Mechanisms for Tropical Tropospheric Circulation Change in Response

1 Annual-mean tropospheric circulation change in global warming is studied by 2 comparing the response of an atmospheric general circulation model (GCM) to a spatial-3 uniform sea surface temperature (SST) increase (SUSI) with the response of a coupled 4 ocean-atmosphere GCM to increased greenhouse gas concentrations following the A1B 5 scenario. In both simulations, tropospheric warming follows the moist adiabat in the 6 tropics, and static stability increases globally in response to SST warming. A diagnostic 7 framework is developed based on a linear baroclinic model (LBM) of the atmosphere. 8 The mean advection of stratification change (MASC) by climatological vertical motion, 9 often neglected in interannual variability, is an important thermodynamic term for global 10 warming. Once MASC effect is included, LBM shows skills in reproducing GCM results 11 by prescribing latent heating diagnosed from the GCMs. 12 MASC acts to slow down the tropical circulation. This is most clear in the SUSI run 13 where Walker circulation slows down over the Pacific without any change in SST 14 gradient. MASC is to decelerate the Hadley circulation but spatial patterns of SST 15 warming play an important role. Specifically, the SST warming is greater in the Northern 16 than Southern Hemisphere, an inter-hemispheric asymmetry that decelerates the Hadley 17 cell north but accelerates it south of the equator. The MASC and SST-pattern effects are 18 on the same order of magnitude in our LBM simulations. The former is presumably 19 comparable across GCMs while SST warming patterns show variations among models in 20 both shape and magnitude. Uncertainties in SST patterns account for inter-model 21 variability in Hadley circulation response to global warming (especially on and south of 22 the equator).