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 Hitchman2 co ncluded that there is a barrier inhibiting the transport of stratosphe ric air from the tropics to middle latitude, raising the question of h ow stratospheric material that has been transported from the troposphe re is subsequently conveyed to higher latitudes. Here we present globa l 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 transpor t. But superimposed on this background structure we also see planetary -scale 'tongues' of tropical stratospheric air extending out into midd le latitudes, and time sequences show irreversible mixing from the tro pics into middle latitudes. Such episodes could be responsible for tra nsporting significant quantities of stratospheric air across the tropi cal barrier.