The representation of easterly waves (EWs) over the east Pacific Ocean (PEWs) in Coupled Model Intercomparison Project phase 6 (CMIP6) simulations is examined. Models are assessed based on their ability of reproducing PEW-related precipitation and its evolution. The leading patterns reveal a large spread in the representation of PEW structure and amplitude. A comparison of the models with the most realistic PEWs with those with unrealistically weak PEW skill showed that the more accurate simulations more effectively capture the mean state and EW thermodynamic structures over the northeast Pacific. Particularly, good models exhibit realistic PEW precipitation, weaker meridional mean state humidity gradients, larger mean state precipitation over the Panama Bight, and realistic convective moisture adjustment timescales. These results underscore the importance of accurately representing the sensitivity of PEW convection to anomalous moisture for the realistic representation of the waves.

The moist processes of the Madden-Julian Oscillation (MJO) in the Coupled Model Intercomparison Project Phase 6 models are assessed using moisture mode theory-based diagnostics over the Indian Ocean (10°S-10°N, 75°E-100°E). Results show that no model can capture all the moisture mode properties relative to the reanalysis. Most models satisfy weak temperature gradient balance but have unrealistically fast MJO propagation and a lower moisture-precipitation correlation. Models that satisfy the most moisture mode criteria reliably simulate the background moist static energy (MSE) and low-level zonal winds compared to models that satisfy the least amount of criteria. The MSE budget associated with the MJO is also well-represented in the good models rather than in the poor models. Our results show that capturing the MJO's moisture mode properties over the Indian Ocean is associated with a more realistic representation of the MJO simulation and thus can be employed to diagnose MJO performance.

The governing thermodynamics of the Madden-Julian Oscillation (MJO) are examined using sounding and reanalysis data. On the basis of four objective criteria, results suggest that the MJO behaves like a moisture mode –a system whose thermodynamics is governed by moisture– only over the Indian Ocean. Over this basin, the small effective gross moist stability, i.e., a slow moist static energy (MSE) export by convection, and slow convective adjustment timescale allows moisture modes to exist for all zonal wavenumbers except 1. Elsewhere, the faster-propagating wavenumber 1-2 components are more prominent and the effective gross moist stability is higher, preventing weak temperature gradient (WTG) balance to be established and causing temperature and moisture to play similar roles in the MJO’s thermodynamics.

A moist static energy budget (MSE)-based transformed Eulerian mean framework is applied to a two-layer model to understand how eddies impact the mean tropical circulation during boreal summer. For fixed sources and sinks of MSE, the mean overturning circulation weakens in proportion to the eddy moisture flux divided by the normalized gross moist stability (NGMS), a measure of the moist static stability. Since the NGMS exhibits values between 0.1-0.2 in the rainy areas of the tropics, the eddy moisture fluxes do not have to be large to flatten the column moisture gradient, weaken the trade winds, and cause ITCZ break down. Based on these findings, it is suggested that the recently-documented convective aggregation-disaggregation cycles are mediated by eddy activity. A comparison of days in which the mean circulation is weak versus those that are strong in ERA5 data are largely in agreement with the theory.

A westward-propagating Rossby-like wave signal is found to explain a large fraction of the intraseasonal variance in cloud brightness over the western hemisphere. A series of diagnostic criteria suggest that this wave is a moisture mode: its moisture anomalies dominate the distribution of moist static energy (MSE) and are in phase with the precipitation anomalies; and the thermodynamic equation obeys the weak temperature gradient approximation. The wave propagates westward due to zonal moisture advection by the mean flow and is maintained by radiative heating and meridional moisture advection. These properties compare favorably with the westward propagating Rossby mode in an equatorial beta-plane model with prognostic moisture, mean meridional moisture gradient, and mean zonal wind. These results underscore the importance of water vapor in the dynamics of slowly evolving tropical systems, and the limitations of dry shallow water theory that rely on a “reduced equivalent depth” to represent moist dynamics.