The 2019–20 summer: Australia's summer of extremes
03 March 2020
The 2019–20 Australian summer was a season of two halves.
The first half was exceptionally hot, dry and dusty. Parts of eastern and southern Australia were engulfed in significant bushfires, and smoke haze covered large areas.
The second half of the season saw tropical moisture feeding south, producing well above average rainfall for some areas. While this was great news for many, some parts of the country missed out altogether. Other areas require follow up falls over the coming months to ease the long-term dry, and inland water storages increased only slightly.
Image: Smoke haze over Town Beach, Port Macquarie, New South Wales, was a feature of the landscape in the lead up to, and during, summer. Credit: Vivien Mitchell
Australia's hottest December on record played a big part in producing Australia's second hottest summer in 110 years of records, with warmth across much of the tropical north and east persisting into January and February. Summer nights were also second warmest on record.
For rainfall, summer overall was nearer to normal with total rainfall 8% below the long-term average.
But summer total rainfall hid the significant shift in the climate mid-way through the season.
December—the hot and dry
In the lead up to summer, Australia experienced both very warm and very dry conditions.
The 11-month period from January to November 2019 was both second warmest and second driest on record. December amplified this pattern with almost nationwide extreme heat on several days during the month.
With a delayed onset of the monsoon moving into the southern hemisphere, heat built across northern and central Australia from the start of December and was drawn south by conducive weather patterns. Australia had its hottest day recorded on 18 December, with a nationwide average temperature of 41.88 °C, far above the previous record of 40.30 °C set on 7 January 2013.
Six other days in December 2019 also exceeded this previous record.
To put this in context, the Australia-wide average maximum temperature has exceeded 40 °C for more days in the past two summers than in the preceding 110 years.
The hottest temperature recorded this summer was at Nullabor, South Australia, on 19 December, when it reached 49.9 °C - also a new December record for Australia, and the highest temperature recorded for any month since February 1998.
Another three sites across Australia reached 49 °C during December.
Averaged across the continent, December maximum temperatures were 4.15 °C above average—head and shoulders above the previous December record of +2.41 °C set in 2018. It was also the highest anomaly (i.e., the most above average month) in 110 years.
Maps: Australian Rainfall Deciles December 2019 (left); Temperature Deciles December 2019 (right)
January to February—heavy rainfall for some areas
A noticeable change to the weather came in early January, with the first tropical cyclones of the season and the belated arrival of the monsoon.
Tropical cyclone Blake and Claudia brought heavy rain to much of the Top End and northern and central parts of Western Australia, while the east coast saw heavy falls in northern New South Wales and southeast Queensland.
February brought more tropical moisture to the continent and another two cyclones—Damien and Esther.
Despite this, temperatures across the north remained very warm. The monsoon did not arrive until 18 January, some three weeks later than normal, though almost a week earlier than the previous year. Even after the monsoon arrived it was sporadic, so did little to temper the heat. The Darwin River Dam (Darwin's main water storage) dropped to 54% capacity - the result of two relatively dry wet seasons in a row.
In contrast, in southern parts of Australia, February felt milder than normal. After Melbourne had three days over 40 degrees in December - the most in December since 1897 - it had no days at all over 35 °C in February. This last happened in 1994.
Maps: Australian Rainfall Deciles December 2019–March 2020 (left); Temperature Deciles December 2019–March 2020 (right)
The abrupt change in weather patterns mid-season can be attributed to changes in two key Australian climate drivers: a strong positive phase of the Indian Ocean Dipole (IOD), and a negative phase of the Southern Annular Mode (SAM).
A positive IOD typically brings drier weather to much of southern, central and northern Australia.
The 2019 positive IOD was the strongest since 1997 and affected large parts of the country (and eastern Africa) during the second half of the year. Normally an IOD event ends with the movement of the Asian monsoon into the southern hemisphere in late spring/early summer, but in 2019–20 this movement occurred about a month later than usual. The delayed arrival of the Australian monsoon was likely related to the strength of the IOD itself, which effectively acted as a barrier to the monsoon's southward shift.
The negative SAM, which typically brings drier and hotter than average weather to much of southeast and eastern Australia was triggered by a sudden warming of the stratosphere above Antarctica. This rare event, identified in early September, affected Australian climate from October to December.
Both negative SAM and positive IOD are known to enhance the likelihood of increased springtime heatwaves and fire weather in southeast Australia.
In addition to the IOD and SAM, Australia's changing climate underpinned the drier and warmer weather across southern Australia in winter and spring.
Drought situation and autumn outlook
Has the mid-summer change in weather made a dint in the drought situation, and is it likely to continue through autumn?
Rain has eased the dry in some places, but more is needed. Most of the very large rainfall totals were recorded east of the Dividing Range, while larger totals west of the Divide generally fell in regions that either didn’t feed into water storages, or where soils were so dry there was little runoff.
While Sydney's water storages rose from 42% to 81% during the second half of summer, storages in the northern Murray—Darling Basin only rose from 6% to 12%, - similar to its storage levels at the height of the Millennium drought.
Additionally, while some areas have had good rainfall, others have missed out. Western New South Wales and southwest Queensland endured another dry summer.
The autumn outlook suggests that parts of southern Australia have a slightly increased chance of being wetter than average, while scattered parts of the north may see a drier end to the northern wet season. Autumn days are likely to be warmer than average for most of the country, except for parts of southern Australia, suggesting evaporation will remain above average
This outlook would indicate a gradual easing of the drought in some areas, especially once the wet season in the north ends and takes with it the chance of tropical moisture heading south.
Temperatures are likely to be warmer than average.
Maps: Rainfall outlook March–May 2020 (left); temperature outlook March–May 2020 (right)
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