Mar/28/2017: Dr. Chao-An, Chen’s Speech:
Title: Regional Perspective on Mechanisms for Tropical Precipitation and Circulation Changes under Global Warming
Considering the moisture budget conservation, changes in precipitation can be attributed to a combination of thermodynamic and dynamic components. The thermodynamic component, which is associated with the increase in water vapor in a warmer atmosphere, contributes uniformly and dominantly to global precipitation change. The dynamic component is related to the change in atmospheric vertical motion, which shows large spatial variation but plays an important role in affecting regional precipitation. Several mechanisms associated with thermodynamic and dynamic effects on tropical regional precipitation and circulation changes are discussed here. The upped-ante mechanism describes that the dry advection from subsidence area weakens the convection over the convective margin area and further reduces precipitation. The rich-get-richer mechanism describes that moisture increase at low levels tends to reduce gross moist stability, i.e. an effective static stability. The reduced gross moist stability destabilizes the atmosphere and strengthens ascent in ascending regions, providing a dynamical feedback that can potentially increase precipitation. In a warmer climate, the tropical troposphere is inclined to become thicker. This may lead to an increase of vertical extent of convection (i.e., the deepening of convection) and gross moist stability. The increased gross moist stability reduces ascending motion and changes the vertical profile associated with convection. The above diverse changes in atmospheric circulation are associated with the diverse changes in gross moist stability. In the warmer climate, the different climatological vertical profiles play different roles in adjusting atmospheric stability, energy transport and large-scale circulation, and tend to induce different changes in atmospheric stability. The bottom-heavy (top heavy) structure brings a more (less) unstable condition and is favorable (unfavorable) to the strengthening of the convective circulation. The bottom-heavy structure is associated with shallow convection, while the top-heavy structure is usually related to deep convection. This study suggests a hypothesis and a possible linkage for projecting and understanding future circulation change from the current climate: shallow convection will tend to strengthen tropical circulation and enhance upward motion in a future warmer climate.