Horn of Africa: Dominant Weather Systems in March and the Climate Outlook for May to July 2000

During March 2000, Cold episode conditions (La Niña) persisted in the equatorial Pacific Ocean. On the other hand upper level convergence and positive outgoing long-wave radiation (OLR) anomalies persisted over much of the eastern Africa subregion. Above average sea surface temperatures (SSTs) dominated the Mascarene, St. Helena and Azores regions while near normal SSTs were observed over the Arabian Sea. The tropical cyclone activity in the Indian Ocean over the last few months resembled what was observed in 1984. Consequently much of the eastern African subregion experienced drier than normal conditions save for the southern parts of the subregion which continued to experience above to near normal rainfall.

Analysis from the Climate Prediction Center (CPC), USA, for March 2000 indicates that the Southern Oscillation Index (SOI) decreased from 1.6 in February to 1.0 in March 2000.There was a reduction in cooling of the subsurface waters over eastern equatorial Pacific with Niño 1 + 2, Niño 3 and 3.4 indices reducing from -0.2°C, -0.9°C and -1.5°C in February to 0, -0.4°C and –1.1°C in March 2000 respectively. However, Niño 4 index region had the same anomalies as in the previous month.

La Niña episodes have strong influence over many parts of eastern Africa that receive significant rainfall during March to September period. They are often associated with below normal and/or poorly distributed rainfall over the equatorial areas during March to May season and above normal rainfall over the northern sector of the subregion during June to September season. However, sea surface temperatures over Atlantic and Indian Oceans play a significant role in modulating the La Niña impacts in the subregion.

Most of the available dynamical and statistical model forecasts by the advanced climate centres indicate that the cold episode conditions are likely to persist over the next three months. Colder than normal sea surface temperatures (SSTs) are also projected to continue developing over the western Indian Ocean while warmer than average SSTs are likely to persist over southern Indian Ocean and much of Atlantic Ocean during the next three months.

It is therefore expected that the continuation of the cold episode conditions, coupled with the anticipated SST patterns over western and southern Indian and much of Atlantic Oceans are likely to result into rainfall suppression over the equatorial eastern Africa and rainfall enhancement over parts of the northern sector of the subregion during May to July 2000.

There are enhanced probabilities of below normal rainfall over Somalia, southeastern Ethiopia, much of Kenya, southern and central Uganda, Rwanda, Burundi and northern Tanzania.

High probabilities of near normal rainfall are forecast for Djibouti, western and central Ethiopia, southern Sudan, northern and eastern Uganda and western Kenya while above normal rainfall is the most likely outcome over much of Eritrea, northern Ethiopia and central and northern Sudan.  Southern Tanzania will remain generally dry as expected .