Sensitivity study of advection and diffusion coefficients in a two‐dimensional stratospheric model using excess carbon 14 data

Using the California Institute of Technology/Jet Propulsion Laboratory two‐dimensional transport model, with transport coefficients taken from Yang and Tung (1989), we study the time evolution of excess carbon 14 in the stratosphere and the troposphere from October 1963 to December 1966. The model provides a satisfactory simulation of the observed data. Due to the impulsive nature of its source, initial distributions of excess carbon 14 exhibit large spatial gradients. This permits important constraints on the range of transport coefficients in the lower stratosphere to be derived. The standard model uses the circulation and eddy diffusivity of the year 1980. Large deviations (by factor of 2) from this standard transport are ruled out by our model. A self‐consistently derived K yy which is small (∼10 9 cm 2 s −1 ) in tropical regions, but is larger (∼10 10 cm 2 s −1 ) at higher latitudes is preferred. A K zz as large as 1×10 4 cm 2 s −1 would be inconsistent with the data. Excess carbon 14 is removed from the atmosphere with surface deposition velocities v S = 3 × 10 −3 cm s −1 and v N = 5 × 10 −3 cm s −1 in the southern and northern hemispheres, respectively. The last result is contrary to the current understanding that the oceans are the dominant sink for excess 14 C.