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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.
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).
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, firstname.lastname@example.org
The ices of Greenland and Antarctica bear the fingerprints of a monster: a gigantic volcanic eruption in 539 or 540 A.D. that killed tens of thousands and helped trigger one of the worst periods of global cooling in the last 2,000 years. Now, after years of searching, a team of scientists has finally tracked down the source of the eruption.
The team’s work, published in Quaternary Science Reviews, lays out new evidence that ties the natural disaster to Ilopango, a now-dormant volcano in El Salvador. Researchers estimate that in its sixth-century eruption, Ilopango expelled the equivalent of 10.5 cubic miles of dense rock, making it one of the biggest volcanic events on Earth in the last 7,000 years. The blast was more than a hundred times bigger than the 1980 Mount St. Helens eruption and several times larger than the 1991 eruption of Mount Pinatubo. It dealt the local Maya settlements a blow that forever altered their trajector……
Ice cores from Greenland and Antarctica show spikes of sulfate, a byproduct of large volcanic eruptions, at 536 and either 539 or 540. The two volcanoes were so large and so violent, they launched sulfur gases and particles miles into the sky. Since this material reflected sunlight away from Earth’s surface, it triggered severe global cooling: One 2016 study found that the volcanoes decreased average global temperatures by as much as 3.6 degrees Fahrenheit……
geologists published new evidence that the historical “dust veil” of 536 was caused by a volcano. In the other, researchers announced that the Tierra Blanca Joven extended into marine sediments off the coast of El Salvador. The Tierra Blanca Joven eruption was even bigger than Dull and others thought.…….
Dull’s team also revised their estimate of Ilopango’s size, taking into account the thickness and spread of Tierra Blanca Joven deposits. They say that Ilopango may have even dwarfed the 1815 Tambora eruption, a huge volcanic event that ushered in “a year without a summer” because of the global cooling it caused. Ilopango likely launched up to a million tons of sulfur miles into the sky, high enough for stratospheric winds to spread the aerosols worldwide and trigger global cooling.…..
The actual Research paper
I have started this blog post because today l have found out all major search engines are re routing the search string ‘climate shift’.
If you enter this term into any search engine, it will respond with pages and pages of ‘climate change’
We are being prevented from viewing alternative theories to man made climate change theories or facts, folks.
I will make an attempt to collect some links to climate regime shift sites that focus on natural variability.
I have tried alternatives to google and they ALL redirect the term ‘climate shift’
…You can get around this by..
Using google scholar…
which will accept the string ‘climate shift’ and lead you to alternative research on the reasons for global temperature trends other than AGW
on on the main google search engine page use talking marks on the search string which over rides the ban on the term… climate shift
I was looking at some time series data in particular SH ozone and the AAO(SAM)
I noticed a clear and abrupt change around 1998 and onwards.
OZONE TIME SERIES
AAO Time series
I thought l might check Google to see if researchers had documented a climate shift and the answer was yes.
There have been major climate shifts noted in 1925,1945,1975,1998
Guest post by Pavel Belolipetsky
( BTW..when was searching for information on google for ‘climate shift’, l noted l got next to NOTHING..,It appears google have removed the term and given full priority to the term ‘climate change’.
I think l might change my search engine)
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:
Continued in the comments below
Thanks to a post from Michael Ventrice on twitter the answer is straight forward.