Changes to the Madden-Julian Oscillation with Surface Temperature Warming in Aquaplanet Simulations

Spencer Ressel , Daehyun Kim, Peter Blossey

Interactions between waves and the mean-state in the tropics are of fundamental importance for reliable future projection of tropical waves and their impacts under anthropogenic climate change. The Madden-Julian Oscillation (MJO) is the dominant source of weather variability over many parts of the globe on intraseasonal timescales. In this study, we investigate the influence of mean-state changes on the MJO in three aqua-planet experiments made with the Community Atmosphere Model version 6 (CAM6) by prescribing sea surface temperature profiles to capture three different climate states: a current climate-like state, a colder (-4K) state, and a warmer (+4K) state. We find that the MJO strengthens with surface warming, showing higher spectral power over the MJO band (20-100 day period, zonal wavenumbers 1-3) in both circulation and convection fields. Furthermore, MJO-associated precipitation variability increases more strongly than mean-state precipitation with warming, while the east-west ratio increases by a factor of 3x from -4K to +4K. Using a moisture budget approach, two key mechanisms were found to result in MJO strengthening; first, a reduction in the Gross Moist Stability associated with increases in the mean low-level vertical moisture gradient, and second, a reduced damping associated with meridional moisture advection resulting from an increase in the mean low-level meridional moisture gradient. The reduction in GMS is slightly offset, however, by a reduction in the cloud-radiative feedback parameter with warming. This occurs because the cloud top temperature is nearly invariant with warming despite a sharp increase in precipitation.