The temperature anomalies stretch from the South American coastline all the way to north-east Australia, or to about the 160-degrees east region (see chart, below). Andrew Watkins, manager of the bureau's climate prediction services, said it was too early to assess how strong the event would eventually become. Still, there are signs of some significant heat building up in the eastern Pacific as the typical easterly trade winds have not only stalled, but reversed. "We've seen a bit of a boost in the last few weeks because of those tropical cyclones we saw in the northern hemisphere ... plus the very rare Australian cyclone" in July, Dr Watkins said.

"As a result of the cyclones, and the El Nino itself ... we're actually seeing the winds blowing from the west to the east," he said. "That's lasted for a couple of weeks and the longer it lasts, the more chance you get of things warming up." In parts of the eastern Pacific, readings at 75 metres below the surface are 6 degrees above average, not far shy of the 7-degree anomalies reached in 1982-83 and 1997-98, the two largest El Ninos on record. "We can't say we'll have a strong event but it is a risk that we have to take into account," Dr Watkins said. El Ninos result when typical atmospheric and oceanic circulation patterns in the Pacific change. With easterly trade winds stalling or reversing, rainfall patterns tend to shift, with western regions – such as south-east Asia and eastern Australia – getting below-average rain while western parts of the Americas receive often heavy rains. A strong event doesn't guarantee that Australia will experience exceptionally dry conditions. While the super El Nino of 1982-83 did result in large areas suffering drought, conditions were more mixed during the bigger 1997-98 event.

Temperatures rise There is less uncertainty, though, about the impact the El Nino will have on global temperatures. Both the month of June and the first half of 2015 are already the hottest on record for global sea- and land-surface temperatures, the US National Oceanic and Atmosphere Administration said on Monday. The half-year result eclipsed the comparable period in 2010, when the most recent El Nino to the current one was breaking down, the agency said. Climate experts say the momentum of the global system suggests 2015 will likely top 2014 as the hottest year in records going back to the 1880s, with 2016 potentially warmer still.

Another indication of the current El Nino's strength is that sea-surface heights have shifted about 50 centimetres across the Pacific. Western regions, such as to Australia's north have seen sea-levels drop 20-30cm, while those in the east have risen by a similar amount. That change appears to be the second largest – behind only the 1997-98 event, according to Australia's Integrated Marine Observing System.