Changes to the Madden-Julian Oscillation in Coupled and Uncoupled Aquaplanet Simulations With 4xCO2

The impacts of rising carbon dioxide (CO2) concentration and ocean feedbacks on the Madden-Julian Oscillation (MJO) are investigated with the Community Atmospheric Model Version 5 (CAM5) in an idealized aquaplanet configuration. The climate response associated with quadrupled CO(2)concentrations and sea surface temperature (SST) warming are examined in both the uncoupled CAM5 and a version coupled to a slab ocean model. Increasing CO(2)concentrations while holding SST fixed produces only small impacts to MJO characteristics, while the SST change resulting from increased CO(2)concentrations produces a significant increase in MJO precipitation anomaly amplitude but smaller increase in MJO circulation anomaly amplitude, consistent with previous studies. MJO propagation speed increases in both coupled simulations with quadrupling of CO(2)and uncoupled simulations with the same climatological surface temperature warming imposed, although propagation speed is increased more with coupling. While climatological SST changes are identical between coupled and uncoupled runs, other aspects of the basic state such as zonal winds do not change identically. For example, climate warming produces stronger superrotation and weaker mean lower tropospheric easterlies in the coupled run, which contributes to greater increases in MJO eastward propagation speed with warming through its effect on moisture advection. The column process, representing the sum of vertical moist static energy (MSE) advection and radiative heating anomalies, also supports faster eastward propagation with warming in the coupled run. How differing basic states between coupled and uncoupled runs contribute to this behavior is discussed in more detail. Plain Language Summary Previous studies have suggested that the Madden-Julian Oscillation (MJO) will speed up under global warming. Understanding the MJO's response to rising carbon dioxide (CO2) concentrations is complicated by the need to separate the direct response to CO2 forcing from that associated with increasing temperatures. Ocean coupling may also mediate the MJO's response to warming. We demonstrate using the Community Atmospheric Model Version 5 (CAM5) run in an aquaplanet configuration that increasing CO2 concentrations in isolation from temperature changes has only small impacts on MJO characteristics. However, the temperature changes resulting from a quadrupling of CO2 produce a significant increase in the MJO precipitation amplitude but weaker increase in MJO wind amplitude. Even for the same climatological surface temperature increases, ocean coupling produces greater increases in eastward MJO propagation speed than in the uncoupled model. This is likely because other aspects of the model climate such as mean east-west winds change differently between coupled and uncoupled models. The model winds become more westerly in the coupled versus uncoupled model for the same surface temperature warming, which causes the MJO to speed up more in the coupled run. Other processes responsible for the more rapid eastward propagation of the MJO with warming in the coupled run are diagnosed.