The US space agency Nasa has reported that a satellite monitoring sea levels has spotted Kelvin waves moving eastward along the equator toward the west coast of South America. This is a strong signal that El Niño is forming -- and the implications for South African agriculture could be chilling.

The return of El Niño this year has been forecast for the past few months. Now, a clear sign of its formation has been detected with satellite imagery.

"The most recent sea level data from the US-European satellite Sentinel-6 Michael Freilich indicates early signs of a developing El Niño across the equatorial Pacific Ocean. The data shows Kelvin waves - which are roughly five to 10 centimetres high at the ocean surface and hundreds of miles wide - moving from west to east along the equator toward the west coast of South America," Nasa said in a statement.

"When they form at the equator, Kelvin waves bring warm water, which is associated with higher sea levels, from the western Pacific to the eastern Pacific. A series of Kelvin waves starting in spring is a well-known precursor to an El Niño," Nasa said.

This being Nasa, it's all very hi-tech.

"Satellites like Sentinel-6 Michael Freilich can detect Kelvin waves with a radar altimeter, which uses microwave signals to measure the height of the ocean's surface. When an altimeter passes over areas that are warmer than others, the data will show higher sea levels," Nasa said.

El Niño typically heralds drought in...