Variability and trends of the rainy season in West Africa with a special focus on Guinea-Bissau

Abstract This study provides a novel assessment of trends in rainy season behavior (1981–2020) in West Africa by analyzing high-resolution rainfall data from CHIRPS 2.0. Additionally, it presents the first comprehensive study of rainy season characteristics in Guinea-Bissau, based on an analysis of the country's available observational data, over the same period. The agronomic method was used to determine the rainfall onset, while the soil water balance was used to calculate the cessation of the rainy season. The findings indicate that the onset of the rainy season in West Africa progresses from south to north, beginning around 8°N in May and advancing to approximately 16°N by July and early August. The cessation of the rainy season follows an opposite, north-to-south trajectory, typically starting at 16°N in late September or early October and reaching 8°N by November and early December. This regional pattern is associated with the seasonal movements of the Intertropical Front. From 1981 to 2020, the onset, cessation, and duration of the rainy season in West Africa have exhibited significant variability. Although statistically significant trends are limited, these shifts still affect agricultural planning and crop planting schedules, underscoring the importance of continuous monitoring. The standard deviation in the onset of the rainy season ranges from 4 to 55 days, while the cessation shows less variability, spanning between 3 and 21 days. Meanwhile, the number of rainy days varies from 52 in the northern part of the study area to over 200 days in the southern regions. In Guinea-Bissau, rains typically start between 15–30 June each year, with notable differences between the northern and southern parts of the country. The average cessation occurs between the first and second weeks of November. Evolutionary trends suggest a slightly delayed onset of rainfall, with more stability in the cessation dates, leading to a slight reduction in the length of the rainy season.