Scale Analysis Shows MJO 36-64 Day Period Stems from Equatorial Kelvin Wave Geometry Constraints

The study by Hing Ong applies three geometric constraints to the equatorial primitive equations: meridional narrowness with e-folding latitudes of 7.2-9.6 degrees, zonal planetary scale, and vertical shallowness. Leading-order mass continuity and momentum balances then yield an intrinsic oscillation period of 36-64 days for Kelvin waves, matching the observed MJO band without invoking moist convection or radiation. This purely dynamical result isolates the frequency limit imposed by the flow geometry itself.

Prior MJO theories have emphasized thermodynamic feedbacks or stochastic forcing, yet this analysis shows the timescale is set before those processes enter. It aligns with earlier observational work by Wheeler and Hendon (2004) on MJO propagation but supplies the missing dynamical mechanism that explains why the oscillation cannot occur at synoptic frequencies under planetary geometry. The finding reframes MJO as a filtered Kelvin wave whose speed is capped by meridional trapping.

Improved representation of this geometric constraint in forecast models could reduce tropical precipitation and wind errors at 2-4 week leads, directly benefiting agriculture planting decisions and energy load forecasting in monsoon regions. The open question is how thermodynamics phase-lock to this dynamically enforced scale.

Next steps include testing whether relaxing the meridional-width assumption in GCMs shifts simulated MJO periods outside the observed range, providing a falsifiable check on the scale analysis.