Stratospheric QBO and Rainfall Association in West Africa

Abstract The Quasi-Biennial Oscillation (QBO) is a dominant mode of variability in the equatorial stratosphere, characterized by alternating easterly and westerly zonal wind regimes that descend from higher altitudes toward the tropopause. Despite numerous studies on QBO in the tropical region, its association with rainfall over West Africa is rarely quantified. This study investigates the influence of QBO phases on atmospheric dynamics and rainfall patterns in West Africa, with a focus on phase-dependent modulation of regional climate systems. Through a combination of Causal Correlation Mapping (CCM) and composite analyses, the study reveals a bidirectional causal relationship between QBO phases and West African rainfall. Results suggest that QBO is a strong driver of rainfall pattern over West Africa. This relationship holds for all QBO phases, with the strength of correlation for QBO-rainfall unidirectional coupling reaching a maximum of 0.50 during the easterly phase and 0.83 during the westerly phase. Furthermore, composite anomalies reveal enhanced (suppressed) rainfall during the westerly (easterly) phase over West Africa. The unidirectional forcing of QBO and West African rainfall is dynamically linked with the modulation of tropopause conditions and changes in large-scale circulation patterns in the troposphere. The westerly phase of the QBO (QBOW) exerts a stronger influence on rainfall, driven by enhanced monsoonal circulation, moisture flux, and vertical motion, resulting in increased precipitation over large portions of West Africa. Conversely, the easterly phase (QBOE) is associated with suppressed rainfall due to weakened convective activity, reduced moisture transport, and an anomalous advection of dry air. The analysis further examines the role of QBO in modulating large-scale atmospheric features such as the Intertropical Convergence Zone (ITCZ), West African Westerly Jet (WWJ), and Tropical Easterly Jet (TEJ), as well as interactions with tropical deep convection. Significant variations in tropopause height and cloud cover across QBO phases highlight the complex interactions between stratospheric processes and regional climate dynamics. The findings of this study provide a comprehensive understanding of how QBO-driven stratospheric dynamics influence lower atmospheric processes and contribute to regional climate variability in West Africa.