Understand Changes of the Tropical Tropopause Under Global Warming

Abstract:

Previous model studies has predicted a warmer and higher tropical tropopause as greenhouse gases increases, but the mechanisms of such changes have not been investigated fully. Here we examine changes the tropical tropopause in two idealized experiments simulated by GFDL global climate model AM3: (1) 4xCO2 with fixed sea surface temperature; and (2) an uniform 4K increase of the sea surface temperature with fixed greenhouse gases concentrations. The tropical tropopause becomes warmer in both experiments, but a higher tropopause is only seen in the second case. By examining the heat budget of the tropical tropoapuse, we diagnose the physical processes that are responsible for these changes and quantify their contributions. For the 4xCO2 experiment, the direct radiative effect of CO2 increase contributes the most. For the SST warming experiment, the radiative effect of a warmer troposphere and convection-related processes lead to a warming at 100 hPa, while a stronger Brewer-Dobson circulation and associated changes in ozone lead to a cooling at 60 hPa. This warming-cooling pattern results in a significant upward shift of the tropopause.