El Niño impacts on Africa

Report
from US Agency for International Development
Published on 30 Sep 1997


El Niño-Southern Oscillation

  • Overview of Impact on Weather in Africa
  • Effects in Zimbabwe
  • Effects in Zambia
Other El Niño Links


Overview

Researchers are developing several long-range weather forecasting tools, many of which are based on El Niño-Southern Oscillation (ENSO), which is a cyclical pattern involving the weather and the oceans. If such forecasts prove reliable, they would be quite valuable in providing early warning of drought.

Fueled by solar energy, the Earth's oceans and atmosphere follow a cyclical pattern of change. Sunshine warms the ocean waters each day, and during periods when all of this heat is not lost overnight, the waters begin to warm very slowly. The warming ocean waters, particularly in the large Pacific Ocean basin, warm the air, causing it to rise, eventually leading to changes in global atmospheric circulation patterns (figure). The warm air over the ocean increases evaporation, resulting in cloudiness and rainfall that block the sunshine and eventually lead to cooling of the waters. After ocean temperatures cool to normal levels, the atmosphere also returns to previous circulation patterns, completing the ENSO cycle.

Weather patterns in many parts of the world appear to be related to different phases of the ENSO cycle, and one of the stronger relationships is between the warm-ocean phase of the ENSO cycle (El Niño) and drought in southern Africa. Warm episodes beginning in April or May have a particularly strong relationship with drought in this area. Relationships to weather patterns in other parts of Africa are less clear, although there is a tendency for wetter weather in east Africa during a warm ENSO phase and there are some indications that a severe warm event is related to drought in western Africa.

NOAA has now issued an ENSO advisory that a warm event is developing. Having started in the April - May period and having developed quite rapidly, this may be one of the stronger episodes. FEWS and other groups are monitoring this event carefully to track its development and determine its likely effect on weather and crops.



Effects of El Niño in Zimbabwe

El Niño events beginning in the first half of the calendar year have a strong relationship to below-normal rainfall in virtually all of Zimbabwe during the following rainy season. The relationship is generally stronger in the south than in the north, but it is also strong in an area clustering around Harare and in the Gutu area. The map shows the increased risk of below-normal total seasonal rainfall at stations in Zimbabwe during an El Niño event which begins during the January through June period. For this analysis, the rainfall season begins in the October following the onset of the El Niño and lasts for a year. The El Niño event begins when the moving average of the past five monthly SOI anomaly values becomes negative.



Effects of El Niño in Zambia

El Niño events beginning in the first half of the calendar year have a strong relationship to below-normal rainfall in much of Zambia during the following rainy season. The relationship is generally stronger in the south than in the north. The map shows the increased risk of below-normal total seasonal rainfall at stations in Zambia during an El Niño event which begins during the January through June period. For this analysis, the rainfall season begins in the October following the onset of the El Niño and lasts for a year. The El Niño event begins when the moving average of the past five monthly SOI anomaly values becomes negative.

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