2 3 Gross Moist Stability and MJO Simulation Skill in Three Full - physics

27 28 Previous studies have demonstrated a link between gross moist stability (GMS) and intraseasonal 29 variability in theoretical and reduced-complexity models. In such simplified models, moisture modes— 30 convectively coupled tropical disturbances akin to the MJO whose formation and dynamics are linked to 31 moisture perturbations—develop only when GMS is either negative or " effectively " negative when 32 considering additional sources of moist entropy. These simplified models typically use a prescribed, 33 time-independent GMS value. Limited work has been done to assess GMS and its connection to 34 intraseasonal variability in full-physics general circulation models (GCMs). 35 The time-mean and intraseasonal behavior of normalized GMS (NGMS) are examined in three 36 pairs of GCMs to elucidate the possible importance of NGMS for the MJO. In each GCM pair, one 37 member produces weak intraseasonal variability while the other produces robust MJOs due to a change 38 in the treatment of deep convection. A highly correlated linear relationship between time-mean NGMS 39 and MJO simulation skill is observed, such that GCMs with less positive NGMS produce improved 40 MJO eastward propagation. The reduction in time-mean NGMS is primarily due to a sharp drop to 41 negative values in the NGMS component related to vertical advection, while the horizontal advection 42 component has a less clear relationship with MJO simulation. Intraseasonal fluctuations of anomalous 43 NGMS modulate the magnitude of background NGMS but generally do not change the sign of 44 background NGMS. NGMS declines ahead of peak MJO rainfall and increases during and after heaviest 45