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 impact of the model-output vertical resolution on the moist static energy (MSE) budget associated with the Madden-Julian Oscillation (MJO) is examined. To achieve this objective, we perform the MSE budget analysis employing reanalysis data at different vertical resolutions: raw output layers, six standard layers as stipulated by the CMIP6 model requirements, high resolution in the upper troposphere, and high resolution in the lower troposphere. Results show that the data with only six layers results in a significant budget residual, consistent with the CMIP6 models observed. Moreover, the data with high resolution in the upper troposphere capture more realistic moist processes than data with high resolution in the lower troposphere. The budget residuals amplify with the rainfall anomalies and correspond to an overestimated vertical MSE advection. The leading bias source is associated with a strong vertical gradient of mean-state dry static energy (DSE) in the upper troposphere, which interacts with anomalous vertical velocity and thus leads to more MSE transport. Unrealistic vertical profiles of mean-state DSE and convection are also observed in the CMIP6 models. Examination of the same analysis using the DYNAMO field campaign shows that budget residuals are evident in low vertical resolution data, yet they correspond to an underestimated vertical MSE advection. The results from these datasets indicate that existing MSE budget residuals in the CMIP6 models are partially attributed to the low vertical resolution data.

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.