Decadal Trends in Seasonal Climatic Variables in Dar es Salaam, Tanzania: A Non-Parametric Approach Using the Mann-Kendall Test

Background Coastal urban cities like Dar es Salaam, Tanzania, are increasingly vulnerable to the adverse effects of climate variability, including urban flooding, heat stress, and changes in water availability. Understanding the evolution of key climatic variables over time is essential for supporting adaptive strategies and sustainable urban development. Methods This study analyzed decadal seasonal trends in rainfall, daytime and nighttime temperatures, and relative humidity using monthly data from January 2014 to October 2024 obtained from the Tanzania Meteorological Authority. The analysis utilized the non-parametric Mann-Kendall trend test and Sen’s slope estimator to detect and quantify monotonic trends across five seasons. Results Statistically significant trends were identified across multiple seasons. Rainfall during the long dry season (JJA) showed an increasing trend with a Sen’s slope of +1.95 mm/year and a p-value of 0.005, indicating a notable deviation from expected seasonal dryness. Also, daytime temperatures during JJA declined significantly with a Sen’s slope of –0.038°C/year (p = 0.001), while nighttime temperatures during the short dry season (JF) also exhibited a significant decreasing trend (Sen’s slope = –0.062°C/year; p = 0.044). Relative humidity exhibited only minor, statistically insignificant fluctuations across all seasons, with the highest z-value observed in OND. Conclusion The findings underscore shifting climatic patterns in Dar es Salaam that deviate from conventional expectations, such as increased precipitation during dry periods and cooling in some seasons. Hence highlighting the need for climate-informed urban planning and infrastructure development and the importance of continued localized climate monitoring to support evidence-based policy and resilience-building measures.