A Mechanism Criterion Replaces Coupling Magnitude for Rainfall Inhibitory Skill Across Three Indo-Pacific Forcing Modes

Pochender Shenigarapu , Sanjeeva Rayudu Ekkaluri

CPAdry — the Conditional Precipitation Anomaly, dry phase — detects rainfall inhibitory mechanism failure across three forcing modes: the MJO (intraseasonal), ENSO, and IOD (interannual). Twenty assessments at eighteen Indo-Pacific sites using two-track methodology establish the governing principle: CPAdry returns meaningful skill only where the forcing mode imposes locally active inhibition — independent of coupling magnitude. At MJO direct-passage sites, diagnostic skill is strong and rising with lead time; at all passive sites, it collapses to chance, with the highest-coupled passive site — Colombo — serving as the central falsifying case. Across the ENSO/IOD arm, four active-mechanism sites spanning warm-pool displacement, Walker relay, thermocline-tilt, and extratropical Rossby wave pathways confirm the same bifurcation with high to moderate skill. Three cross-timescale synthesis findings emerge. First, MJO phase state does not condition ENSO/IOD skill (p = 0.94): ENSO and IOD establish the suppressive umbrella; MJO contributions are absorbed by the accumulation window. Second, a three-layer conditional suppression structure at Colombo reveals that NegIOD alone produces −11.6% suppressive anomaly, while NegIOD combined with active MJO produces −28.4% — neither crosses the −25% threshold alone. Third, this conditional suppressibility is unique to Colombo across the entire non-active network. The coupling-magnitude proxy is replaced by a sign-discriminated suppressive anomaly criterion that correctly classifies all twenty assessments without skill computation.