Is this summer turning out to be exceptionally dry, the catastrophic result of global warming? Or is it just another cyclical dry season as a result of an El Nino effect in the mid-Pacific Ocean?
Traditionally, highveld rain can be measured as starting in earnest from September, and building up gradually towards December with, therefore, October being a bit wetter than September and November being wetter than October. At least that’s what our daily rainfall records show, collected in Bredell on the East Rand since 2000.
On this basis, and bearing in mind that at the time of writing we are only halfway through November, the second half of November is going to have to be exceptionally wet (and that doesn’t look likely from the current long-range forecasts) for this year not to be the second driest we have recorded, the three months of September to November 2002 having been the driest at a total of only 65mm, for to 16 November this year only 78mm of rain has fallen on our plot.
By contrast to recent years, our 78mm is far short of the 210mm which we enjoyed last year, and miniscule compared to the wettest period we have recorded, namely the 339mm which fell from September to November 2016.
What’s the cause of this decline? Back in July the ever-cautious SA Weather Service was predicting a “70% chance” of a moderate El Nino effect in the early summer months. Given that weathermen thrive on complicated statistics in their predictions, that’s their way of saying that they thought, back in July, there was a more than fighting chance of an El Nino affecting our summer rainfall.
What is an El Nino effect? El Nino (Spanish for “Little Boy”)and its counterpart La Nina (Little Girl) are colloquial names given historically by Chilean fishermen to the temperatures of offshore currents which affect Chilean anchovy stocks available to them in December.
These current temperatures, it has been found, are in turn affected by what happens to temperatures of the water in the mid-Pacific and thus the terms have been appropriated by oceanographers who more portentiously use the term El Nino Southern Oscillation, or Enso, which is defined as “a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean.”
Changes of a degree in the ocean temperature, or of even a few parts of a degree, affect the amount of moisture in the atmosphere, atmosphere which is carried by the prevailing westerly winds over South America, and then on prevailing westerly winds over South America, and then on to Africa, and even on to Australia.
And it’s that moisture in the atmosphere which is dumped periodically as rain.
The oscillation in temperature is, of course, not absolute, either one way or the other, in the same way that a pendulum swings back and forth. Some years temperatures may not vary from the average at all, other years there may only be a slight variation and in others it may be extreme.
Thus, the effect is not absolute either. Some years may be average, others a little wetter, or drier, than average. And others extreme.
The problem is predicting what’s coming, and here the scientists are on more shaky ground.
The first difficulty they face is gathering temperature data in sufficient amounts from an area as vast as the central Pacific Ocean.
Next, while it would be convenient if a pattern of cyclical variation could be ascertained in the data collected over time, sadly this appears not necessarily to be the case. Thus, until further underlying research is completed they are not able to say, for example, that if Year 1 is a strong El Nino, Year 3, or 5, or 6, will be a La Nina.
The best we can glean at present, therefore, is some prediction of what might happen in the upcoming summer based on ocean temperature readings in midwinter. Hence the weathermen’s “70% chance” prediction in July.