El Nino southern oscillation based rainfall forecasts in southern Africa. II. daily forecasts could help summer planting decisions in Zimbabwe

Analysis of historical data of daily rainfall for Harare showed that El Nino Southern Oscillation (ENSO) impacts on the timing of break-of-dry season and follow-up rainfall as well as the frequency of rain events during summer (Nov-Mar). The Southern Oscillation Index (SOI) was a useful indicator of the timing and frequency of these events, defined as 40mm of rainfall in 3 days. Median dates for the first and second events were 1 December and 17 December respectively; ENSO contributed to a shift in these dates of about 2 weeks. The percent chance of receiving break-of-season rainfall by 1 December during La Nina years was 67% compared to 30% in El Nino years. Maize is a staple food in Zimbabwe and delaying planting after 1 December can result in lower yields. Communal farmers could benefit from this apparent ENSO signal in Zimbabwe by adjusting crop variety, planting time and fertilizer rate. However, poor spatial coverage of long-term (>80 years) daily rainfall data in Zimbabwe limits widespread application.