Spatial distribution of unidirectional trends in climate and weather extremes in Nile river basin

摘要

The recent finding of the influence of long-term persistence (LTP) in time series on trend significance has made the past findings of climatic trends in the Nile river basin (NRB) disputable. Four versions of the Mann-Kendall test including the latest one which considers the LTP in time series have been used in this study to distinguish the unidirectional trend from natural variability of climate in NRB. The gridded Princeton global meteorological forcing data having 1-day and 0.25 degrees temporal and spatial resolution, respectively, for the available period 1948-2010 was used. The results showed that the number of grid points showing a significant change in climate and weather extremes reduced drastically when LTP in time series was considered. The annual rainfall was increasing only at some locations in the main Nile and Atbara sub-basins at a rate of 0.26-26.4mm/decade while decreasing in Sobat sub-basin up to -76.6mm/decade. The maximum temperatures were increasing in the main Nile, Atbara, Blue Nile, Bahr Elgazal, and Bahr Eljabel at a rate of 0.09-0.48 degrees C/decade, while the minimum temperatures were increasing in most parts of the NRB by 0.17-0.50 degrees C/decade. Among the weather extremes, a significant trend over a large part of NRB was found for extreme rainfall days (-0.53-0.75day/decade), cold nights (-6.05-3.26days/decade), heat waves (0.29-2.00days/decade), and cold waves (-4.05-1.15day/decade).