Tropical stratospheric circulation deduced from satellite aerosol data

THE dispersal of volcanic material from large tropical eruptions provides insight into the circulation of the lower stratosphere. Here we infer stratospheric motions from zonal mean cross-sections of satellite observations of the aerosol layer taken during 1979–81 and 1984–91. By examining the aerosol distribution following volcanic eruptions in the tropics, we find that poleward transport occurs readily at altitudes within a few kilometres above the tropopause, whereas in the altitude range of 21–28 km, aerosols tend to remain within 20° of the Equator. We further deduce that the aerosol distribution in this upper regime is controlled by the phase of the quasibiennial oscillation. When the easterly shear is present, aerosols are lofted over the Equator, whereas when the westerly shear is present, descent relative to the mean stratospheric circulation occurs over the Equator. From the aerosol distributions, we suggest that the tropical stratosphere may be regarded as a temporary reservoir for trace constituents entering the stratosphere through the tropical tropopause.

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