Northern Hemisphere Extratropical Tropopause in Radiosonde Data

The spatial structure of the northern Hemisphere extratropical tropopause is examined using radiosonde data from 1980 to 2007. For the stations identified to have consistent observations for the analysis period, the seasonal mean, intraseasonal variability, and long-term trends of three tropopause variables are calculated: these are the tropopause pressure (TP), tropopause temperature (TT), and tropopause inversion layer amplitude (TILa). It is found that, during summer, the seasonal mean and intraseasonal variability of all variables are to a large degree zonally symmetric. A local maximum are found only in the intraseasonal variability over the Asia, associated with the occurrence of double tropopauses. A weak zonal structure is also found in the winter-time TP and TT with an exception over storm-track region, where intraseasonal variability reaches a maximum. A very different spatial structure, however, is observed in the winter TILa. Both the seasonal mean and intraseasonal variability exhibit distinct maxima over western North America. As these maxima are not present in the TP and TT, it suggests that the formation and maintenance mechanisms of the extratropical TIL may not be explained by the balanced dynamics, and would be different from those of the TP and TT. It is further found that the long-term trends of tropopause variables are generally weak and not statistically significant. Nonetheless, they have quite homogeneous spatial distribution in both seasons.