Components of rainy seasons’ variability in Equatorial East Africa: onset, cessation, rainfall frequency and intensity
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The inter-annual and spatial variability of different rainfall variables is analysed over Equatorial East Africa (Kenya and northeastern Tanzania). At the station level, three variables are considered: the total precipitation amount (P), the number of rain days (NRD) and the daily rainfall intensity (INT). Using a network of 34 stations, inter-station correlations (1958–1987) are computed for each of these variables. The spatial coherence of monthly or seasonal P and NRD is always much higher than that of rainfall intensity. However, large variations in spatial coherence are found in the course of the seasonal cycle. Coherence is highest at the peak of the short rains (October–December) and low during the long rains (March–May), except at its beginning. The inter-annual variability of the onset and cessation of the rains is next considered, at the regional scale, and the study extended to 2001. In the long rains, the onset and cessation dates are independent of NRD and INT during the rainy season. Hence, the long rains seasonal rainfall total depends on a combination of virtually unrelated factors, which may account for the difficulty in its prediction. However, the onset, which exhibits a large inter-annual variability and a strong spatial coherence, has a prime role. Conversely, in the short rains, though the onset is again more decisive than the cessation, the different intra-seasonal descriptors of the rains are more strongly inter-related.
This study is a contribution to the PICREVAT project, funded by the “Agence Nationale de la Recherche” under its VMCS program. The authors wish to thank the Kenya Meteorological Department, Nairobi, the Tanzania Meteorological Agency, Dar-Es-Salaam, as well as Patrick Valimba (Department of Water Resources Engineering, University of Dar-Es-Salaam), for providing a large part of the data used in this study.
- Akonga J, Downing TE, Konijn NT, Mungai DN, Muturi HR, Potter HL (1988) The effects of climatic variations on agriculture in central and eastern Kenya. In: Parry ML, Carter TR, Konijn NT (eds) The impact of climatic variations on agriculture. Kluwer, Dordrecht, The Netherlands, pp 121–270Google Scholar
- Camberlin P, Okoola RE (2003) The onset and cessation of the ‘long rains’ in eastern Africa and their interannual variability. Theor Appl Climatol 75:43–54Google Scholar
- DMC (2000) DEKAD 19 report, 1–10 July, 2000. Ten day bulletin no. DMCN/01/337/19/07/2000. Drought Monitoring Centre, Nairobi, KenyaGoogle Scholar
- Ilesanmi OO (1972) An empirical formulation of the onset, advance and retreat of rainfall in Nigeria. J Trop Geogr 34:15–33Google Scholar
- Jackson IJ (1972) Mean daily rainfall intensity and number of rain days over Tanzania. Geogr Ann 54A(3–4):36–375Google Scholar
- Nicholson SE (1996) A review of climate dynamics and climate variability in Eastern Africa. In: Johnson TC, Odada EO (eds) The limnology, climatology and paleoclimatology of the east African lakes. Gordon and Breach, Toronto, pp 25–56Google Scholar
- Oettli P (2008) Précipitations et relief en Afrique orientale et australe: modélisations statistiques et géostatistiques. Ph.D. thesis, Université de Bourgogne, p 287Google Scholar
- Traoré SB, Reyniers FN, Vaksmann M, Kouressy M, Yattara K, Yorote A, Sidibe A, Kone B (2000) Adaptation à la sécheresse des écotypes locaux de sorgho du Mali. Sécheresse 11:227–237Google Scholar
- Wairoto JG, Nyenzi BS (1987) The unseasonal heavy rains in Kenya during January 1979. In Proceedings of the First Technical Conference on Meteorological Research in Eastern and Southern Africa, KMD, Nairobi, 6–9 January, pp 173–178Google Scholar
- Wang J, Anderson BT, Salvucci GD (2007) Stochastic modeling of daily summertime rainfall over the southwestern U.S. Part II: intraseasonal variability. J Hydrometeorol 8:938–951Google Scholar