The tropopause is a layer which marks the boundary between the troposphere and stratosphere, and a fundamental characteristic of the tropopause is a change in static stability (temperature lapse rate) across the interface. The double tropopauses (DTs), found when calculating the tropopuase using lapse rate definition, are associated with a characteristic break in the tropopause near the subtropical jet, wherein the tropical tropopause extends to higher latitudes, overlying the lower tropopause, as seen in Fig. 1. The locations of DTs are related with baroclinic Rossby wave breaking in the subtropical upper troposphere and lower stratosphere (UTLS) region, and DTs are considered to be responsible for enhanced transport of air mass and chemicals between the troposphere and stratosphere. Pollutions, e.g. CO in the troposphere and Ozone in the stratosphere may exchange in these region and influence the global climate. Also, the location and height of the DTs may be a sensitive indicator of anthropogenic climate change. So information on the climatological characteristics of DTs and the origin of air sandwiched within DTs is crucial for qualitative estimates of human impacts on climate. 

Dr. WU Xue and Prof. Lü Daren from the Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences have analyzed the climatological features of the DTs using ERA-Interim data from 2003 to 2012, and found that DTs are permanent or semi-permanent in the midlatitudes, and high DT frequency bands move poleward in winter and equatorward in summer, which is consistent with the seasonal movement of the subtropical jet. Based on their statistics, the second tropopause is found at about 100 hPa in the subtropics and at slightly lower altitudes in sub-polar regions. 

Moreover, the origin of air sandwiched between the first and second tropopause of DTs was studied with a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code (Version 3) diabatic trajectory model. The results show that, in the lower or middle troposphere, air is transported into the DTs from lower latitudes, mainly in the tropics. The dominant source regions are mainly areas of deep convection and steep orography, e.g., the western Pacific and Himalayan Mountains, and they show strong seasonality following the seasonal shift of these strong upwelling regions (see Fig. 2).