Changes of MJO propagation characteristics under global warming

The change of Madden–Julian Oscillation (MJO) propagation and periodicity characteristics under global warming is investigated using two selected CMIP5 models. It is found that the MJO period tends to be shorter, while its eastward propagation tends to be accelerated. Meanwhile the main MJO activity center shifts eastward toward the central equatorial Pacific. Two factors are possibly responsible for the increased eastward phase speed of the MJO. The first is attributed to the increase of atmospheric static stability that accelerates equatorial Kelvin wave speed. The second is attributed to the increase of zonal asymmetry of MJO-scale moist static energy (MSE) tendency, which is contributed to the combined effect of anomalous circulation and mean MSE gradient. A theoretical framework is constructed to understand the relative role of anomalous heating and mean static stability changes in determining MJO-scale circulation change. It is found that their effect is phase dependent. During the initial warming phase the circulation change is primarily controlled by the heating change, whereas during the later warming phase it is primarily controlled by the static stability change. The eastward shift of the main MJO activity center is possibly caused by the occurrence of an El Niño-like mean SST change and associated vertical overturning circulation change in the tropical Pacific.