Zimbabwe + 7 more

Southern African Regional Rainfall Outlook Seasonal Update No. 5

Format
Situation Report
Source
Posted
Originally published


April-May-June (AMJ) 2001 forecast summary
The likelihood of normal to above-normal rains remains in the northern half of the sub-region, Mauritius, the coastal areas of South Africa and southern Namibia for the period May to June 2001 while the central and southern parts are likely to have below-normal rainfall during the same period. The period is also dry over the bulk of the southern region.

Sea surface temperature anomalies and Southern Oscillation Index

The warming in the equatorial Eastern Pacific increased from -0.7 in January 2001 to -0.5 by the end of February 2001 ( Figure 1 ). This is further signaling the weakening of the La Niña phase. However, the sea surface temperatures influence on the rainfall pattern over southern Africa might not be dominant since the period coincides with the end of the rainy season especially over the southern parts of the sub-region. The Southern Oscillation Index (SOI) ( Figure 2 ) remains positive and has reached its highest value since the beginning of the year 2000.

Observed significant weather systems

Dry conditions that characterized the whole of January over the southern half of the sub-region came to an end in February. Instead a low pressure system that was quasi-stationary over north Mozambique shifted southwards introducing wet conditions for Zimbabwe, southern Mozambique, eastern half of South Africa, northern Botswana and Namibia. Drier conditions persisted though, over western South Africa, south-western Botswana, much of Namibia, northern Malawi and Tanzania.

February 2001 rainfall summary

During the month of February there was a significant improvement in rainfall over the whole of Zimbabwe, southern tip of Mozambique, northern Botswana and northwest Namibia ( Figure 3a ). The highest rainfall amounts of between 401 and 500 mm for the month occurred in northwest and eastern Zimbabwe and central Mozambique. The rest of the region received between 100 and 400 mm of total monthly rainfall.

Zimbabwe, central and southern Mozambique, southern Zambia and Malawi, northwestern Botswana and a small area both along the eastern coast of Tanzania and the interior northeast of South Africa recorded about 125% of normal rainfall ( Figure 3b ). The rest of the region recorded normal rains with the exception of western South Africa, southwestern Botswana, much of Namibia, northern Malawi and its surroundings and the northern tip of Tanzania where 74% of normal rains occurred.

The rainfall season for much of the central and southern parts of the sub-region will be virtually over by the end of April as the cool and dry season commences due to the proximity of the sub-tropical high pressure belt. At the same time, the northern sections of DRC and Tanzania will continue to have some active precipitation during this time because of the presence of the ITCZ. However, the coastal areas of South Africa will mostly be affected by frontal systems associated with rain from time to time.

Much of the southern half of the region receives below 100 mm of rainfall during the April to June period with the exceptions of the coastal areas of southern Namibia, South Africa and much of Mozambique ( Figure 4 ). The total rainfall for the period increases northward due to the presence of the ITCZ.

Methodology

In the climate outlook update the rainfall regionalisation apply to the station network available at the DMC which were used for the forecast period covering April, May and June 2001. Four homogeneous rainfall zones were demarcated ( Figure 5 ).

The factors taken into account were the current weak La Niña and the sea surface temperatures (SST’s) over much of the tropical Indian and the Atlantic Oceans and their influence on the regional ocean-atmospheric systems. There was also blending with products from other sister centers and local expert interpretation in order to arrive at the outlook provided below.

Outlook

Zone I: (DRC, Tanzania, Mozambique, northern tip of Zambia, northeastern half of Angola and extreme eastern parts of Zimbabwe) Normal to above-normal rainfall is expected.

Zone II: (much of Zambia, Namibia and Zimbabwe, Malawi, Botswana, northern half of South Africa and southeastern half of Angola) Likelihood of below-normal rainfall is expected.

Zone III: (western and coastal areas of South Africa, Lesotho and Swaziland) Normal to above-normal rainfall is expected.

Zone IV: (Mauritius ) Normal to above-normal rainfall is expected.

Map caption

The numbers for each zone indicate the probabilities for rainfall in each of the three categories, above normal, normal and below normal. The top number indicates the probability of rainfall occurring in the above normal category, the middle number is for the normal category and the bottom for the below normal category. In case of the Zone I for example, there is 30% probability for rainfall occurring in the above normal category: 45% probability in the normal category: and 25% probability in the below normal category. It is emphasized that boundaries between zones should be considered as transition zones.

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