An investigation into dry and wet spell characteristics over Zambia and into the onset of the rainy season
Master Thesis
2005
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University of Cape Town
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Abstract
Recurring dry/wet spells and decreased agricultural productivity during the last decade in the southern and northern parts of Zambia point to the need for a clearer understanding of these events, their frequencies and their possible connection to large-scale climate modes. Apart from having a high number of dry spells during the December-February (DJF) core rainy season, the southern part of Zambia is frequently subjected to late onset dates and short durations of the rainy season whereas the northern and northwestern region frequently have high numbers of wet spells which in most cases lead to flooding. Using CMAP and station data, rainfall variability in terms of dry spells, wet spells, onset and cessation dates of the Zambian rainy season and the associated circulation anomalies are investigated. The study also looks at relationship between these extreme events with Nin03.4 SST anomalies. Low level easterly anomalies over Zambia are enhanced during seasons with high dry spells. As a result, there is reduced moisture penetration into Zambia from the Angola low and reduced low level moisture convergence over the country and hence increased dry spells. During the seasons with high numbers of wet spells, there are low level westerly anomalies over southern Angola and western Zambia implying a strong moisture influx from the tropical South East Atlantic and increased convergence over Zambia. It was shown that El Nino (La Nina) events typically result in above (below) average DJF dry spell frequency in Zambia. Early onset dates over the northern parts of Zambia were observed to occur during strong El Nino seasons and the strength of the observed relationship with Nin03.4 SSTs seems to be stronger for years during the 1980s than those during the 1990s. Early onset dates tend to occur for years with higher than average dry spell frequency in the following DJF season. This suggests that early onset may be a disadvantage in that it often leads to more dry spells during the subsequent peak growing period of the season.
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Includes bibliographical references (leaves 113-124).
Reference:
Hachigonta, S. 2005. An investigation into dry and wet spell characteristics over Zambia and into the onset of the rainy season. University of Cape Town.