New understanding of the drivers behind hot and dry conditions over Australia’s north-east

Posted by BCG on 14th March 2019

Predicting temperature extremes and dry conditions over spring and summer is vital for agriculture, water supplies, bushfire risk and human health. But to make accurate predictions the Bureau of Meteorology needs to better understand the climate drivers behind such extremes.

Bureau researchers have recently identified how wind patterns in the stratosphere around Antarctica can drive hot and dry conditions over southern Queensland and northern New South Wales.

This research was undertaken by the Forewarned is Forearmed (FWFA) project, which is part of the Federal Government Rural R&D for Profit program. FWFA is supported by the Bureau, a range of Rural R&D Corporations, Universities and state government agencies.

While most weather systems like storms, rain and high pressure systems are found in the troposphere, the relatively turbulent first layer of the atmosphere, the stratosphere above it is known for its stable air flows. Commercial airplanes target this layer to find jet streams for a smooth and efficient flight.

A key feature of the stratospheric circulation is the development of the wintertime polar vortex, whereby Antarctic circumpolar westerly winds (extending up to 40-50 km altitude) seasonally strengthen from autumn to winter as the polar cap region seasonally cools. The vortex weakens and breaks down during late spring as the polar cap warms up. In some years the vortex warms up and breaks down early, which can lead to hot and dry conditions on Australia’s surface during late spring-summer.

The early weakening of the polar vortex results in a strong downward air flow and a lack of clouds over eastern Australia (Figure 1) via another large-scale circulation that Australian farmers already know – a negative Southern Annular Mode (SAM) according to Bureau researcher Dr Eun-Pa Lim.

“A negative SAM is responsible for bringing hot and dry conditions to eastern Australia in our warm seasons” says Dr Lim. “Anything we can do to improve our ability to predict SAM will help people on the land to prepare for and manage these conditions. The long time–scale of the polar vortex weakening, which spans several months, means if we can capture it in our model, we can potentially predict low SAM conditions during late spring as early as late winter.”

Figure 1. A strong weakening of the polar vortex and the associated negative Southern Annular Mode leads to an abnormally strong downward air flow and a lack of clouds (orange indicates less than average cloud cover) over eastern Australia, which results in higher than average temperatures and dry conditions. This plot shows the cloud cover variation in late spring-early summer following the early break down of the polar stratospheric vortex.

But just what is a polar vortex and how does it influence Australia’s climate?

The abnormal weakening of the polar vortex and its downward coupling in spring to summer can be visualised in Figure 2. During winter the westerly winds associated with the SH polar vortex are stronger than usual in the upper stratosphere (as shown in orange), which can allow more atmospheric waves to propagate from the lower atmosphere into the stratosphere. Because these vertically propagating waves act to weaken the upper stratospheric westerlies, the polar vortex starts to weaken from early spring. As the vortex weakens over time, the weakening signal descends (shown in blue). The impact is felt at ground level from October to January.

Figure 2. When the abnormal polar vortex weakening happens, generally the vortex is abnormally strong during winter (shown in orange) and then weakens rapidly in spring. The process of weakening of the westerly winds descends after September (blue). Exact timings of the strengthening of the winter polar vortex and its subsequent weakening in spring to early summer can vary year-to-year.  Wind (m/s) is measured as being stronger or weaker than the average.

Impact on Australia’s climate

Developing an index to measure polar vortex weakening and strengthening has been a vital part of the Bureau’s recent research and has made it possible for them to study the impact of these events on Australia’s seasonal conditions.

“The Bureau has developed the stratosphere-troposphere (S­‑T) coupled mode index to identify these events and quantify their strength,” says Dr Lim. The index is based on monthly average wind data (1979-2017) over the Antarctic sub-polar region (55° to 65° South) at all available vertical levels from the surface to 50km altitude.

