After months of anticipation, the Bureau of Meteorology officially declared an El Niño on June 16.
El Niño is a naturally occurring variation in temperature and winds across the Pacific Ocean that can influence weather around the globe.
During El Niño, sea surface temperatures in the central and eastern Pacific warm up and trade winds - which typically blow from east to west along the equator - weaken. As a result, a region of strong storm activity known as the Walker Circulation shifts east over the Pacific Ocean, drawing moisture and clouds away from Australia.
Past El Niño events have coincided with some of the driest and hottest weather in Australian history.
Making El Niño official
El Niño events occur about every three to seven years, and can last anywhere from six months to two years. They typically ramp up in winter and spring, before easing in autumn.
The likelihood of El Niño has been in the news for months, but the Bureau of Meteorology only just officially declared it active. That's because there is a specific set of criteria that must be met.
Scientists must observe at least three of the following:
Sea surface temperatures in the central Pacific Ocean must be greater than 0.8°C above average
The trade winds that blow east to west across the Pacific have to be weaker than average for the past four months
The Southern Oscillation Index , which measures the difference in atmospheric pressure between Tahiti and Darwin, must be lower than -7. This tells us whether the region of strong storm activity is closer to Darwin or Tahiti
The majority of global seasonal forecasting models must predict that ocean temperatures in the Pacific will stay warm for at least three months.
What does this mean for Australia?
Importantly, an El Niño declaration is not a forecast. Rather, it's a statement on the current conditions in the Pacific Ocean.
On average, past El Niño events were associated with warmer than normal maximum temperatures across Australia, particularly in winter and spring. They were also linked to drier than average winter and spring conditions, especially in eastern Australia.
Western Australia is not as affected by El Niño because, particularly compared to eastern states, it's only indirectly influenced by Pacific Ocean conditions. The north of the country, however, tends to experience fewer tropical cyclones on average and a delayed start to the monsoon season during El Niño.
It may seem counterintuitive, but El Niño can lead to colder minimum temperatures and therefore more frost. This is because we tend to see less cloud cover during El Niño, and nighttime clouds act like a blanket that stops heat from escaping to space.
However, global average temperatures tend to be hotter during El Niño. And we often see record breaking years coinciding with El Niño.
It's worth noting, human-made greenhouse gas emissions are the main driver of rising global average temperatures. However, El Niño can tip these temperatures to record breaking levels.
Why are people talking about a 'super El Niño'?
You may have seen reports of a potential "Super El Niño". A "super" or "strong" El Niño refers to events where the sea surface temperatures in the Central Pacific Ocean are about 2°C warmer than normal.
As of June 14, the sea surface temperatures in this region were 0.92°C above average . However, forecasting models suggest temperatures could exceed the 2°C threshold by late winter.
However, a "super El Niño" will not necessarily lead to "super droughts" or "super bushfires". That's because, in Australia, the strength of an El Niño event is not related to the severity of its impacts.
The 2002 El Niño event was weak, but was still associated with widespread drought and severe bushfires in some parts of Australia. In contrast, the strong El Niño of 2015 affected rainfall patterns differently across the country.
El Niño is not the whole story
El Niño is just one ingredient in the recipe of Australian weather.
Our weather is influenced by El Niño in the Pacific Ocean as well as the Indian Ocean Dipole , sometimes known as El Niño's cousin to the west. Australia's weather is also shaped by shifts in the jet stream to the south - which impacts how many cold fronts reach Australia - and tropical storms in the north.
Now that El Niño has been declared, scientists will keep a close eye on what happens in the Indian Ocean. If waters off Australia's northwest cool over winter, the rest of the year may be quite dry. But if the waters off northwest Australia get warmer, it's less likely a drought will develop.
In short, many factors must coincide to cause severe droughts and bushfires in Australia. Currently, this is not happening. So the Bureau's long-range forecast - which considers all these factors as well as El Niño - offers the most accurate information about Australia's ever-changing weather.
Kimberley Reid receives funding from the Australian Research Council.
