Interannual climate variability and shifting climate-stress regimes in Western Nigeria: Evidence from trends, anomalies, and compound events

Climate assessments require evidence not only of changes in individual climatic variables but also of how thermal and moisture stressors combine over time. This study assessed climate-stress reorganization across Kwara, Ogun, Ondo, and Oyo states in Western Nigeria from 1982 to 2023 using NASA POWER data. Trends were evaluated with Mann–Kendall, modified Mann–Kendall, and Sen’s slope statistics; standardized anomalies and Pettitt tests identified departures and distributional shifts; and daily stress indicators, compound-event metrics, a Compound Agroclimatic Stress Index (CASI), and ordinary kriging characterized stress pathways and spatial gradients. Minimum temperature increased significantly in all states by 0.18–0.29 °C/decade, whereas maximum temperature showed no significant trend. Rainfall increased significantly in Ogun (109.02 mm/decade) and marginally in Oyo (64.76 mm/decade), while relative humidity increased significantly in Ogun, Ondo, and Oyo by 0.69–1.08 %/decade. Surface soil wetness increased significantly in Ogun and Oyo by 0.04 and 0.02/decade, respectively. Anomalies shifted from predominantly negative minimum-temperature and moisture departures early in the record toward more frequent positive departures after the early 2000s. Pettitt tests identified significant change points in 12 of 24 state-variable series, including minimum-temperature breaks in all states during 2000–2002. Atmospheric dry-heat days declined significantly across all states by 2.07–2.86 days/decade, while extreme-rain-plus-wet-soil days increased in Ogun and Oyo and marginally in Kwara. CASI declined in three states because weakening dry-heat and soil-deficit components outweighed increasing hot-night and wet-excess pathways. Western Nigeria is therefore experiencing a spatially uneven reorganization of climate stress rather than uniform warming, drying, or wetting.