Climate Change and the Middle Atmosphere. Part III: The Doubled CO2Climate Revisited

The response of the troposphere–stratosphere system to doubled atmospheric CO2 is investigated in a series of experiments in which sea surface temperatures are allowed to adjust to radiation imbalances. The Goddard Institute for Space Studies (GISS) Global Climate Middle Atmosphere Model (GCMAM) warms by 5.1°C at the surface while the stratosphere cools by up to 10°C. When ozone is allowed to respond photochemically, the stratospheric cooling is reduced by 20%, with little effect in the troposphere. Planetary wave energy increases in the stratosphere, producing dynamical warming at high latitudes, in agreement with previous GCMAM doubled CO2 simulations; the effect is due to increased tropospheric generation and altered refraction, both strongly influenced by the magnitude of warming in the model’s tropical upper troposphere. This warming also results in stronger zonal winds in the lower stratosphere, which appears to reduce stratospheric planetary wave 2 energy and stratospheric warming events. The dynamical changes in the lower stratosphere are weakened when O3 chemistry on polar stratospheric cloud effects are included at current stratospheric chlorine levels. Comparison with the nine-level version of the GISS GCM with a top at 10 mb shows that both the stratospheric and tropospheric dynamical responses are different. The tropospheric effect is mostly a function of the vertical resolution in the troposphere; finer vertical resolution leads to increased latent heat release in the warmer climate, greater zonal available potential energy increase, and greater planetary longwave energy and energy transports. The increase in planetary longwave energy and residual circulation in the stratosphere is reproduced when the model top is lifted from 30 to 50 km, which also affects upper-tropospheric stability, convection and cloud cover, and climate sensitivity.