The Effect of SST Pattern Change on Future MJO Activity

The Madden Julian Oscillation (MJO) consists of convection and wind anomalies that propagate eastward in the tropics with a timescale of 30-90 days. How the MJO may change in a warmer climate has implications for the subseasonal modulation and prediction of extreme events. Decadal variability in the climate system causes patterns of sea surface temperature (SST) change in the tropical Pacific and associated precipitation, humidity, and wind change pattern changes to vary from one decade to the next in future projections. Understanding uncertainty in potential MJO changes over the 21^st Century in the context of this decadal variability is the primary motivation for this investigation. Eighty members with the SSP370 radiative forcing scenario from the NCAR Community Earth System Model 2 (CESM2) Large Ensemble are used. The projected pattern of SST and associated moisture pattern changes across ensemble members can be weighted more toward the central or eastern equatorial Pacific, especially in earlier parts of the 21^st Century, although becomes strongly El Niño-like later in the century. While MJO precipitation amplitude generally increases, ensemble members with stronger MJO precipitation amplitude changes are characterized by stronger El Niño-like east Pacific warming and a strengthened meridional moisture gradient. As interpreted through moisture mode theory, greater east Pacific warming supports stronger MJO precipitation amplitude by enhancing propagation through a stronger meridional moisture gradient, and enhancing MJO amplitude through a stronger vertical moisture gradient. Changes to MJO wind amplitude with warming are less clear. The investigation supports the hypothesis that the details of projected SST and moisture pattern changes are important for regulating future MJO precipitation amplitude.