Understanding El Niño
A brief explanation of the weather system worsening our dry conditions.
When Aeolus, the Greek god of wind, decides to slow down the easterly trade winds over the Pacific Ocean, we are in trouble.
The El Niño-Southern Oscillation (ENSO) is a naturally occurring phenomenon that involves fluctuating ocean temperatures in the equatorial Pacific.
Every two to 10 years the pressure gradient force that raises the easterly trade winds in the Pacific region, weakens, causing the trade winds to relax, sea temperatures to change and ultimately plunging south eastern Africa, Indonesia and parts of Australia and parts of South America into drought.
El Niño (the dry phase of ENSO) normally lasts between 12 and 18 months and is the reason KZN is being held hostage by the current drought.
Under normal conditions, the high pressure cell over the eastern Pacific Ocean (west coast of South America) and the low pressure cell over the western Pacific (Indonesia and east coast of Australia) create great differences in air pressure, creating the easterly trade winds in the Pacific region (wind always blows from a region of high pressure to low pressure).
The easterly trade winds in the tropical Pacific cause warm water to pile up in the western Pacific. Consequently there is an upwell of cold water in the eastern Pacific to replace the displaced water. This is the wet phase of ENSO and brings rain to Indonesia, parts of Australia and South America and south east Africa. When the wet phase of ENSO is strong, it is called La Niña and may bring floods to the regions mentioned.
During an El Niño year, the pressure difference between the high pressure cell over the eastern Pacific and the low pressure cell over the western Pacific decreases, causing the easterly trade winds to weaken. The heaped up warm surface water flows back to the eastern Pacific, raising the sea surface temperature in that region by between one and four degrees Celsius, causing a global switch in air pressure and rain.
Local effect
The North Coast and Zululand are worst affected by El Niño, while the Midlands, South Coast and Mpumalanga are usually spared the brunt of it.
In the sugarcane industry, rainfall distribution is more important to yields than total rainfall. The South African Sugarcane Research Institute (SASRI) said in a published information sheet that the warm (dry) phase of ENSO nine out of 10 times brought drought but only six out of 10 dry seasons produced low yields. This indicated that less rain but in strategic places can still produce a high yield.
SASRI said rainfed sugarcane (not irrigated) depended on soil water availability through the growing season and soil water availability is determined by where it rains, not necessarily how much.
The current El Niño phase is expected to continue until April 2016 and SASRI said farmers could reduce wasteful evaporation by covering the soil in a residue mulch layer (trash blanket). They said looking after soil ensures deep roots, further increasing cane’s chances of surviving a drought.
