A Systematic relationship between the representations of convectively coupled equatorial wave activity and the Madden-Julian oscillation in climate model simulations

Abstract

The relationship between a model's performance in simulating the Madden-Julian oscillation (MJO) and convectively coupled equatorial wave (CCEW) activity during wintertime is examined by analyzing precipitation from 26 general circulation models (GCMs) participating in the MJO Task Force/Global Energy and Water Cycle Experiment (GEWEX) Atmospheric System Study (GASS) MJO model intercomparison project as well as observations based on the Tropical Rainfall Measuring Mission (TRMM). A model's performance in simulating the MJOis determined by how faithfully it reproduces the eastward propagation of the large-scale intraseasonal variability (ISV) compared to TRMMobservations. 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, and westward and eastward inertio-gravity waves), which are defined by the ratios ofwave variances of specific wavemodes to the "total" variance, and 2) exhibit strongerCCEWvariances in association with the eastward-propagating ISVprecipitation anomalies for these high-frequencywavemodes. 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.