Australia has posted its hottest end to any year as the impact of one of the biggest El Ninos on record began to be felt across the continent.
Mean temperatures were 0.36 degrees above the previous record for the October to December period, capping what was Australia's fifth-hottest year since the Bureau of Meteorology began keeping national figures in 1910.
"For temperatures, it was a year of two halves - a relatively cool part of the year and then an extremely warm second half," Blair Trewin, senior climatologist at the bureau, said.
Spring was the standout season, with the past three September-November periods comprising the hottest trio on record. Such conditions have led to a busy fire season across southern Australia with a couple of months of summer still to run.
Among the major capitals, Sydney had its third-warmest year on record, just behind the record heat in 2013 and 2014. Statewide, temperatures were 1 degree above average, making it the seventh-hottest year since records began.
Melbourne was also on the warm side, with maximum temperatures ranging between 0.5 and 1 degree above average across the city. Victoria, too, was 1 degree warmer than average for maximums, making it the seventh-hottest year.
Perth was the standout state capital for warmth, recording its equal hottest year on record for maximums, matching 2011 and 2012. Statewide temperatures lagged only 2013 for record heat.
Brisbane had near-average temperatures for the year, while statewide temperatures were the third-warmest on record.
Three big exceptional heatwaves stood out - in March across northern Australia, and in October and December across the south. Tasmania was one place to have a cool winter and late-season snow across northern NSW and into Queensland was another cold weather extreme.
For the final three months of 2015, average mean temperatures were 1.93 degrees above the 1961-90 average, easily eclipsing the previous record of anomaly 1.57 degrees set just a year earlier. October itself was 2.89 degrees above the norm - the most for any month in the 106 years of records.
Warming to come
The monster El Nino in the Pacific, which rivals the 1997-98 and 1982-83 events, appears to have peaked in recent weeks, the bureau said on Tuesday.
The event, which may not break up until the autumn, will most likely give Australian temperatures a relatively warm start to 2016 - notwithstanding the unusually cool and wet week now under way across the eastern seaboard.
In the trailing year of El Ninos, "the first half of the year is often significantly warmer than average", Dr Trewin said.
For 2015 as a whole, area-averaged mean temperatures were 0.83 degrees above the 1961-90 average. Maximum temperatures were 0.96 degrees above average, the sixth hottest on record, the bureau said.
The climate change signal is evident in Australia's warming trend, building on the natural fluctuations, scientists say.
Historically, El Ninos - or their opposite, La Ninas - add or subtract about 0.5 degrees to or from national temperature averages.
"The baseline you're starting from is 0.5 degrees above the 1961-90 average," Dr Trewin said.
Eight out of Australia's 10 warmest years have happened since 2002, the bureau said. Only one year in the past decade was a below-average year, in 2011, during a big La Nina event.
Globally and in Australia, temperatures have risen about 0.9 degrees over the past century, with rising greenhouse gas emissions from burning fossil fuels, clearing land and agriculture among the primary causes. The warming trend prompted almost 200 nations to agree last month at the Paris climate summit to keep temperature increases to "well below 2 degrees" compared with pre-industrial times.
In coming weeks, international agencies are likely to declare 2015 to be the hottest year on record, and Britain's Met Office has said 2016 may be even warmer as the effects of the El Nino continue to influence the world's weather.
"Our research from 2015 shows that record-breaking hot temperatures over the last 15 years outnumber new cold records by a factor of 12-to-one [in Australia]," Sophie Lewis, a research fellow at the Australian National University and the ARC Centre of Excellence for Climate System Science, said.
"This dramatic increase in hot records in recent years is not random; it is linked to human-caused climate change," Dr Lewis said. "Combined with strong El Nino conditions, we should be prepared for hot conditions to continue in Australia in 2016."
Rainfall influences
Nationally, rainfall was about 5 per cent below average - with the distribution far from even as would be expected over the island continent.
Three areas were particularly dry: south-eastern Australia, inland Queensland and south-western Western Australia:
El Nino years are marked by drier-than-usual conditions across much of eastern and southern Australia. During El Ninos, typically westward-blowing Pacific winds stall or are reversed, and rainfall patterns tend to shift eastwards away from Australia and south-east Asia.
During 2015, though, record warmth in the Indian Ocean provided a supply of moisture that helped counter the El Nino influence for winter rainfall in areas such as NSW, Dr Trewin said.
The bureau doesn't issue annual rainfall or temperature projections. A factor, though, will be whether the El Nino makes way for more neutral conditions or reverts, as some scientists predict, to a La Nina.
During La Nina years, rainfall tends to be above average for much of Australia and temperatures are cooler. Such years also typically include more tropical cyclones than usual.
