Stratosphere-troposphere exchange in a changing climate simulated with the general circulation model MAECHAM4

Tracer simulations of Be-7 and Be-10 have been performed with the general circulation model MAECHAM4 for present and future climatic conditions in order to study possible trends of stratosphere-troposphere exchange (STE). These model simulations suggest an intensification of the mean meridional circulation above 100 hPa in a warmer climate. At extratropical tropopause levels, it increases poleward of 60degrees and between 30degreesS and the equator. This is attributed to shifts of the transport pattern. Consequently, large-scale STE there increases with time, whereas it decreases between 60degreesS and 30degreesS and equatorward of 60degreesN. STE changes also occur due to changes in eddy activity. Our model results suggest that in a warmer climate cyclonic and blocking-like variability is lower than today poleward of 30degreesN, and cyclonic activity is stronger at Southern Hemisphere midlatitudes. If the amount of air exchanged per cyclone or cutoff low between stratosphere and troposphere does not change in a warmer climate, our results suggest a decrease of STE poleward of 30degreesN and an increase at Southern Hemisphere midlatitudes due eddy activity. Extratropical changes in the distributions of Be-7 and Be-10 are dominated by changes in eddy activity. Due to their short tropospheric lifetime, they are more sensitive to the shorter timescale process. Eddy activity does not play any role in STE in the tropics. Large-scale STE changes there do occur due to changes in the mean meridional circulation. In the troposphere, the zonal mean Be-10/Be-7 ratio increases in the Southern Hemisphere and decreases in the Northern Hemisphere as a consequence of STE and tropospheric transport changes. Thus, one has to be cautious in using this ratio as an exclusive measure for STE changes because changes in the tropospheric processes can also affect the climatological mean ratio