Australia's climate over the past 12-months
The past 12 months (July 2017 to June 2018, or equivalently the 2017–18 financial year) highlights Australia's reputation as a land of climate extremes. The northwest, in particular, has seen unusually wet conditions, whereas below average rainfall occurred over much of the east of the continent, with some areas slipping into worsening rainfall deficiencies. Maximum (daytime) temperatures were highest on record for Australia as a whole, with minimum (night time) temperatures generally above average, except in the northwest (Table 1).
Dry in the east and southwest; wet in parts of the north and west
Australia's total rainfall during the 2017–18 financial year was 441 mm, 5% below average. However, such an average conceals much of the detail, with a strong contrast in rainfall from west to east (Figure 1).
Rainfall was below average across much of the east of the continent, including areas of very much below average covering large parts of New South Wales, South Australia, and southern Queensland. Smaller areas of below average rainfall affected the southwest, despite heavy summer rainfall in this region. Financial year rainfall was above average in a band from the Top End through northwest Western Australia and into southeast Western Australia, continuing a pattern of higher rainfall in this region that has become more prevalent in recent decades.
Serious to severe rainfall deficiencies (lowest 10% and 5% of historical observations) for the 12 months since July 2017 cover many areas across northern New South Wales into southern and central Queensland, parts of central and eastern South Australia, and coastal parts of East Gippsland. With the exception of East Gippsland, most of those regions recorded less than 60% of their average rainfall during the 2017–18 financial year.
Warmest maximum temperatures on record for 2017–18 financial year
The rise in Australia's temperature over recent years is well documented, and broadly follows the pattern seen in global land and sea surface temperature data.
Australia's mean daily maximum temperature for the 2017–18 financial year was the warmest on record, with the entire country having mean maximum temperatures that were above average. It was 1.46 °C above the average over the 1961–90 baseline period, and more than 0.1 °C warmer than the previous financial year record (2015–16).
Night time (minimum) temperatures were not as extreme, with some cooler than average temperatures in the country's northwest. 2017–18 still ranked as Australia's equal sixth-warmest financial year on record for mean minimum temperatures (the warmest, 2015–16, was almost half a degree warmer again).
Combining day and night temperatures for an overall mean temperature (average of daily maximum and minimum temperatures) saw 2017–18 rank as our second-warmest financial year on record, 0.19 °C behind the record set in 2015–16.
Within the overall pattern of warmth, a number of cool spells affected Australia. Southeast Australia experienced cooler than average nights for a long period spanning June to September. Clear skies associated with a persistent strong high pressure ridge across the country contributed to warm, sunny days and chilly nights during winter.
Late 2017 saw the development of a weak La Niña in the tropical Pacific Ocean, which is typically associated with cooler than average daytime temperatures across southern Australia (making the high daytime temperatures particularly remarkable).
The climate in 2017–18 was dominated by intra-seasonal drivers rather than a strong influence from the Pacific or Indian Oceans. The financial year began with a positive Southern Annular Mode (SAM) and strong subtropical ridge, suppressing rain-bearing low pressure systems and cold fronts and resulting in below average winter rainfall. Later in 2017, a positive phase of the SAM likely contributed to a significant rain event in southeastern Australia at the start of December.
The El Niño–Southern Oscillation (ENSO) was neutral during winter 2017, but the tropical Pacific Ocean cooled steadily from mid-winter and La Niña was declared at the start of December. It is unusual for La Niña to develop so late in the year. La Niña declined during February 2018 and ended in early March. The weak and short-lived La Niña had relatively little effect on Australian rainfall patterns over the summer, but may have contributed to above average temperatures in the far southeast of the country as weather patterns slowed and moved further south.
The Indian Ocean Dipole (IOD) remained neutral during 2017–18, however there was a west-to-east temperature gradient across the Indian Ocean, with sea surface temperatures (SSTs) during spring that were near average to cooler than average around Western Australia south of the Kimberley. This temperature gradient likely contributed a weak drying influence on Australia's climate as it limited the supply of moisture to the atmosphere across the country.
Overall, the oceans around Australia were much warmer than average, particularly around the north and east. For the Australian domain, preliminary data (July 2017 to May 2018) show an anomaly of +0.56 °C, making the year the third warmest on record.
