Changes in tropical circulation in a warming climate

Precipitation is experiencing a great change under global warming. Two major processes induced changes in precipitation are thermodynamic and dynamic contributions. The former is related to changes in water vapor and the latter is associated with changes in vertical velocity. Since water vapor increases almost everywhere, the thermodynamic contribution is determined by the corresponding mean vertical motion: positive for ascents and negative for descents, which creates a 'wet-get-wetter and dry-get-drier' tendency. The dynamic contribution, on the other hand, is much more complicated, especially on a regional basis. For a global average, all climate models in the CMIP3 and CMIP5 show a robust weakening of tropical circulation, while observed changes in tropical circulation are inconsistent among data sets. A fundamental explanation for changes in tropical circulation is related to atmospheric stability. In a warmer climate, the upper troposphere warms up faster than the lower troposphere, so dry static stability increases. Thus, the atmosphere should become more stable and the corresponding tropical circulation weakens. However, a more physically-sound index, moist static stability, shows very scattering changes in global-warming simulations. In other words, the atmosphere can become either more stable or more unstable if considering the effect of water vapor. Here we introduce a new effect associated with convection depth by proposing another index, gross moist stability, to measure changes in atmospheric stability under global warming. The gross moist stability, an effective static stability, is vertical integral of the vertical gradient of moist static energy weighted by pressure velocity. It depends not only on the vertical gradient of dry static energy, which is equivalent to dry static stability, but also on moisture vertical distribution and convection depth. The effects of dry static energy and convection depth stabilize the atmosphere, while the moisture effect tends to destabilize the atmosphere. Because tropical convection closely follows a moist adiabatic process, there is a strong cancellation between the effects of dry static energy and moisture. This leads to the dominance of the effect of convection depth in atmospheric stability.