Impact of the 25-70 day intraseasonal oscillation on extreme rainfall distribution over Central Africa

Abstract

This paper investigates the relationship between the intraseasonal oscillation (ISO) and extreme rainfall patterns in Central Africa during the March-April-May (MAM) season. Using advanced statistical techniques on CHIRPS and TAMSAT precipitation data from 1983 to 2019, we analyze the inter-annual variations of the 25-70 day ISO spatial structure and its impact on rainfall distribution and extreme rainfall indices. Result shows a marked and consistent year-to-year variation in the spatial distribution of the wave amplitude within the region, enabling a clear distinction between years characterized by positive, unfavorable, mixed, or neutral oscillations. This classification was established using Empirical Orthogonal Function analysis and a Lanczos band-pass filter to isolate the 25–70-day signal, complemented by an assessment of the interannual spatial distribution of wave amplitudes. These years significantly influence differently, the spatial distribution of the seasonal rainfall anomalies. Composite rainfall anomalies revealed significant inter-annual rainfall variability, with distinct spatial patterns associated with ISO phases. A statistically significant spatial correlation (exceeding 0.4; p < 0.05, based on Student’s t -test), was observed between ISO variations and rainfall, especially in the eastern region. Analysis of the impact rate demonstrated a more nuanced distribution in CHIRPS data compared to TAMSAT. Extreme rainfall indices, calculated using ETCCDI methods, exhibited spatial disparities, with dry zones in the north and south contrasting with wetter coastal areas and Lake Victoria. Composite extreme rainfall index anomalies highlighted varying influences depending on the region and index. Positive ISO years are generally characterized by a decrease in consecutive dry days (CDD) alongside an increase in consecutive wet days (CWD) and rainfall intensity indices (RR1, RR20, R95ptot, and SDII), particularly along the Atlantic coast and northwestern Ethiopia. This pattern undergoes a near-symmetrical inversion during unfavorable ISO phases, demonstrating a robust bi-modal influence on the spatial distribution of dry spells and precipitation extremes across Central Africa. In contrast, mixed ISO years transition toward a complex latitudinal stratification of anomalies, while neutral phases exhibit a generally weakened and inverse spatial signature, except over mountainous and lacustrine regions where local topographic effects appear to dominate the RR1 response. Collectively, these results reveal that neutral ISO years typically display anomaly patterns opposite to those of mixed years, with the notable exception of rainfall frequency. Understanding these phase-specific relationships is crucial for water resource management in Central Africa, as it enables improved forecasting and mitigation of extreme hydrometeorological events