In late September, unseasonably warm conditions were reported across much of the country. Exceptionally warm days towards the end of the month resulted in monthly temperature records being set for Victoria, New South Wales, and Queensland.
SSTs in the southern Tasman Sea were above average throughout 2017, and rose sharply from November. SSTs were record-warm from December through January and while the peak of the event had passed by February 2018, SSTs remained above average into June. The exceptionally warm Tasman Sea was associated with above average air temperatures in Tasmania and coastal areas of mainland southeast Australia. For the financial year as a whole, mean SSTs across the Tasman Sea were the warmest on record.
Tasmania and Victoria experienced an unusually long run of warm days and nights during November 2017. Many records were set for runs of consecutive days and nights above temperature thresholds, and Tasmania had its warmest November maximum temperature on record by more than one degree.
Abnormally warm conditions persisted through large parts of Australia during the first half of April 2018. The heat, which was more characteristic of mid-summer than mid-autumn, was unprecedented in many areas in April for its intensity, its persistence or both. The spatial extent of the heat was also exceptional, with above-average maximum temperatures extending almost nationwide on each day during the first ten days of the month.
Early 2018 saw the rapid emergence of dry conditions in parts of northern and western Victoria, southeast South Australia, parts of northern New South Wales, and southern Queensland. Soils in those regions rapidly dried out early in 2018 as temperatures stayed above average and rainfall during the first few months of the year was below average.
The northern wet season (October to April) across Australia's tropical north was very much above average for much of the Northern Territory Top End and parts of northwest Western Australia. Elsewhere, mainly average rainfall was recorded. Flooding occurred periodically, with the highest rainfall for the year occurring at Port Douglas during March (Table 3).
Temperatures at Australia's Antarctic bases of Mawson and Davis have been around half a degree warmer than average over the last twelve months. April's temperatures were over three degrees above average; together with a relatively mild May and November, this more than made up for the very cold months in late winter. Further east at Casey, temperatures were around half a degree cooler than average; even the mild April could not compensate for the string of cold months at the start of the year. Casey's temperature dropped to −33.8 °C on 28 September, whilst at the other extreme Davis reached a high of +8.3 °C on 5 January.
On sub-Antarctic Macquarie Island, temperatures were around half a degree above average, although they were about half a degree cooler than the record warm 2016–17 financial year. Each month from August to March was warmer than the long-term average. Total precipitation of 1200.6 mm was more than 20% above the long-term average, and included both the wettest September on record and one of five wettest days in the Island's 70 years of observations when 45.6 mm of precipitation fell in the 24 hours to 9 am on 24 June. Precipitation has been generally increasing at Macquarie Island over the last few decades.
Is it better to report over calendar years or financial years?
The Bureau of Meteorology routinely reports on Australia's climate each month, each traditional season, and each calendar year. But the climate does not always follow those periods — seasonality in climate varies markedly across Australia and there is no-one period that is universal for all regions and climate features. For example, the climate in the north is more characterised by a "wet season" (typically October to April) and a "dry season" with the first of these occurring across two calendar years
Reporting on climate over 12-month periods that do not match the calendar year is not new. The Bureau of Meteorology's water reports (National Water Account and water assessments) discuss the climate over the water year, also defined as the 12 months from July to June. The July to June period cannot be universally applied, but reports can capture many climate drivers and impacts that may occur over calendar year boundaries and be seamlessly integrated with our water reports.
More information and outlook
For more information, find area average data and timeseries for the financial year period, with maps for the 12-months periods ending June available from recent conditions. This current and historical climate information allows for comparison of climate impacts from one year to the next, and aligns with other reporting processes that occur over financial year periods.
Although only six months of the 2018 calendar year are complete, January to June was the second-warmest such period for monthly maximum temperature and fifth-warmest for the monthly mean temperature. The January to June rainfall total was close to average, however it was particularly dry across southeast and southwest Australia, while average to above average across the remainder of Australia; a pattern not too dissimilar to that for the 2017–18 financial year. With the ENSO Outlook currently at an El Niño watch, there is a 50% chance of El Niño developing during spring.