Linking Hadley Cell Instability to Slow Equatorial Motions in Reanalysis and CMIP6 Models

Abstract A recent theory–originally proposed for tropical depression (TD)‐type waves–is extended to the Madden‐Julian Oscillation (MJO) and convectively‐coupled equatorial Rossby (ER) waves across the Indo‐Western Pacific region using reanalysis and Coupled Model Intercomparison Project Phase 6 (CMIP6) data. This framework posits that waves grow from a Hadley Cell‐linked instability, quantified by a metric tied to the product of the meridional gradient of background vertical velocity and planetary vorticity. Results show that wave activity, diagnosed via wave‐filtered moisture variance and poleward eddy moisture fluxes, is strongest in MJO and ER waves when the instability metric peaks. CMIP6 models with a higher instability metric exhibit stronger Southern Hemisphere MJO and ER waves. While significant in both, the correlations are stronger for ER waves. These results suggest that ER waves are driven by the same mechanisms as TD‐type waves, whereas other processes not accounted for in this framework are also important for the MJO.