El Niño – its impact on Subtropical crops

Andrew Sheard (Subtrop) 

Fruit farmers in South African know all too well the vagaries of our harsh climate and current weather conditions. These play an important role in flowering, fruit set and final yield of our crops. Recent press coverage has warned of the return of the dreaded El Niño effect this summer. A recent forecast by the South African Weather Service ( 9 July 2009) predicts that the 2009/10 El Niño event will be moderately strong but emphasizes that it’s intensity does not indicate how much the event will influence rainfall & temperature in Southern Africa during the summer months.

What is El Niño?

It is the warming of sea-surface temperatures in the equatorial Pacific Ocean which influences atmospheric circulation, and consequently rainfall and temperature in specific areas around the world. The term comes from the Spanish translation, "the boy child", which Peruvian anchovy fishermen used to describe the appearance of a warm ocean current off the South American coast around Christmas. There have been approximately 14 El Niño events between 1950 and 2003 with the 1997/98 event being the strongest recorded.  South Africa appeared to escape its full impact.

El Niño is often mentioned in conjunction with La Niña which is the cooling of sea-surface temperatures in the equatorial Pacific Ocean. The changes in the Pacific Ocean are represented by the term "El Niño/La Niña", while changes in the atmosphere are known as the "Southern Oscillation". These terms cannot be separated, resulting in the term ENSO (El Niño Southern Oscillation) being used which refers to a combination of the terms. The scientific definition of ENSO is as follows: When the three-month running mean of the surface sea temperature in the Nino 3.4 region of the Pacific Ocean are greater than or equal to 0.5°C, there is a good chance of an El Niño event taking place. When the anomalies are smaller than or equal to -0.5°C, there is a good chance of a La Niña event taking place. During normal (non-ENSO) years (Fig. 1), an upwelling of cold water along the Peruvian coast causes relatively cold water to occur along the west coast of South America. The cold water then flows westward along the equator to Australia, being heated along the way by the tropical sun. These normal conditions make the western Pacific about 3°C to 8°C warmer than the eastern Pacific.  

  

Figure 1: Normal conditions in the Pacific Ocean and the atmosphere (Source: http://www.weathersa.co.za/)

During an El Niño year (Fig. 2), the area of warm water (usually over the western tropical Pacific near Australia) cools down and the warm water is displaced eastward to the central Pacific. The upwelling of cold water off the Peruvian coast is suppressed and the surface sea temperatures in this region become warmer than usual.  

Figure 2: Conditions in the Pacific Ocean and the atmosphere during El Niño  (Source: http://www.weathersa.co.za/)

What impact will ENSO/El Nino have on farming and our weather over the period of November to January 2010?

Below normal rainfall is being anticipated over the eastern parts of SA up until mid/late January 2010 (Fig. 3a). The conditions are expected to improve towards February 2010. Maximum temperatures are expected to be above normal for the period with heat wave conditions being predicted (Fig. 3b). The current heat wave conditions being experienced in the Limpopo/Mpumalanga areas may be an indication of more to come. It should be noted that ENSO only explains approximately 30% of the rainfall variability, meaning that other factors also play a role in predicting seasonal rainfall & temperatures. The 1997-98 was the strongest recorded El Niño, but not all of South Africa received below-normal rainfall. Some regions received above average rainfall because of moist air coming in from the Indian Ocean.

  
Figure 3: Predicted rainfall (a) and max temperatures (b) over Africa and South Africa during November 2009 to January 2010 (Source: http://iri.columbia.edu/climate/forecast/net_asmt/) 

Based on the current conditions and the forecasted hot dry conditions, farmers are advised to approach this summer season with caution.  Farmers growing dryland orchards are advised to consider some form of supplementary irrigation (i.e. water tankers) and mulching, if possible, to prevent excessive fruit drop, particularly during the normal November fruit drop period. Conditions appear to be a lot more favourable for irrigated orchards due to the relatively good dam levels in most areas but water should be used sparingly & conserved where ever possible.The drier conditions will hopefully negate certain fruit diseases such as anthracnose & Bacterial black spot which are both rain-related diseases.

References & Sources:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/nino_regions.html http://www.weathersa.co.za/References/elnino.jsphttp://www.info.gov.za/speeches/2009/09092113051001.htm