Evidence of Synoptic-Scale Moisture Modes in a Convection-Permitting Aquaplanet Experiment

Westward-propagating disturbances are fundamental for organizing tropical convection and modulating precipitation. While classic theories attribute their existence to dry dynamics with a reduced effective static stability, recent evidence suggests moisture-flow interactions are the primary driver. Isolating these mechanisms in nature is difficult; therefore, this study investigates them using a convection-permitting aquaplanet simulation. Previously, aquaplanet experiments identified ambiguous “advective disturbances” that shared the same wavenumber-frequency space as Doppler-shifted equatorial Rossby waves and tropical depressions, but seemingly lacked the rotational characteristics of those waves. This study tests the hypothesis that these advective disturbances are manifestations of “moisture modes,” while also testing if their mechanisms can be explained by moisture-vortex instability. Using a composite approach, we find that these systems exhibit coherent cyclonic circulation coupled with moisture and precipitation anomalies. Moisture variability dominates the rainfall and thermodynamic variability of these disturbances, supporting our hypothesis. While vorticity and moisture budgets support the growth of these disturbances via moisture-vortex instability, their decay is driven by non-linear eddy processes not accounted for in existing theory. These findings resolve previous ambiguity regarding aquaplanet wave classification and demonstrate that synoptic-scale westward-propagating moisture modes are fundamental tropical phenomena arising from the interaction between convection and meridional moisture gradients.