The index allows the Bureau to measure whether the polar vortex weakening is progressing at its usual pace. A high index means unusual weakening which leads to faster vortex breakdown. The strongest weakening event occurred in 2002 (Figure 3), which was related to the strongly negative SAM in spring 2002 that is believed to have played a more important role in driving hot and dry conditions than the relatively weak El Niño observed in the same year.

Figure 3. The Bureau have developed the S-T coupled mode index to identify polar vortex weakening (in red) and strengthening (in blue) events. The strongest vortex weakening event on record occurred in 2002 (based on monthly average wind data from April 2002 to March 2003 compared to wind data of all years).

Hot conditions

The Bureau’s S-T coupled mode index also highlights other less dramatic, but still significant, polar vortex weakening/strengthening events. By comparing historic temperatures and rainfall for the October to January period with the index the Bureau have found a very strong correlation between the polar vortex weakening and hot and dry seasonal conditions in southern Queensland and northern NSW.

For instance, maximum temperatures (Tmax) in the nine polar vortex weakening years (Index ≥ 0.8) were 1.2°C to 1.8°C warmer over southern Queensland and northern NSW than in the other 29 years studied between 1979 and 2017 (Figure 4). At the same time rainfall was 0.4 to 1.2 mm per day lower; that’s around 12 to 36mm a month.

Figure 4. The October to January mean maximum daily temperature is between 1.2°C and 1.8°C warmer and rainfall is 0.4 to 1.2 mm/day lower over southern Queensland and northern NSW during the nine identified polar vortex weakening years than in all the other 29 years.

“When you consider the seven hottest years (the top 20 per cent) – they are over four times more likely to occur when it is a polar vortex weakening year than a non-weakening year,” says Dr Lim.

“The research demonstrates that the Antarctic polar vortex is an important driver of heat and rainfall extremes in subtropical eastern Australia during late spring to summer.”

“The Bureau’s new ACCESS-S seasonal forecast system has a high level of skill in predicting S-T coupling from the beginning of September, which will improve our ability to predict temperature and rainfall extremes for the spring and early summer in polar vortex weakening years,” she said.

This result of the Bureau’s research implies that if land managers can be warned in September that they are likely to face hot and dry conditions through to January due to the polar vortex weakening, it will put them in a better position to make timely decisions such as how to manage livestock numbers, pastures and their supply of supplementary feed.

Lastly, since the beginning of spring 2018 the stratospheric polar vortex has been stronger than usual, which is likely to have somewhat mitigated the hot and dry conditions promoted by the development of El Niño over Queensland and northern NSW. This was something we could be thankful for during the tough dry spring of 2018”

Eun-Pa Lim, 03 9669 4000,


Complete record of droughts at Manilla NSW from 1884.

By ‘surly bond’,an independent climate analyst at Manilla NSW , Australia.

Posted on ‘weather zone forum’ July 2019

Red colour marks times when rainfall was in the lowest one percent of occurrences. The graph is novel, in that the beginning of drought is marked as well as the end.

There were just six great droughts lasting more than one year: 1902, 1911, 1940, 1946, 1965, 2018.
Very long droughts persisted for forty years from 1910 to 1950, then they ceased for the next forty years.
The pattern does not match a model of climate change that supposes that droughts have been getting (a) more frequent or (b) more extreme.

Details are in posts such as:


What caused the record heat in Europe -June 2019?

Thanks to a post from Michael Ventrice on twitter the answer is straight forward.


2018/19 : the second-warmest financial year on record in Australia

Climate of the 2018–19 financial year ..Report by BOM–19 financial year

‘…it was the second-warmest financial year on record’


.My comment… Since when are ‘financial years’ the  time frame for the primary seasonal report?




Global temperature anomaly 2019

Some scientists suggesting a global cooling with the solar downturn in the next 30yrs or more .

I won’t be around for that long but why not take a few medical records of planet earth temperature anomaly this year.

