El Nino conditions come as many inland regions are already quite dry. Credit:Chad Clark The last El Nino event ended in March 2010 and was relatively mild, with above-average summer rain countering the below-normal winter-spring rainfall totals that preceded it. Calling an El Nino this early in the year is also unusual. The most recent May El Nino declaration was in 1993, six events ago, the bureau said. Rainfall drop

While El Nino events are not aways associated with droughts, most are. The biggest drop in rainfall typically comes during the key winter-spring planting season for eastern Australia. Relatively clear skies mean daytime temperatures across much of the country tend to be above average which, when combined with dry conditions, mean bushfire seasons arrive earlier and are more active than usual. Australia's temperatures are about one degree above average in the second half of the calendar year during El Nino events, adding to the background warming signal from climate change, Dr Jones said. The warm conditions can also bring forward crop development – water availability permitting – leaving them vulnerable to late-season frosts, climate specialists say. Reduced moisture can also lead to lower-than-usual snowfalls. "Most of the poorer ski seasons are associated with El Nino events," Dr Jones said.

Thresholds crossed Tuesday's declaration came as all four of the bureau's criteria for declaring an El Nino were met, as noted in its fortnightly update released on Tuesday. Abnormally warm sea-surface temperatures have been observed in all five regions of the equatorial Pacific monitored by meteorologists.

While the threshold level is 0.8 degrees, all regions were more than one degree above normal – the first time all weekly values have exceeded such a mark since February 1998, the bureau said. The "super El Nino" event of 1997-98 resulted in a spike in global temperatures that set a record that has only been marginally topped in 2005, 2010 and 2014. Climate specialists say El Ninos add 0.1-0.2 degrees to global temperatures, making it likely that 2015 and 2016 may challenge the new high set only last year. Another criterion for an El Nino is that the normal east-to-west trade winds are weaker than average. In fact, the winds were reversed, blowing eastwards for five days to May 10, in a sign that the atmosphere has "coupled" with the changes in ocean temperatures, reinforcing them. A third criterion is that the Southern Oscillation Index – a gauge of pressure differences between Darwin and Tahiti – has persistent readings of at least minus-7. The latest bureau update, shows the SOI dropped to almost minus-10 in the past two weeks. The final criterion, also reached, is that a majority of the eight major climate models from around the world used by the bureau show continued anomalous warmth in the equatorial Pacific.

In fact, all eight of the international models indicate the central Pacific will warm further in coming months and may be prolonged, the bureau said. "All surveyed models indicate that NINO 3.4 [region] will remain above El Nino thresholds through the southern winter and at least well into spring," the bureau said, adding "it is too early to determine with confidence how strong this potential El Nino could be".

'Intolerable' strain on the farm While the impacts of an El Nino vary, the tendency towards lower-than-average rainfall across eastern Australia is a common feature for the bulk of the 27 events recorded since 1900 by the bureau. Federal agriculture minister Barnaby Joyce moved to downplay the threat, saying "just because you have an El Nino event doesn't mean that it is ruled in concrete that you are going to have a dry period".

"In fact we have had a rather wet autumn in many areas along the East Coast," he said. Unfortunately for many farmers, though, a belt stretching from inland Queensland through much of northern and western NSW into western Victoria is already experiencing serious to severe rainfall deficiencies. Farmers in the Walgett and Brewarrina areas of NSW, for instance, are already suffering from a one-in-50 year drought after three dry years, said Fiona Simson, president of the NSW Farmers Association. "A fourth year is nearly intolerable for these farmers," Ms Simson said, adding that at least 1000 families were in dire straits. The declaration of an El Nino by the bureau is "going to be very worrying for them when they are already quite distressed and their farms are already quite dry", she said.

Richard Anderson, chair of the Victorian Farmers Federation's Water Council, said the current cold front had done little to ease dry conditions in the state's north-west or north of the Great Divide in general. "There's only been cold weather. There's been no substantial rain," Mr Anderson, who is also a farmer based west of Echuca, said. Farmers will be concerned if the Victorian government starts to pump water on the north-south pipeline from the Goulburn River, taking water for Melbourne that would otherwise flow into the Murray River. Given Melbourne's current storage levels "there'll be no need to ship that water", he said. Water storage levels across the Murray-Darling Basin are averaging about 40 per cent, Lynette Bettio​, a senior bureau climatologist, said. Sydney's reservoirs, though, are about 92 per cent full after recent large east coast lows and other rain, while Melbourne's have eased below 70 per cent in the past month or so.

The worst El Nino years for Australian droughts have come when conditions in the Indian Ocean reinforce the dry set-up being influenced by Pacific weather patterns. During such years, temperatures in the eastern Indian Ocean – particularly off north-west WA – are cooler relative to those in the west near Africa. Those conditions, known as a positive Indian Ocean Dipole – are not currently in place. Nevertheless, El Nino is likely to be the dominant influence on Australia during the second half of the year, the bureau said.