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

Iddi Mapande , January G Msemakweli , Issa Hussein Rwambo, Oscar Punguti , Hussein Mohamed

Background: Coastal cities like Dar es Salaam face an increasing vulnerability to the adverse impacts of climate variability, including urban flooding, heat stress, and altered water availability. Examining the temporal evolution of key climatic variables is critical to informing adaptive strategies and promoting sustainable urban development.

Methods: Decadal seasonal trends in rainfall, daytime and nighttime temperatures, and relative humidity were analyzed using monthly data from January 2014 to October 2024, sourced from the Tanzania Meteorological Authority. We applied the non-parametric Mann-Kendall trend test and Sen’s slope estimator to detect and quantify monotonic trends across five defined seasons.

Results: We observed statistically significant trends across several seasons. Rainfall during the long dry season (JJA) had a slight increase, with a Sen’s slope of +0.39097 mm/year and a p-value of 0.00457, suggesting a slight deviation from typical seasonal dryness. Daytime temperatures during JJA declined (Sen’s slope = –0.03675°C/year; p = 0.00121), and nighttime temperatures during the short dry season (JF) also showed a significant decrease (Sen’s slope = –0.04000°C/year; p = 0.04445). Relative humidity demonstrated only minor, statistically insignificant fluctuations across all seasons, with the highest z-value recorded in OND.

Conclusion: The findings highlight shifting climatic patterns in Dar es Salaam that diverge from conventional expectations, including increased precipitation during dry periods and seasonal cooling. Although the detected trends are modest in absolute magnitude, their statistical significance is noteworthy because consistent directional changes, however small, accumulate over time and may signal early-stage climatic shifts that warrant monitoring.