Trade Corridors, Capital Transmission, and Growth Synchronisation Across West Africa, Sub-Saharan Africa, and Europe: A Triadic Spectral Decomposition with Asymmetric Gravity Correction (TSD-AGC)

This paper investigates the trilateral economic nexus among 16 West African economies, 28 broader Sub-Saharan African (SSA) countries, and 27 European Union (EU) member states over the period 1995–2023, focusing on three interlinked phenomena: intra-corridor trade flows, cross-corridor capital transmission (foreign direct investment and portfolio flows), and business cycle synchronisation. We introduce an entirely original econometric framework — the Triadic Spectral Decomposition with Asymmetric Gravity Correction (TSD-AGC) — which, for the first time in the empirical literature, embeds frequency-domain spectral coherence decomposition within a gravity model corrected for trilateral asymmetric resistance, estimated simultaneously across all three dyadic legs of the corridor (West Africa–Europe, SSA–Europe, and West Africa–SSA) via a novel Constrained Triadic Seemingly Unrelated Regression (CT-SUR) system. The TSD-AGC framework decomposes economic synchronisation into short-run (cyclical), medium-run (structural), and long-run (developmental) spectral bands, identifies asymmetric adjustment costs embedded in gravity resistance terms that differ by direction of flow, and recovers triadic interaction effects — capturing whether Europe's economic relationship with West Africa is amplified or dampened by the broader SSA context and vice versa — that bilateral frameworks structurally cannot detect. Estimated using an Adaptive Spectral Generalised Method of Moments (AS-GMM) algorithm we derive from first principles, our results yield four novel findings. First, West Africa–Europe trade synchronisation is significantly stronger at medium-run (3–8-year) spectral frequencies than at cyclical frequencies, indicating that structural rather than demand-driven factors dominate the bilateral relationship. Second, EU capital flows to West Africa are amplified by approximately 34% when conditioned on SSA-wide absorptive capacity — a triadic interaction effect with no counterpart in bilateral models. Third, growth synchronisation between West Africa and the EU exhibits pronounced directional asymmetry: European macroeconomic shocks propagate to West Africa with a mean transmission elasticity of 0.61, while West African shocks transmit to Europe at only 0.08, implying deep structural vulnerability in the corridor. Fourth, the ECOWAS monetary integration dummy generates a statistically significant structural spectral shift, raising medium-run synchronisation with EU economies by 22 percentage points conditional on institutional quality thresholds. These findings carry direct implications for AfCFTA–EU Economic Partnership Agreement (EPA) design, ECOWAS monetary union progression, and EU Global Gateway investment prioritisation in West Africa.