Angola + 3 more

Southern African Regional Rainfall Outlook Seasonal Update No. 3


February-March-April (FMA) 2001 forecast summary
Normal to above normal rainfall is expected over much of the northern half of the sub-region, for the period February to April 2001 (FMA). Most of the southern half is likely to receive normal to below normal rainfall as the period coincide with the retreat of the ITCZ towards the north.

Sea surface temperature anomalies and Southern Oscillation Index

The sea surface temperature (SST) anomalies for the equatorial Eastern Pacific are projected to remain negative until the end of March when they are expected to become positive, further weakening the La Nina. The December 2000 SST anomalies (NINO 3.4) were near average but warmer than in December 1999 ( Figure 1 ).

Meanwhile, the Southern Oscillation Index (SOI) though remaining positive decreased from its highest value of 2.0 in November 2000, to about 0.7 in December 2000. The December 2000 value is lower than that of 1999 but higher than that of 1991 at the same time ( Figure. 2 ). Negative SOI is associated with low rainfall in southern Africa as shown with the 1991 SOI tracing below

Observed significant weather systems

From October 2000 up to mid December 2000, the middle level subtropical high pressure system was significantly weak and this allowed the monsoon trough to get established over southern Africa. The moisture associated with the monsoon trough resulted in significant rainfall over much of the region except for Namibia, Botswana, southern Zimbabwe and western parts of South Africa. Towards the end of December 2000 the middle level subtropical high pressure system started to intensify thus suppressing rainfall activity over the southwestern parts of the sub-region. The development of a low pressure system in the Mozambique channel in early January 2001 instigated the strengthening of the subtropical high further reducing the influx of moisture over Namibia, Botswana South Africa and southern Zimbabwe. The high pressure system also restricted the southern limits of the ITZC over Angola, Zambia, Malawi, northeastern Zimbabwe and the central and northern parts of Mozambique for the greater part of January.

December 2000 rainfall summary

During the month of December 2000 the northern half of the sub-region was generally wet while the southern half was relatively dry ( Figure 3a) . Southern Angola, Namibia, South Africa, the southern tip of Mozambique, Botswana and southern Zimbabwe received less than 100mm cumulative rainfall while the rest of the region received above 100mm of rainfall. The highest (>200 mm) rainfall was recorded in Zambia, eastern Angola, the southern tip DRC, parts of southern Tanzania, Seychelles and northern Mozambique.

An analysis of Figure 3(b) shows that the DRC, northern Angola, most of Zambia, western Tanzania, northern and southern Malawi, central Mozambique, northern and eastern Zimbabwe, southern Botswana, Lesotho, eastern South Africa and Swaziland received normal (75 to 125%) rainfall during the month of December. Below normal (<75%) rainfall was received over southern Angola, Namibia, northern Botswana, southern Zimbabwe, southern Mozambique and western South Africa. Above normal (>125%) rainfall was registered in Seychelles, eastern Tanzania, northern Mozambique, central Malawi, north-western Zambia and north-central South Africa.

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 February, March and April 2001. Seven homogeneous rainfall zones were demarcated ( Figure 4 ).

The factors taken into account were the current weakening La Nina 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: (much of DRC and northern Tanzania) Normal to above normal rainfall is expected.

Zone II: (northern Angola, southern DRC, southern Tanzania, northern Zambia, Malawi, northern Mozambique and northern Zimbabwe ) Normal to above normal rainfall is expected.

Zone III: (south-eastern Angola, south-western Zambia, north-eastern Namibia, northern Botswana, southern Zimbabwe, southern Mozambique, north-eastern South Africa and Swaziland) Normal to below normal rainfall is expected.

Zone IV: (southwestern Angola, most of Namibia, southern Botswana, northern and south-eastern South Africa and Lesotho) Normal to below normal rainfall is expected.

Zone V: (the Cape Province of South Africa) Normal to above normal.

Zone VI: (Mauritius) Normal to above normal.

Zone VII: (Seychelles) Normal to above normal.

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 Seychelles (Zone VII) 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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