Senwes Scenario April / May 2019 | Page 66

GRAIN BROKERS El Niño years What happened in the past? This article is the third in a series where we look at the background and development of ENSO (El Niño Southern Oscillation) events. The previous edition looked at the specific classification of ENSO in the form of a weak El Niño event.  By Frans Dreyer Manager: Senwes Grain Brokerage O ne of the findings in the pre- vious article was that history indicates that rainfall during weak El Niño years may even be above average during the planting period from September to December, but it may change to below average cumulative rainfall from January to March during the tra- ditional mid-summer drought period. Rainfall until the end of February once again con- firmed the unique nature of every production season. Figure 1 effectively reflects the opposite trend in respect of rainfall until the end of February 2019, with below average rain which realised during the pre-season period. This realisation could, however, mean that the intensity of the existing weak El Niño even may be stronger than initially expected. It would therefore be sensible to specifically look at the historic impact of a stronger El Niño season on rainfall and the concomitant yield. A strong to very strong El Niño event is identified when sea surface temperatures are warmer than normal, but specifically 64 SENWES SCENARIO | AUTUMN 2019 Figure 1: Rainfall comparison. above a certain degree Celsius band. Sea surface temperatures are expressed as a norm by means of monthly Oceanic Nino Index (ONI) values. Every monthly ONI- value is the 3-month average of the sea surface temperature in the Niño 3.4 area. This classification was sub-divided over time to indicate the expected intensity of an El Niño or La-Niña event. The division is made Source: Thys Grobbelaar in 0.5° Celsius intervals and a strong El Niño is indicated when sea surface temperatures are 1.5° Celsius warmer than normal. This classification makes it possible to categorise previous seasons according to the type of season experienced, as summarised in Table 1. Looking at El Niño and La-Niña events over the long term, the actual impact