The MJO in a Warmer World—Projection Using a MJO-resolving Coupled Model ECHAM5-SIT

Whereas studies of the future MJO rely on numerical models, great difficulties remain in accurately simulating the MJO using state-of-the-art climate models (Slingo et al., 1996; Waliser, 2003; Lin et al., 2006).  For example, most of the CMIP5 models still poorly simulate the MJO and only one of the 20 models analyzed is able to simulate a realistic eastward propagation of the MJO (Hung et al., 2013).  To this account, it is thus impossible to study the impact of global warming on the MJO using many of these models.

Here we use a new climate model, ECHAM5-SIT, which is the AGCM ECHAM5 oupled to a one-column TKE ocean model SIT. ECHAM5-SIT is listed among the eight best models at simulating the MJO and four best at simulating convectively coupled wave spectra in a recent inter-comparison of 27 models [Jiang et al., 2015]. The model configuration allows proper simulation of upper-ocean temperature variations, while maintaining a realistic model mean state. The coupling substantially improves simulation of the MJO to have realistic strength, period, and propagation speed (Tseng et al, 2014; Jiang et al., 2014).

We compare the MJO between two climate states at the end of 20th and 21st century for its structure and propagation characteristics to infer the changes. In a warmer climate predicted for the end of the century, the MJO increases in amplitude (by ~30%) and frequency, showing a more circumglobal propagation tendency. The MJO spatial extent becomes enhanced, deeper and more zonally extended but meridonally confined. A stronger vertical tilting structure in diabatic heating, moisture and convergence fields is seen.  Our findings indicate that these changes result from an intensification of the frictional-wave CISK mechanism via the coupling of dynamical and thermodynamic response to the warming. The warming and moistening of the mean state contribute to the enhanced deep convective heating, driving a stronger forced Kelvin wave-like perturbation. This reinforces the frictional low-level convergence, leading to larger shallow convective heating, and therefore to a faster development and enhancement of the deep convection in MJO.