlivat (1986) are less than those from other experiments using other formulas. Although the global annual mean transfer velocities obtained using the formulas of Nightingale et al. (2000), Ho et al. (2006), and Sweeney et al. (2007) are quite close to each other, the transfer velocity from the formula of Nightingale et al. (2000) in regions with high wind speed is smaller than those from the latter two formulas because of differences in the formulas, leading to a relatively small strength in the main uptake and storage area. The formula of Wanninkhof is formally the same as those of Ho et al. and Sweeney et al. except for the proportional coefficient, and thus it results in a significantly better simulation of the distribution of the CFC-11 water column inventory in the Southern Ocean. However, its maximum value is still slightly smaller than the data-based estimate. The simulated global accumulative uptake with the transfer velocity from Wanninkhof is 8% smaller than the data-based estimate. The relative difference in the oceanic CFC-11 uptake between two experiments that used the formulas of Wanninkhof and Liss and Merlivat, respectively, on data from individual years decreases overall with time until it is about 2% in 2007. This demonstrates that for a short time period (10 years) from the beginning of the simulation, the sensitivity of CFC-11 uptake to the air-sea gas transfer velocity is greater, whereas during a long-term simulation, the oceanic uptake of tracers depends more on the physical model. In addition, the simulated CFC-11 results are compared with cruise data for two sections. The results show that the different transfer velocities used in these experiments produce certain differences in the vertical distributions of CFC-11 in the main uptake regions. For our OGCM, the simulated CFC-11 results from Wanninkhof's formula for air-sea gas transfer velocity are closer to the data-based estimates than those from other formulas.

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History
  • Received:October 10,2011
  • Revised:May 14,2012
  • Adopted:
  • Online: October 25,2012
  • Published: