Abstract:
Changes in rainfall onset and cessation dates are critical for improving decision making
and adaptation strategies in numerous socio-economic sectors. An objective method of determining
onset and cessation date is employed over Southern West Africa (SWA) in this study. The method is
applied over 34 years of the quasi-global rainfall dataset from the Climate Hazards Group InfraRed
Precipitation with Stations (CHIRPS) and five High Resolution Model Intercomparison Project
(HighResMIP) model datasets under the Coupled Model Intercomparison Project Phase 6 (CMIP6)
experiment. Generally, a strong agreement exists between CHIRPS and the HighResMIP models
in capturing the behaviour of seasonal rainfall over SWA, with models able to capture the bimodal
rainfall season. The ability of models in capturing onset and cessation dates as observed in CHIRPS
shows the strength of these models in representing the short break between the two wet seasons
that is otherwise known as the ‘Little Dry Season’. Patterns observed in the onset and cessation
dates over the SWA region are consistent with the northward and southward displacement of the
Intertropical Convergence Zone (ITCZ). The seasonal timing of the models shows good agreement
with observations such that most mean onset/cessation dates agree within 26 days. While IPSLCM6A-ATM-HR, a model among the five HighResMIPs used in the study, best agrees with CHIRPS in
representing onset and cessation dates during the unimodal rainfall season, no one model best agrees
with CHIRPS during the bimodal season, with models outperforming each other in representing
onset/cessation dates with little variation.
