A Systematic Relationship between Convectively Coupled Equatorial Wave Activity and the Performance of the Madden-Julian Oscillation in Climate Model Simulations

As the dominant mode of the tropical intraseasonal variability, The Madden-Julian Oscillation (MJO) has been believed to be a key source of untapped predictability for the extended-range forecast in both the tropics and the extratropics. Despite the apparent importance, the simulation of the MJO has remained a challenging task. In addition to the MJO, the convectively coupled equatorial waves (CCEWs) also compose a substantial fraction of the tropical sub-seasonal variability. In this study, we examined possible relationships between models' MJO performance and their representation of the CCEWs in 26 GCMs that participated in the MJOTF/GASS MJO model intercomparison projects, so as to provide more insight into the simulation of the MJO.

The relationship between a model's MJO performance and the CCEW activity during wintertime is examined by analyzing precipitation from 26 GCMs as well as TRMM observations. A model's performance in simulating the MJO is determined by how faithfully it reproduces the eastward propagation of the large-scale intraseasonal variability (ISV) compared to TRMM observations. Results suggest that models that simulate a better MJO tend to: 1) have higher fractional variances for various high-frequency wave modes (Kelvin, mixed Rossby–gravity, westward inertio-gravity, and eastward inertio-gravity waves), which are defined by the ratios of wave variances of specific wave modes to the “total” variance; 2) exhibit stronger CCEW variances in association with the eastward propagating ISV precipitation anomalies for these high-frequency wave modes. The former result is illustrative of an alleviation in the good MJO models of the widely reported GCM deficiency in simulating the correct distribution of variance in tropical convection, i.e., typically too weak (strong) variance in the high (low) - frequency spectrum of the precipitation. The latter suggests better coherence and stronger interactions between these aforementioned high-frequency CCEWs and the ISV envelope in good MJO models. Both factors likely contribute to the improved simulation of the MJO in a GCM.