Long-term changes in the thermodynamic structure of the lowermost stratosphere inferred from ERA5 reanalysis data

Abstract. The lowermost stratosphere (LMS) plays an important role for stratosphere-troposphere coupling and the Earth’s radiation balance. This study investigates the effects of long-term changes of the tropopause and the lower stratospheric isentropic structure on the mass of the LMS. We use ERA5 reanalysis data from 1979–2019, focusing on changes after 1998, which marks the anticipated beginning of stratospheric ozone recovery. The trend analysis is performed with a dynamic linear regression model (DLM), capable of modeling non-linear trends.

According to our study, isentropes in the lower stratosphere (here 380–430 K) show upward trends in the tropics and downward trends in the extratropics, consistent with a strengthening of the Brewer-Dobson circulation. In the Northern Hemisphere (NH), we find that the extratropical tropopause is rising at a mean rate of −1 hPa/decade. For the Southern hemispheric (SH) extratropics, the lapse rate tropopause shows a downward tendency of up to +2 hPa/decade after 1998. The tropical tropopause and the cold point is rising with an average trend of roughly −0.5 hPa/decade, consistent with increasing potential temperatures. Additionally, we find that the tropical tropopause in the NH has expanded polewards by 0.5° latitude.

Consistent with the upward and poleward trend of the NH tropopause, the mass of the LMS is decreasing by 2–3 % between 1998–2019 if a fixed isentrope (380 K) is chosen as the upper LMS boundary. This mass decline disappears if dynamical upper LMS boundaries are used, that take the upward trends of the tropical tropopause into account.