Comparison of Global-Scale Lagrangian Transport Properties of the NCEP Reanalysis and CCM3

Abstract

The global-scale transport properties of the NCEP reanalysis winds for the period from 1979 to 1997 are compared to a standard simulation with version 3 of the NCAR Community Climate Model (CCM3) forced by observed sea surface temperatures for the same period. The transport properties of each dataset are defined by the climatological Green's function for the mass conservation equation for a conserved, passive tracer. Characterizing the atmospheric circulation in terms of material transport provides a very different view of the circulation than standard Eulerian-mean statistics. The Green's functions are estimated from large numbers of Lagrangian (kinematic) particle trajectories computed by using the NCEP and CCM3 resolved winds. Generally the Green's functions computed from the two datasets agree well. The transport circulation is dominated by two thermally direct cells, one in each hemisphere. There is a substantial seasonal cycle in the transport, particularly in the Tropics. From a transport point of view, the atmosphere can be divided into three regions: the Southern Hemisphere extratropics, the Tropics, and the Northern Hemisphere extratropics. Particle dispersion within each region is relatively rapid, while exchange between the regions is slower. There are partial barriers to transport between the Tropics and extratropics. Differences between the transport characteristics of NCEP and CCM3 are most noticeable in the Tropics, where CCM3 has stronger subsidence in the ITCZ compared to NCEP. The transport circulation is slightly faster in NCEP than in CCM3. Interhemispheric transport rates computed from the Green's functions are compared with measurements of long-lived trace species from the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment (ALE/GAGE) network. The ALE/GAGE station data for the northern and southern extratropics give an interhemispheric time lag of ?1.8 yr for long-lived tracers such as CFCs. Fitting the transport data to a three-box model gives interhemispheric time lags of ?1.8 and ?2 yr, respectively, for NCEP and CCM3.