Variability and trends in the PV-gradient dynamical tropopause

The dynamical tropopause as a transport barrier between the tropical upper troposphere and extratropical lowermost stratosphere is characterized by steep gradients in potential vorticity (PV) along an isentropic surface. Hence, the latitudinal separation between the dynamical tropopause in the Northern and Southern hemispheres can be used as a metric of upper tropospheric width for assessing climate change impacts. Here, we calculate the PV gradient-based dynamical tropopause from different meteorological reanalyses (ERA5, ERA-Interim, JRA-55, MERRA-2) and investigate its climatology, variability and long-term trends. Our results show a large seasonal cycle in the dynamical tropopause, with larger PV values and a poleward movement in summer. The climatological tropopause PV values are substantially different between different reanalyses, but the tropopause latitude is similar. Significant inter-annual variability in the dynamical tropopause latitude is related to El Niño Southern Oscillation (ENSO), is much weaker for the Quasi-Biennial Oscillation (QBO), and is robustly represented in reanalyses. In particular, El Niño causes equatorward shifts of the dynamical tropopause, hence a decrease of upper tropospheric width. Long-term trends in the dynamical tropopause exhibit a distinct vertical structure with poleward shifts below 340 K potential temperature, equatorward shifts between 340 K to 370 K and poleward shifts between 370 K to 380 K, implying an expansion of tropospheric width at lower levels, narrowing at upper levels and expansion near the tropical tropopause. Therefore, the dynamical tropopause as a metric for tropospheric width at a given level appears consistent with a widening of the tropics found from other metrics at lower levels, and furthermore shows a concurrent narrowing of the tropical upper troposphere.