Impact of the 25-70 Day Intraseasonal Oscillation on Extreme Rainfall distribution over Central Africa

This paper investigates the relationship between the intraseasonal oscillation (ISO) and rainfall patterns in Central Africa during the March-April-May (MAM) season. Using CHIRPS and TAMSAT precipitation data from 1983 to 2019, we analyzed the inter-annual variability of ISO spatial structure and its impact on rainfall and extreme rainfall indices. Empirical Orthogonal Function (EOF) analysis classified years into positive (10 years), negative (10 years), mixed (6 years), and neutral (11 years) ISO types. Composite rainfall anomalies were constructed based on these classifications. Results revealed significant inter-annual rainfall variability, with distinct spatial patterns associated with positive and negative ISO years. A significant spatial correlation (over 0.4) was found between ISO variations and rainfall, particularly in the eastern region. Analysis of the impact rate of ISO years showed 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 based on positive and negative ISO years demonstrated varying influences depending on the region and index. Positive ISO years generally saw a decrease in consecutive dry days (CDD) and an increase in consecutive wet days (CWD), extreme rainfall intensity (RR1, RR20, R95ptot, SDII) along the Atlantic coast and northwestern Ethiopia. Neutral ISO years often displayed opposite trends to mixed years, except for the RR1 index. Understanding these relationships is crucial for water resource management in Central Africa, enabling better forecasting and mitigation of extreme rainfall events.