Long-term Changes in the Thermodynamic Structure and the Mass of the Lowermost Stratosphere

The lowermost stratosphere (LMS) plays an important role in stratosphere-troposphere coupling and the Earth’s radiation balance, containing strong gradients of ozone and water vapor. Increasing concentrations of greenhouse gases and the recovery of stratospheric ozone are expected to impact stratospheric thermodynamics and circulation, also affecting the LMS structure. We investigate the long-term evolution of the LMS mass, i.e., the combined effect of changes in the location of the tropical and extratropical tropopause and the lower stratospheric potential temperature structure. Identifying LMS mass trends in turn can improve our knowledge of lower stratospheric column ozone and water vapor trends that, despite their radiative importance, are still insufficiently understood.

Our study is based on ERA5 reanalysis data spanning the time period 1979–2019 and compared with ERA-Interim, MERRA2 and JRA3Q. The focus is on changes after the year 1998 which marks the anticipated beginning of stratospheric ozone recovery. The trend analysis is performed with a dynamic linear regression model, able to account for non-linear trends and identify potential trend reversal dates.

We find the tropical lapse rate tropopause and the cold point to exhibit an upward trend. Therefore, we propose a dynamical upper LMS boundary surface accounting for this trend, rather than approximating the upper boundary with the 380 K isentrope. We define the upper LMS boundary by the isentropic surface corresponding to the average potential temperature at the tropical tropopause. Outside the tropics, we compare a thermal and dynamical tropopause definition. In the northern hemisphere, we find the mass of the LMS to be decreasing between 1998–2019 if a fixed isentrope (380 K) is chosen as the upper LMS boundary. This mass decline is significantly weaker if dynamical upper LMS boundaries are used which take the upward trends of the tropical tropopause into account. In the southern hemisphere, the extratropical lapse rate tropopause shows a downward tendency after 1998, leading to increasing LMS mass. These LMS mass trends can be expected to have implications on the LMS ozone and water vapor budget.