MJO Phase-Response Diagnostic Skill Reflects Convective Regime Contingency Beyond Coupling Strength Across Tropical Sites

Pochender Shenigarapu , Sanjeeva Rayudu Ekkaluri

MJO phase composites are the standard tool for building tropical rainfall diagnostic frameworks. The coupling strength between the MJO phase and local rainfall is routinely used to justify their application. Whether coupling strength alone guarantees diagnostic skill — or whether the nature of the underlying convective regime is the additional governing condition — has not been examined. We introduce CPAdry, the Conditional Probability Anomaly for strongly suppressive MJO phases, and evaluate it across seven tropical sites spanning the Indo-Pacific and tropical Africa. Where the MJO exerts direct convective envelope control — Darwin (η² = 7.5%) and Dili (η² = 6.7%) — CPAdry achieves robust precursor skill at 5–15 day lead times (AUC 0.69–0.76). Where remote teleconnections govern, or where MJO-rainfall coupling is structurally absent, skill is negligible regardless of coupling magnitude. The sharpest demonstration is Colombo — the strongest-coupled site (η² = 11.2%) — which yields AUC indistinguishable from random (0.50). Coupling strength is necessary but insufficient. Convective regime contingency is the additional governing condition. These results formalise the active-versus-passive modulation principle: CPAdry detects failures of direct local MJO inhibition but not the absence of remotely modulated convective enhancement. This asymmetry holds within individual sites — suppression failure AUC 0.69–0.76, enhancement failure AUC 0.52–0.55 — and is structural: a designed regime-conditioning falsification test failed to recover wet-phase skill (the best conditioned AUC was 0.569, with the remaining AUCs 0.06 below the rolling baseline). A coupling–contingency threshold near η² ≈ 5–6% demarcates the transition from direct-envelope-passage to remote-teleconnection regimes. Below it, skill collapses to chance level regardless of coupling magnitude. Independent Bureau of Meteorology gauge validation at Darwin corroborates and modestly strengthens all reanalysis-derived estimates. Projected changes in MJO amplitude, propagation speed, and Maritime Continent barrier penetration under warming are likely to shift this threshold geographically — making the η² ≈ 5–6% boundary condition a practical benchmark for monitoring the climate-driven evolution of MJO diagnostic utility.