Browsing by Author "Reason, C.J.C."

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    Evolution of the 2002-2004 drought over northern South Africa and potential forcing mechanisms
    (Academy of Science of South Africa (ASSAf), 2005) Reason, C.J.C.; Phaladi, R.F.
    We consider the evolution of the 2003/04 summer drought over northern South Africa to wetter than average conditions by the end of the season. This season was neutral (that is, it did not coincide with either an El Niño or a La Niña event) and yet recorded wellbelow-average rainfall for at least the first four months of the summer (October–January) with a transition to above-average rainfall by March. Previous work investigating other neutral summers with significantly below-average rainfall over the region (1951/52, 1967/68, 1981/82) have indicated that, in contrast to El Niño dry summers, there are mid-latitude circulation anomalies south and southwest of South Africa that lead to an increase (decrease) in the advection of cool, dry (warm, moist) South Atlantic (South Indian) air masses over South Africa and hence dry conditions. It is shown that a similar situation occurred in October–December 2003 but these circulation anomalies started to break down in January 2004, resulting in a transition towards above average rainfall by the end of summer. Although the basic mechanism (modulated air mass advection) behind the early summer drought is clear, the midlatitude circulation anomalies that lead to this situation for each significantly dry neutral summer (1951/52, 1967/68, 1981/82, 2003/04) are somewhat different. As a result, early identification of these patterns and potential forecasting of dry conditions prior to the start of summer is difficult.