Mechanisms for Global Warming Impacts on Madden-Julian Oscillation Precipitation Amplitude

Mechanisms that cause changes in Madden-Julian oscillation (MJO) precipitation amplitude under global warming are examined in models from phase 5 of the Coupled Model Intercomparison Project. Under global warming in representative concentration pathway 8.5, MJO precipitation intensifies in most models relative to current climate while MJO wind circulations increase at a slower rate or weaken. Changes in MJO precipitation intensity are partially controlled by changes in moisture profiles and static stability. The vertical moisture gradient increases in the lower half of the troposphere in response to the surface warming, while the vertical static stability gradient increases due to preferential warming in the upper troposphere. A nondimensional quantity called alpha has been defined that gives the efficiency of vertical advective moistening associated with diabatic processes in the free troposphere, and has been hypothesized by previous studies to regulate MJO amplitude. The term alpha is proportional to the vertical moisture gradient and inversely proportional to static stability. Under global warming, the increased vertical moisture gradient makes alpha larger in models, despite increased static stability. Although alpha increases in all models, MJO precipitation amplitude decreases in some models, contrary to expectations. It is demonstrated that in these models more top-heavy MJO diabatic heating with warming overwhelms the effect of increased alpha to make vertical moisture advection less efficient.