Abstract
The trajectories of passive tracers in the global atmosphere were statistically analyzed with emphasis on the exchange paths crossing the boundary between the Northern and Southern Hemispheres. Twice-daily three-dimensional wind data for January and July 1989 from the European Centre for Medium-Range Weather Forecasts were used for the trajectory calculation.
The statistics of the latitudes of tracers indicated that the partial barrier between the Northern and the Southern Hemispheric air masses was at 15°S and 5°S in January and at 10°N and 30°N in July. Tracers moving from the northern side of the boundary to the southern side and those moving in the opposite direction were selected with a criterion for the trajectory. The trajectories were classified into three types: one passing through only the lower part of the troposphere; one passing through only the upper part of the troposphere; and one going into the tropics from the lower troposphere, lifted to the upper troposphere in the ITCZ, and entering the other hemisphere. The trajectory from the summer to the winter hemisphere consists of an upper-only path and a lifted path, whereas the trajectory from the winter to the summer hemisphere consists of a upper-only path and a lower-only path.
Although individual trajectories were sensitive to the methods of computation, a set of trajectories was insensitive to them. The regional characteristics of the trajectories were discussed by making a comparison among the three trajectories obtained from instantaneous wind fields, and from twice-daily, and monthly-averaged wind fields.