It might be a good exercise just to keep a tab’

This site gives an 8-14 day forecast

Forecast period May 10th to 17th 2019


A study of significant heat or drought in Australia

I will just collect information and post links.


SUMMER 2018/2019

A persistent pos AAO…Stalled synoptic pattern…with the Tasman high centred west of NZ..,.. jetstreams well south and zonal ( not wavy).. monsoonal activity mainly up in the far nth qld

map source:

Three-monthly rainfall deciles for Australia

Comments and information from weatherzone forum  >>>>>>>>

Rainfall deciles… Archive …. Australia.


KEN KATO..said  “Above is what Australia’s hottest summer on record looks like in the decile maps covering the last 3 months.”


Scroll down to the comments for all other entries

Click the heading of this post if the comments aren’t loaded



Years with more than 22 DRY DAYS during SUMMER on the mid to North coast of NSW Australia.

SEABREEZE from weatherzone forum in Australia .

Has taken the time to note the years that have had more than 22 dry 

A lot of work has been put into this and the results are interesting.

5 cities have been selected

Coffs Harbour, Grafton, Yamba, Casino  and Lismore

Grafton records are from 1871 to 2018   !!!

Quote from ‘SEABREEZE’ from south west rocks on the coast of NSW


I’ve looked to see how the current dry spell compares to previous ones during the summer-time along the NSW north coast.
It looks like extended dry spells during the summer were more common before the mid 1950s and have been pretty much absent since then until 2018. If this is a cyclical pattern, I would be expecting extended dry spells during the summer-time to occur from time-to-time over the decades ahead.
The length of the dry spell is a record for summer-time at Coffs, though not yet for the other towns.
The threshold for an extended dry spell for the towns listed below being more than three weeks (22 days or more) without any recordable rain.Dry Spells of 22 days or more at Coffs Harbour during or partly during summer (since 1900):
23 Dec 2018 to present – 26 days and counting
22 Dec 1993 to 14 Jan 1994 – 24 days
20 Jan 1952 to 11 Feb 1952 – 23 days
23 Feb 1943 to 17 Mar 1943 – 23 days
30 Jan 1939 to 20 Feb 1939 – 22 days
14 Nov 1913 to 5 Dec 1913 – 22 days
13 Dec 1902 to 4 Jan 1903 – 23 days


Dry Spells of 22 days or more at Grafton during or partly during summer (since 1871):
23 Dec 2018 to present – 26 days and counting
19 Jan 1952 to 11 Feb 1952 – 24 days
22 Feb 1943 to 17 Mar 1942 – 24 days
28 Jan 1926 to 23 Feb 1926 – 27 days
15 Jan 1915 to 6 Feb 1915 – 23 days
23 Dec 1904 to 15 Jan 1905 – 24 days
19 Dec 1902 to 11 Jan 1903 – 24 days
3 Dec 1893 to 1 Jan 1894 – 30 days
25 Feb 1883 to 24 Mar 1883 – 28 days
18 Nov 1881 to 18 Dec 1881 – 31 days
26 Nov 1876 to 17 Dec 1876 – 22 days
19 Nov 1875 to 14 Dec 1875 – 26 days

Dry Spells of 22 days or more at Yamba during or partly during summer (since 1877):
26 Dec 2018 to present – 23 days and counting
4 Jan 2018 to 27 Jan 2018 – 24 days
21 Dec 1954 to 11 Jan 1955 – 22 days
20 Jan 1952 to 11 Feb 1952 – 23 days
20 Dec 1932 to 11 Jan 1933 – 23 days
4 Feb 1926 to 26 Feb 1926 – 23 days
12 Jan 1912 to 8 Feb 1912 – 28 days
17 Jan 1900 to 8 Feb 1900 – 23 days
20 Jan 1892 to 15 Feb 1892 – 27 days
30 Jan 1886 to 24 Feb 1886 – 26 days
12 Nov 1883 to 5 Dec 1883 – 24 days
30 Dec 1879 to 23 Jan 1880 – 25 days

