Mechanisms for Tropical Tropospheric Circulation Change in Response to Global Warming

The annual-mean tropospheric circulation change in global warming is studied by comparing the response of an atmospheric general circulation model (GCM) to a spatial-uniform sea surface temperature (SST) increase (SUSI) with the response of a coupled ocean-atmosphere GCM to increased greenhouse gas concentrations following the A1B scenario. In both simulations, tropospheric warming follows the moist adiabat in the tropics, and static stability increases globally in response to SST warming. A diagnostic framework is developed based on a linear baroclinic model (LBM) of the atmosphere. The mean advection of stratification change (MASC) by climatological vertical motion, often neglected in interannual variability, is an important thermodynamic term for global warming. Once MASC effect is included, LBM shows skills in reproducing GCM results by prescribing latent heating diagnosed from the GCMs. MASC acts to slow down the tropical circulation. This is most clear in the SUSI run where the Walker circulation slows down over the Pacific without any change in SST gradient. MASC is used to decelerate the Hadley circulation, but spatial patterns of SST warming play an important role. Specifically, the SST warming is greater in the Northern than Southern Hemisphere, an interhemispheric asymmetry that decelerates the Hadley cell north, but accelerates it south of the equator. The MASC and SST-pattern effects are on the same order of magnitude in our LBM simulations. The former is presumably comparable across GCMs, while SST warming patterns show variations among models in both shape and magnitude. Uncertainties in SST patterns account for intermodel variability in Hadley circulation response to global warming (especially on and south of the equator).