Potential impact of tropopause sharpness on the structure and strength of the general circulation

The core of the atmospheric jet stream sits near the tropopause, the interface between troposphere and stratosphere (see schematic above). This tropopause-level jet acts as a waveguide for Rossby waves; their propagation and dissipation characteristics near the tropopause in turn affect the strength and latitude of the jet. One potentially important controlling feature that in previous work on jet characteristics did not receive much attention is the tropopause sharpness (primarily due to sharpness of the potential vorticity, PV, gradient). Tropopause sharpness is in part controlled by the sharpness of the stratification contrast between troposphere and stratosphere, which is proportional to the strength of the tropopause inversion layer (TIL, a layer of locally enhanced stratification just above the tropopause, grey shading in schematic). Here, we analyze interannual variability in both reanalysis and climate model output and find that a stronger TIL is associated with an equatorward jet shift. This relationship also holds for variations across climate-mean states from different models. Experiments with a mechanistic general circulation model (GCM) with an enforced stronger TIL show that the associated strengthened tropopause-level PV gradient (panel a above) promotes an intensified general circulation and an equatorward-shifted jet (panel b above).

Caption

Schematic (left) illustrating the link between tropopause sharpness, the tropopause-level wave guide and Rossby wave propagation. Panels a) and b) on the right show zonal-mean time-mean fields of the PV gradient (a) and zonal wind (b) from mechanistic GCM experiments with control run values in contours and differences arising from an enforced stronger TIL in colors.

Reference

Boljka, L., and T. Birner, 2022: Potential impact of tropopause sharpness on the structure and strength of the general circulation. npj Clim. Atmos. Sci., 5:98.