Dry Spells of 22 days or more at Casino during or partly during summer (since 1879):
24 Dec 2018 to present – 25 days and counting
4 Jan 2018 to 28 Jan 2018 – 25 days
19 Jan 1945 to 11 Feb 1945 – 24 days
1 Feb 1926 to 26 Feb 1926 – 26 days
15 Nov 1906 to 12 Dec 1906 – 28 days
22 Dec 1904 to 12 Jan 1905 – 22 days
19 Dec 1902 to 9 Jan 1903 – 22 days

Dry Spells of 22 days or more at Lismore during or partly during summer (since 1884):
25 Dec 2018 to present – 24 days and counting
18 Jan 1952 to 12 Feb 1952 – 26 days
11 Dec 1920 to 3 Jan 1921 – 24 days
19 Jan 1915 to 9 Feb 1915 – 22 days
3 Nov 1913 to 5 Dec 1913 – 33 days
5 Dec 1893 to 1 Jan 1894 – 28 days


end quote



As there was a 66 yr gap between the last dry spell ( 1952 – 2019) , l suggesteds to seabreeze the possibility of a link with the AMO which is linked closely to natural oscialltions in global temperature.

So l downloaded a time series graph of the AMO  from

I overlayed the dryspell years from the Grafton NSW Australia BOM data that seabreeze had compiled above.

amo vs dry days mid coast nsw

Of interest

5  of 7  Quasi intervals between   9 to 11 years …  ( consider the schwabe cycle perhaps)

The other 2 intervals  were of  5 and 25 years. The 5 year interval was the commencement of the 77 yr  dry spell pattern and commencement of an AMO cycle

The 25 yr interval was at the end of the AMO cycle

There is a break of 66 years ( 1952 – 2018) between extended summer dry spells . This break coincides with another AMO cycle very closely but with what appears to be a post offset of about 10 years of the max AMO peak.


My hunch is that….

The summer dry spells are linked to solar cycles( Schwabes) AND the AMO

But there needs to be another cycle in play that switches on or off that connection.

and so with all research.

The search continues..   in to the future


Comments and suggestions are welcome below






A Stalled synoptic pattern ..Australian summer 2018_2019

stalled synoptic pattern nov_dec_ jan 2018_2019

The synoptic pattern above ……stalled over Australia from about NOV 2018 , all through DEC 2018 and is still current at time of writing. ( Jan 11th 2019)

Persistent ridging on the eastern quadrant of Australia.

Contracted westerly belt .. Which is not unusual for summer

A big dip in tropical isobars right down to 35 s

Cyclone  Penny ( Jan 2019) could not transition south due to ridging in the Tasman blocking

The stalled pattern caused heatwaves as hot air from the interior was directed southward

Very hot temperatures in the interior as no cold fronts or cooler air from the south advected.

Storm lines persisted in the same place as the interior troughing axis remained quasi stationary.

Sydney  were included in many places with severe storms almost daily.

Noting the AAO/SAM had been positive since the first week of November 2018 and was still positive at time of writing 11th jan 2019. That is approximately 63 days so far

6_1_2019 mslp corelated with positive aao

The monsoon trough has not been over the top end of Australia until recently

Thanks to BOM and ACCESS model



source news article

HOT! HOT! HOT! Where the heatwave will strike today

Brisbane: 39C
Birdsville (Qld): 47C
Warwick (Qld): 46C
Tamworth (NSW): 44C
Moree (NSW) 46C
Bourke (NSW) 47C

BOM forecaster Dean Narramore told AAP.

“We saw on Saturday an incredible amount of records broken in NSW and that heat is just moving north for Sunday.”

Four Queensland towns set their highest ever recorded temperatures on Saturday as Brisbane reached 37C.

The city is tipped to reach 39C on Sunday, while several towns in southwestern Queensland, including Birdsvillle, could hit 47C.