Angola + 2 more

Southern African Regional Rainfall Outlook Seasonal Update No. 1


Seasonal Update
November-December 2000-January 2001 (NDJ) forecast summary

Normal to above normal rains are expected over much of the sub-region for the period of November, December 2000 and January 2001. The northern half of the region is expected to have above normal rainfall with the exception of northern Tanzania which has a likelihood of normal to below-normal rainfall. The southern half of the region should still expect the likelihood of normal to above normal rainfall. This is largely consistent with the Southern Africa Regional Climate Outlook given in Gaborone, September 2000.

Sea surface temperature anomalies and Southern Oscillation Index

The sea surface temperature (SST) anomalies for the equatorial Eastern Pacific (NINO3_4) continue to be near the average category and are expected to remain so for the next three months. Fig. 1 compares the drought year of 1991 in the Nino3_4 region with the current state up-to September 2000. The SST’s for NINO3_4 in 1991 remained generally above 0.5 (El Nino) as compared to the current weak La Nina (-0.4). Fig.2 indicates that the state of the SST anomalies (NINO3_4_00) by the end of September were slightly higher than in 1999 (NINO3_4_1999). Above normal rainfall was received in most areas within the region during the 1999/2000 season.

Fig. 1 also shows Southern Oscillation Index (SOI) comparison of the current situation with that of the 1991. This heralded the drought of 1991/92. The SOI for September 1991 was about -1.8 as compared to +1.0 for September 2000.

Figure 1 SST & SOI anomalies for 1991 and 2000

A positive SOI is favourable for good rains in most of southern Africa. The SOI was slightly below normal in June and July 2000 (Fig. 2). However, by the end of September (SOI_00) it had recovered to become positive (1.0 higher than Sep. 1999).

Figure 2 SST & SOI anomalies for 1999 and 2000

Weather systems

During the forecast period the Inter-tropical-Convergence-Zone (ITCZ) is anticipated to slowly and erratically move southward as part of its seasonal migration. This should allow the persistence of the monsoon trough over the region. Meanwhile, the westerly cloud bands rooted in the mid-latitude weather systems will periodically traverse mostly the southern portions of the subcontinent. The combination of these factors will sustain an influx of moisture for precipitation across the region. Analysis of the current circulation trends and outputs from global models also suggest a transition from winter to summer weather systems during the early part of the period. The conditions favour enhanced precipitation over the region.

October rainfall summary

During the month of October moisture associated with the ITCZ has been affecting most of the western and northern parts of the region. The southward penetration of moisture through Botswana and Namibia to South Africa is being facilitated by frequent frontal intrusions across the south seas. Below normal rainfall was received in most SADC countries during the first and second 10 day periods (dekads) of October. However, the DRC, northwestern Tanzania, and northern South Africa received above normal rainfall during the second 10 day period (dekad). At the same time, southern Tanzania, Lesotho, eastern South Africa received above normal rainfall during the first dekad of the month.

It has to be emphasized however that it is still quite early in the season to draw much conclusion from these rainfall anomalies. These can change dramatically once the rainfall season commences in earnest throughout the region. Currently most of the region is experiencing some good rains.

Methodology

In the climate outlook update the rainfall regionalisation apply to the station network used for the November-December-January period and there are three zones. The factors taken into account were the current state of the La Nina and the sea surface temperature over much of the tropical Indian and the Atlantic Oceans. Also taken into account were the circulation patterns across southern Africa and the adjacent Oceans, and their influence on the weather systems over the region.

Outlook for November-December 2000-January 2001

Zone I: (southern Tanzania, DRC, northern Malawi, northern and western Zambia, Angola, Namibia’s Caprivi Strip and the north-east coastline, Botswana’s north-eastern corner and northern Mozambique) Above normal rains expected.

Zone II: (northern Tanzania) Normal to below normal rainfall expected.

Zone III: (central Zambia, Zimbabwe, South Africa, central and southern Mozambique, much of Botswana and Namibia, southern Malawi, Lesotho, Mauritius and Swaziland) Normal to above normal rainfall expected.

Figure 3 Outlook for NDJ

The numbers in Fig. 3 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 and the bottom below normal. In case of Tanzania (Zone II) for example, there is 20% probability for rainfall occurring in the above normal category; 45% probability in the normal category; and 35% probability in the below normal category. It is emphasized that boundaries between zones should be considered as transition zones.

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