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 each dataset. The tropopauses derived from the radiosonde data are used to validate the ERA-derived tropopauses and to investigate the sharpness of the tropopause. The validation reveals that the ERA data are well suited for the determination of the tropopause. A comparison between the thermal and the dynamical tropopause shows a very good agreement except for polar winter, and there is clear evidence that the dynamical criterion is more appropriate in winter.The results show that the annual cycle of the polar tropopause can be classified into three different patterns. A single wave with a tropopause pressure maximum in winter and a minimum in summer is typical for the subpolar parts of eastern Siberia and North America. A double wave with pressure maxima in spring and autumn and minima in summer and winter is found above northern Europe, western Siberia, and generally at high Arctic latitudes. Finally, Antarctica exhibits a reversed single wave with a pressure maximum in summer and a minimum in winter. Tropopause temperatures are generally highest in summer and lowest in winter, but the amplitude of their annual cycles shows distinct differences. It is lowest in those regions where a single pressure maximum in winter is present and largest in the Antarctic. A comparison between the tropopause pressure and the temperatures in 500 and 100 hPa reveals that the tropopause pressure is closely related to the temperature difference between 500 and 100 hPa. A large temperature difference corresponds to a low tropopause pressure and a small temperature difference to a high tropopause pressure. The sharpness of the tropopause, that is, the change in vertical temperature gradient across the tropopause, is found to be highest in summer and lowest in winter. Its annual cycle and its regional differences are primarily determined by the mean temperature gradient above the tropopause because it varies much more strongly than the gradient below the tropopause.