Winter 2015 the coolest in decades for parts of the southeast
Some of the headlines this winter—‘Bitter cold snap’, ‘Icy cold front to hit much of Australia’, ‘Australia’s sunshine state covered in snow’, etc.— may have implied that Australia just survived one of its coldest winters on record.
For some it was on the cool side. Victorians shivered through a mean temperature (the average of the maximum and minimum) of just 8.3 °C; the coldest since the last big El Niño year of 1997. Melbournians had the coldest winter in nearly 30 years (1989) and the coldest night in nearly 20 years. Tasmania had its coldest winter in nearly 50 years with a State mean temperature of just 5.8 °C (Table 1).
Snow did fall in ‘unusual’ places, including the Granite Belt of Queensland, the ranges near Adelaide, Melbourne and Sydney, and even settled on the beaches south of Hobart. Pictures of surfers standing in snow near Hobart, and the ‘Welcome to the Sunshine State’ sign surrounded by snow, reinforced a view of a cold harsh winter. In the Alps, a late start to the ski season turned into a series of blizzards, bringing smiles to skiers in the southeastern states.
But the station recordings actually show a quite different story. They also highlight a number of challenges that are well understood by climatologists, but can be easily lost in personal experiences and local news reporting. These challenges include: considering local conditions within the national picture; how we calculate and characterise a ‘record’; and how recent experiences influence our long-term understanding of climate.
Australia is a big country
It is hardly surprising that people may extrapolate cool conditions in southeast Australia or snowy photos on their television to imagine cold conditions across the whole of the country.
Winter 2015 temperatures for Australia as a whole were actually above the long-term average, continuing the trend which has seen only two below average seasons since 2000 (Table 1). The national average winter temperature was 0.8 °C above the 1961–90 reference period (Figure 1), making it the ninth warmest winter on record. While these values are drawn from the Bureau’s carefully homogenised 100 station temperature data set, the same findings follow from using the entire Bureau network of nearly 700 temperature stations.
It was the second-warmest winter on record for Western Australia, with mean temperatures 1.2 °C above normal. In Queensland, temperatures for the season were 1.2 °C above normal, ranking as the State’s tenth-warmest winter on record. New South Wales had winter temperatures only slightly (+0.2 °C) above normal, but these were sufficient to make winter 2015 the eighteenth consecutive winter with above-normal temperatures.
Below-normal winter temperatures were largely confined to the southeast of the country, in Tasmania, Victoria, southern South Australia, and parts of southern New South Wales (Figure 2). It was a particularly cold winter in Tasmania, the one State to have a markedly cool anomaly. Mean winter temperatures for Tasmania were 0.6 °C below normal, making it the coldest winter for the State as a whole since 1966.
How do we characterise records?
Other challenges when trying to determine how hot or cold conditions fit into a longer term context arise when comparing records at different weather stations or considering stations with a short length of record.
Stations with shorter records will tend to ‘break records’ more easily or more frequently, but that record cold or record warm temperature may not be significant in the context of the broader climate. Further challenges can arise when a new station sits in particularly unusual (unreprepsentative) location such as an alpine peak. Climate scientists typically deal with these challenges by limiting the reporting of records to stations with at least a few decades of data, commonly 30 years.
Stations that have changed locations but are then compared to determine long-term context need to be carefully considered. We know that stations only a few kilometres apart can have very different climates, be it wetter, warmer, colder or more variable. Well known examples include changes in elevation (e.g. the move of the Macedon Victoria observations from the top to the bottom of the mountain) and moves away from the coast (e.g. the move of observations in Albany from the waterfront in town to the airport several kilometres inland). These changes are well understood and are mostly accounted for by using homogenised (adjusted) datasets when characterising long-term records, but are easily missed in real-time reports and local perception. Comparing data from two stations with the same name can lead to quite wrong conclusions about climate conditions.
The recent move of the Melbourne city site causes some difficulties in putting the winter of 2015 into context. At the Melbourne ‘city site’, observed winter temperatures were the lowest since 1989 but this comparison is not entirely valid. The observation site moved from La Trobe Street to Olympic Park in 2013. Overlapping data show that the new site is about half a degree cooler in winter than the old La Trobe Street site. When the site changes are taken into account, it is likely the conditions were colder in Melbourne in the winter of 1997. We estimate that the actual temperature was only (about) 0.1 °C cooler than the 1961–1990 average, so the winter was just slightly below the long-term average.
What is ‘normal’ in a changing climate?
The coolest winter in Melbourne (and Victoria) since 1997 may give the impression that conditions were notably cold. However, both the Victorian and Melbourne temperature anomalies were not significant in the longer-term context. This can be seen if we plot the number of times per decade that Victoria has experienced a cooler winter than that just passed (temperature anomaly of −0.26 °C; Figure 3).
It is likely that people have become accustomed to the milder winters of recent years, and hence what they experience as normal and below normal has altered. Similar long-term trends are evident across all Australian Statesas a result of the steady warming trend since the middle of last century. We know that animals and plants respond to these changes, for example by altering their time of flowering. It is likely that humans also become accustomed (one might say ‘adapted’) to the milder conditions.
A changing climate affects more than meteorological recordings, with impacts on phenomena such as snow and frost. The recent settling snow in Hobart had extensive media coverage. While snowfall data have limitations, they do show a marked decline in snow in Hobart over the last century—the first two decades of the 20th century saw 33 snowfalls with seven settling in Hobart, whereas the first 15 years of the 21st century have brought just seven snowfalls and one settling. These numbers suggest that the frequency of snow falling has declined by more than half over this period.
There is no substitute for good data
Winter 2015 highlights again the highly variable nature of the Australian climate. We saw record heat in parts of the north and west, and record cold in parts of the southeast.
As our perceptions can be skewed towards recent experience, we need access to good, long-term data to assist our decision making. Whether planning a holiday, purchasing a heater, pouring a slab or planting a crop—our weather, climate and water data is freely available online to help you make the right decisions.
|State||Maximum temperature ( °C)||Minimum temperature ( °C)||Mean temperature ( °C)||Rainfall (mm)||Notes|
|Australia||22.65 (+0.83)||9.37 (+0.75)||16.01 (+0.79)||53.6 (−16%)|
|Western Australia||23.77 (+1.44)||10.21 (+0.95)||17.00 (+1.20)||48.4 (−20%)||Mean and maximum temperature both 2nd highest on record (after 1996)|
|Northern Territory||26.77 (+0.68)||12.02 (+0.17)||19.40 (+0.43)||13.5 (−25%)|
|South Australia||19.10 (+0.26)||6.08 (+0.40)||12.60 (+0.34)||44.4 (−20%)|
|Queensland||24.77 (+0.96)||11.26 (+1.46)||18.01 (+1.21)||38.1 (−26%)|
|New South Wales||16.18 (+0.03)||4.67 (+0.40)||10.43 (+0.22)||125.2 (+8%)|
|Victoria||12.66 (−0.37)||3.91 (−0.16)||8.29 (−0.26)||152.8 (−25%)||Maximum temperature lowest since 1989, mean lowest since 1997. Rainfall lowest since 2006.|
|Tasmania||9.45 (−0.62)||2.14 (−0.63)||5.80 (−0.62)||369.0 (−16%)||Mean temperature lowest since 1966 and 6th lowest on record, maximum lowest since 1992, minimum lowest since 1995.|
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