Shift in cold-point tropopause trends derived from radiosonde, satellite and reanalysis data

Abstract

The tropical tropopause layer is the transition region between the well-mixed convective troposphere and the radiatively controlled stratosphere and plays a crucial role in air mass transport between these layers. In this paper, we present updated trends of tropopause and lower stratospheric temperature from radiosondes, GNSS-RO data, and the reanalyses ERA5, JRA-3Q and MERRA-2. Given its importance in determining the concentration of water vapour entering the stratosphere, we focus on temperature trends at the cold-point tropopause (CPT), which we determined from radiosonde observations after correcting for time-varying biases. Radiosonde and GNSS-RO data show a significant shift from strong cold-point cooling for 1980-2001 to warming for 2002-2023. Reanalysis datasets reproduce the robust change in the tropical tropopause temperature trends and furthermore show opposite trends in tropical upwelling for 1980-2001 compared to 2002-2023, consistent with the cold-point and lower stratosphere temperature changes. The shift in cold-point temperature trends around 2000 suggests a regime shift in the dominant mechanism controlling CPT temperatures, from ozone-depleting substance-induced dynamical changes before 2000 to greenhouse-gas-induced radiative warming with some dynamical contributions after 2000. While the role of dynamical changes after 2000 is not completely clear, this regime shift suggests that in the absence of strong dynamically induced cooling trends, radiative warming could dominate the cold-point temperature trends and thus stratospheric water vapour entry values.