On one hand, it’s an immensely complicated and chaotic climactic phenomenon. But on the other, El Niño is also one of the most predictable weather systems in the world.
‘El Niño is the cornerstone of seasonal forecasting,’ Adam Scaife, professor from the Met Office’s Hadley Centre, tells Geographical. ‘It’s one of the most predictable things on timescales of a few months ahead. So it’s not difficult to make a prediction, but it is difficult to be precise.’
The last major El Niño event came back in 2009–10, and the recent declaration by forecasters such as the Australian Bureau of Meteorology, the American Climate Prediction Centre and the UK’s Met Office has led to speculation about what the next few months may have in store as a result.
This year, multiple theoretical climate models and forecast centres – taking into account the external impacts of ‘atmospheric chaos’ – are indicating a high risk of El Niño conditions building throughout 2015. ‘With the current event, things like the Indian monsoon, tropical West Africa, the maritime continent [Indonesia], and Australian impacts are all appearing in current forecasts – all of those regions are at increased risk of drought,’ says Scaife.
‘We have two tools that we can use to estimate the regions that are most at risk,’ he continues. ‘One is the historical observational record, which goes back perhaps 100 years. And the other is the actual theoretical climate model that we can use to make predictions.’ He outlines how the last century of weather data – including several El Niño occurrences, based on their two to seven year cycles – can be combined with our latest technology to predict not only when El Niño events are imminently going to occur, but where in the world there will then be an increased risk of unusual weather behaviour as a result.
El Niño events occur when the prevailing wind – blowing west across the Pacific, from South America to Australasia – weakens. Instead of pushing the Pacific’s warm water across towards Australasia, the whole ocean experiences higher temperatures, often between 1°C and 2°C higher than normal. These changes in water temperatures can affect air and water currents right across the Pacific, which in turn can then impact upon other weather systems around the world.
As Scaife highlights, warmer air over the Pacific will rise into the upper troposphere, then descend both in the east and the west – the tropical Atlantic and the Indian basin respectively. As it descends, it suppresses the air underneath, preventing clouds forming and rain falling. Therefore, while the open Pacific Ocean tends to experience more rainfall than in a normal year, these areas end up drier.
‘Because it’s coming down over the Indian region, then in the summer to autumn – when the Indian monsoon is active – it suppresses the monsoon convection and kills off the rainfall,’ Scaife continues. ‘So in most El Niño events, the Indian monsoon is suppressed. This is only around perhaps ten per cent or so of the total rainfall in the monsoon season, but that ten per cent is a big change. It affects agriculture, it causes breaks in the monsoon when there’s no rainfall at all, and if that lasts too long then obviously crops wilt and die. So there are all sorts of impacts on agriculture.’ Similar forces in the tropical Atlantic have the potential to indirectly increase the risk of drought in West Africa, and across the Sahel region.
Some other key regions which have historically experienced drought as a result of El Niño events include Australia (especially the northeastern state of Queensland), Indonesia and surrounding islands, and South Africa. However, South Africa is also an example of how these events can defy predictions; the biggest El Niño event in the modern era – in 1997–98 – failed to create any significant drought in the country, despite a high probability that it would. ‘It’s only an increasing risk,’ emphasises Scaife. ‘That’s a very important point. It’s not that the forecasts are wrong, what it means is that the risk is increased.’
But it isn’t only drought that El Niño can create. The initial warming of the Pacific can create a dramatic bleaching event of coral reefs, as happened in 1997–98, when ocean temperatures were recorded as being up to 3°C higher than normal. And while the northern part of South America can experience an increased risk of drought during an El Niño year, the event can also create anomalies such as increased rainfall right by the coastline, in countries such as Peru. ‘There’s also a possible alleviation of the Californian drought,’ continues Scaife. ‘Again it’s not guaranteed, but at least it’s a force pushing in the right direction. So there are some positive impacts.’