Changes in the structure and propagation of the Madden-Julian Oscillation with increasing CO2 in the NASA GISS general circulation model

The influence of the basic state on the structure and propagation of the Madden-Julian Oscillation (MJO) is investigated in this study. Using the NASA/Goddard Institute for Space Studies (GISS) Model E2 General Circulation model, coupled to a slab ocean model, a set of long-term simulations are conducted by varying CO2 concentration from half to quadruple the current level. With increasing CO2, the modeled MJO is characterized by increases in its zonal scale, vertical extent, eastward propagation, and increases in the amplitudes of anomalous moisture, precipitation, vertical motion and outgoing longwave radiation (OLR). The relationships between the changes in the simulated MJO and changes in the background state are investigated based on a recent theoretical framework in which column integrated moisture is central to MJO dynamics. The increasing amplitude in the moisture and precipitation fields with increasing CO2 can be attributed to the vertical gradient of mean moisture increasing more quickly than the environmental dry static stability, which leads to a reduction in the environmental gross moist stability. The amplified precipitation anomalies cause an amplification of the OLR anomalies even though cloud-radiation feedbacks (defined per unit precipitation) weaken with increasing CO2. The increasing rate of propagation can be explained by the increasing horizontal and vertical gradients of mean moisture, which leads to more rapid moistening by horizontal and vertical moisture advection. Many of the characteristics of the simulated MJO are consistent with the notion of the MJO being a type of moisture mode in which the anomalous wind field in the wave induces eastward propagation through horizontal and vertical advection of moisture.