Stratospheric transport from the tropics to middle latitudes by planetary-wave mixing

TRANSPORT of air from the troposphere to the stratosphere takes place mainly in the tropics¹. By studying satellite records of the dispersal of volcanic aerosols from tropical eruptions, Trepte and Hitchman ² concluded that there is a barrier inhibiting the transport of stratospheric air from the tropics to middle latitude, raising the question of how stratospheric material that has been transported from the troposphere is subsequently conveyed to higher latitudes. Here we present global maps of nitrous oxide and water mixing ratios obtained by the Upper Atmosphere Research Satellite. We see strong latitudinal gradients in these trace species, confirming the existence of a barrier to transport. But superimposed on this background structure we also see planetary-scale 'tongues' of tropical stratospheric air extending out into middle latitudes, and time sequences show irreversible mixing from the tropics into middle latitudes. Such episodes could be responsible for transporting significant quantities of stratospheric air across the tropical barrier.