The tropopause in the polar regions

The polar and subpolar tropopause in both hemispheres is investigated using the ECMWF Reanalysis (ERA) data from 1979 to 1993 and radiosonde data from 1989 to 1993. Both the thermal and the dynamical criteria are applied to e ach dataset. The tropopauses derived from the radiosonde data are used to v alidate the ERA-derived tropopauses and to investigate the sharpness of the tropopause. The validation reveals that the ERA data are well suited for t he determination of the tropopause. A comparison between the thermal and th e dynamical tropopause shows a very good agreement except for polar winter, and there is clear evidence that the dynamical criterion is more appropria te in winter. The results show that the annual cycle of the polar tropopause can be class ified into three different patterns. A single wave with a tropopause pressu re maximum in winter and a minimum in summer is typical for the subpolar pa rts of eastern Siberia and North America. A double wave with pressure maxim a in spring and autumn and minima in summer and winter is found above north ern Europe, western Siberia, and generally at high Arctic latitudes. Finall y, Antarctica exhibits a reversed single wave with a pressure maximum in su mmer and a minimum in winter Tropopause temperatures are generally highest in summer and lowest in winter, but the amplitude of their annual cycles sh ows distinct differences. It is lowest in those regions where a single pres sure maximum in winter is present and largest in the Antarctic. A compariso n between the tropopause pressure and the temperatures in 500 and 100 hPa r eveals that the tropopause pressure is closely related to the temperature d ifference between 500 and 100 hPa. A large temperature difference correspon ds to a low tropopause pressure and a small temperature difference to a hig h tropopause pressure. The sharpness of the tropopause, that is, the change in vertical temperature gradient across the tropopause, is found to be hig hest in summer and lowest in winter. Its annual cycle and its regional diff erences are primarily determined by the mean temperature gradient above the tropopause because it varies much more strongly than the gradient below th e tropopause.