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Abstract A recent theory proposes that tropical depression (TD)‐type waves grow by flattening the mean meridional moisture gradient, consequently weakening the Hadley Cell through a poleward moisture flux. To evaluate this theory, we investigate the seasonality of TD‐type waves and their relation to the Hadley Cell in ERA5 and Coupled Model Intercomparison Project Phase 6 (CMIP6) models. On the basis of the theory, a Hadley Cell instability metric is defined whose variability is largely determined by the background meridional moisture gradient and the sensitivity of rainfall to moisture fluctuations. Results show that both TD‐type wave column moisture variance and eddy moisture fluxes peak when the Hadley Cell instability metric is a maximum. These conditions typically occur when the mean meridional precipitation gradient is strongest and the Hadley Cell is weak and narrow. CMIP6 models that exhibit higher Hadley Cell instability metric simulate stronger TD‐type wave activity in the Northern Hemisphere. 

Abstract Caribbean easterly waves (CEWs) propagate in an environment that is distinct from that of other easterly waves since it exhibits substantial westerly vertical wind shear. In spite of this distinction, their structure, propagation, and growth have not received much attention. A linear regression analysis reveals that these systems exhibit features consistent with moisture modes that are destabilized by moisture–vortex instability. They exhibit large moisture fluctuations and are in weak temperature gradient (WTG) balance, and moist static energy (MSE) growth is partly driven by meridional mean MSE advection by the anomalous winds. However, its circulation tilts vertically against the mean shear, a feature that is often associated with baroclinic instability. To reconcile these differences, a linear stability analysis employing a moist two-layer model is performed using a basic state that resembles the Caribbean Sea during boreal summer. The unstable wave solution from this analysis exhibits a structure that resembles observed CEWs. Excluding the upper troposphere from the stability analysis has little impact on the propagation and growth of the wave, and its circulation still exhibits a westward tilt in height. Thus, baroclinic instability is not the main growth mechanism of CEWs despite their structural similarity to baroclinic waves. Instead, the instability is largely rooted in how the lower-tropospheric circulation interacts with water vapor, as expected from moisture mode theory. These results suggest that tilting against the shear should not be used as the sole diagnostic for baroclinic instability. Baroclinic instability is unlikely to be a primary driver of growth for most oceanic tropical-depression-type waves, in agreement with previous work. Significance StatementThe environment in which Caribbean easterly waves propagate has a vertical wind shear that is like that seen in the midlatitudes, with winds becoming more westerly with height. Furthermore, the center of low pressure of the waves shifts toward the west, as in deepening midlatitude weather systems. This wave structure and shear is different from easterly waves that occur in other regions. However, in spite of the similarity to midlatitude weather systems, we show that Caribbean easterly waves mostly grow from moisture transports in the lower atmosphere. Thus, in spite of the distinct environment and wave structure, Caribbean easterly waves are driven by the same processes as other tropical easterly waves. These results underscore the importance of water vapor in driving tropical circulations. They also indicate that the processes that govern the growth of midlatitude weather may be of less importance in the tropics, even in regions that suggest otherwise. 

Abstract 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 a stronger MJO. The background moist static energy (MSE) and low‐level zonal winds are more realistic in the models that satisfy the most criteria. The MSE budget associated with the MJO is also well‐represented in the good models. Capturing the MJO's moisture mode properties over the Indian Ocean is associated with a more realistic representation of the MJO and thus can be employed to diagnose MJO performance.