Abstract
Investigating the influence of sea surface temperatures (SSTs) on seasonal rainfall is a crucial factor for managing Ethiopian water resources. For this purpose, SST and rainfall data were used to study a wide range of inhomogeneous areas in Ethiopia with uneven distribution of rainfall for both summer (1951–2015) and spring (1951–2000) seasons. Firstly, a preliminary subdivision of rainfall grid points into zones was applied depending on spatial homogeneity and seasonality of rainfall. This introduced new clusters, including nine zones for summer rainfall peak (July/August) and five zones for spring rainfall peak (April/May). Afterward, the time series for each zone was derived by calculating the rainfall averaged over grid points within the zone. Secondly, the oceanic regions that significantly correlated with the Ethiopian rainfall were identified through cross-correlations between rainfalls averaged over every homogeneous zone and the monthly averaged SST. For summer rainfall as a main rainy season, the results indicated that the Gulf of Guinea and southern Pacific Ocean had a significant influence on rainfall zones at a lag time of 5–6 and 6–7 months. Besides, for summer rainfall zones 8 and 9 at lag time 5–6 months, the common SST regions of the southern Pacific Ocean showed the opposite sense of positive and negative correlations. Thus, the difference in SSTs between the two regions was more strongly correlated (r ≥ 0.46) with summer rainfall in both zones than others. For the spring season, the results indicated that SST of the northern Atlantic Ocean had a strong influence on spring rainfall zones (3 and 5) at a lag time 6–7 months, as indicated by a significant correlation (r ≥ −0.40). Therefore, this study suggests that SSTs of southern Pacific and northern Atlantic oceans can be used as effective inputs for prediction models of Ethiopian summer and spring rainfalls, respectively.
Highlights
Rainfall variability across time and space affects all aspects of human activity, especially agricultural economies and social activities
The average cross-correlations between and within zones were tested for various times for the optimal selection of a proper number of zones, as shown in Tables 2 and 3, with a reasonable homogeneity delineating a new classification for Ethiopia into nine summer and five spring zones
In case of high spatial and temporal variability of Ethiopian rainfall, it would be crucial to delineate boundaries and define the country into homogenous zones, as the optimal zonation would be valuable for hydrologic modeling, prediction, ecological, and climate classification, or for any other analysis [66,67]
Summary
Rainfall variability across time and space affects all aspects of human activity, especially agricultural economies and social activities. Water 2020, 12, 55 onset, and cessation times, and length of growing season [3,4], but sometimes, rainfall can be temporally varied from days to decades in terms of the direction and magnitude of rainfall trends over regions and seasons [5,6] These spatiotemporal variations in rainfall are attributed to, in Ethiopia, the variable altitudes [3] and over the Pacific, Atlantic, and Indian oceans [7,8,9], the variable sea surface temperatures (SSTs), including the interannual and interseasonal variations in the strength of monsoon over the Arabian Peninsula [4,7,8]
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