Inter Annual Variability of Onset and Cessation of the Long Rains in Kenya

Abstract

This study examined the spatial and temporal variability of the onset and cessation of long rains (MAM). The daily rainfall data used in the study was obtained from the Kenya Meteorological Department (KMD. Both graphical and statistical methods were applied in the analyses. Most of the stations indicated the onset month to be March. However, a few stations indicated February as their onset month while a few others had April as the onset month. Further analysis indicated a link between performance of the seasonal rainfall and the onset date. Thus positive (negative) rainfall anomaly was associated with early (late) onset. The results from the analysis showed that over most stations, the mean onset is the 74th day (14th march) with an average standard deviation of 24 days. The mean cessation day was day 150 (29th of May) with an average standard deviation of 21 days. The highest variability was observed in dry areas. The study showed that over most parts of the country there is very high likelihood of below normal rainfall when the onset is late. However, over the north western part of the country (region 1) which receive it seasonal rainfall over a short period, the rainfall performance may not be affected by the late the onset It was also noted that over most regions there is high chances of near normal rainfall occurring when the onset is normal. The chances of below normal rainfall occurring when the onset is normal are low. It was noted that whereas the chances of below normal rainfall occurring when there is an early onset were minimal, The western region (region 12) represented by Elgon had sub-stantive chance of having below normal rainfall even with early onset. The onset over this region is associated with the low level westerlies from Congo, while the performance of the seasonal rainfall over this region is largely linked with the characteristics of the ITCZ.