On the relative importance of radiative and dynamical heating for tropical tropopause temperatures

Abstract

AbstractThe tropical tropopause layer (TTL) shows a curious stratification structure: temperature continues to decrease beyond the level of main convective outflow (∼200 hPa) up to the cold point tropopause (∼100 hPa), but the TTL is more stably stratified than the upper troposphere. A cold point tropopause well separated from the level of main convective outflow has previously been shown to be consistent with the detailed radiative balance in the TTL even without dynamical effects. However, the TTL is also controlled by adiabatic cooling due to large‐scale upwelling within the Brewer‐Dobson circulation, which creates the extremely low stratospheric water vapor content via freeze drying. Here we study the role of water vapor and ozone radiative heating on the detailed temperature structure of the TTL based on idealized single‐column radiative‐convective equilibrium simulations. An atmosphere without adiabatic cooling due to upwelling results in much higher stratospheric water vapor content; the resulting altered radiative heating structure is shown to push the TTL in a regime of radiative control by water vapor. The TTL structure is furthermore shown to be strongly sensitive to the altitude where ozone sharply transitions from tropospheric to stratospheric values. Adiabatic cooling due to upwelling is found to reduce the radiative control by water vapor, resulting primarily in a negative transport‐radiation feedback. Conversely, the radiative control by ozone is enhanced due to upwelling—a positive transport‐radiation feedback. The particularly strong ozone radiative effect may explain about half of the reported spread in cold point temperatures (∼10 K) in current climate models.