A rising trend of double tropopauses over South Asia in a warming environment: Implications for moistening of the lower stratosphere

The water vapour variation in the upper troposphere and lower stratosphere (UTLS) is of high significance due to its impact on global warming. In this article, we present an association of occurrence frequency of double tropopauses (DTs) with convective clouds and transport of water vapour in the UTLS over subtropical South Asia using multiple multi-decadal datasets (e.g., radiosonde temperature profiles (1977–2017), Atmospheric Infrared Sounder (2003–2017), ERA-Interim reanalysis (1979–2017) and Microwave Limb Sounder (2004–2016). The diagnostic analysis of temperature, water vapour and potential vorticity indicates that convective clouds occurring during DTs enhance water in the altitude layer near the DTs. DTs are frequent (~5–55%) over the subtropical South Asia (25°–30°N) and associated with an enhancement of water vapour mixing ratios by ~5–40% (0.2–7.5 ppmv) above the lower tropopause. The radiosonde observations show a positive trend (~0.27 ± 0.12 to 0.4 ± 0.2%/year) in the occurrence of DTs during last 45 years, enhancing the moisture during DT days (trend 0.04 ± 0.02 to 0.26 ± 0.24 ppmv/decade above the tropopause). The convective injection of anomalously high water vapour mixing ratios in DT conditions and moistening trends in the UTLS may be consequences of global warming. The increasing trend in the water vapour in the UTLS may enhance long-wave radiation coming back down to warm the troposphere and exacerbate the global warming effect