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

Posted by BCG on 14th March 2019

http://www.climatekelpie.com.au/index.php/2019/03/14/new-understanding-of-the-drivers-behind-hot-and-dry-conditions-over-australias-north-east/

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, eun-pa.lim@bom.gov.au

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Climate shifts…natural variation

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

or

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

“climate shift”

 

 

 

 

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Townsville and Brisbane. Past Rainfall correlated with sunspot number

This research was conducted by ‘RetiredWeatherMan ‘ a retired BOM meteorologist.

‘RWM’ is an active member of Australian weather forums

http://www.theaustralianweatherforum.com/forum/viewtopic.php?f=94&p=111657#p111657

…….

by retiredweatherman » Fri Aug 02, 2019 6:11 pm

The Queensland coastline from Bowen to Townsville has a more west-northwest orientation than the rest,
and even westerly in the Townsville region. The prevailing east-southeast trade winds therefore blow
more parallel to the coastline over this section producing lower rainfalls – the area being known as the
Dry Tropics. A range of hills from Cape Cleveland, northeast of Townsville runs to the south of
Townsville near Majors Creek, further protecting the Townsville region from the Trades. Therefore this
area is ideal to study for monsoonal influenced summer rains, with the monsoon flow being predominately
from the northeast to north. This region is also affected by PDO,ESNO,IOD and at the southern end of
MJO influences. And all these factors whether singly or in tandem can help to influence the rainfall
totals. As well as the coastline orientation, the lack of nearby coastal ranges largely negates rainfall
being boosted by the Trade Flow. My studies have shown these summer rainfalls may also be partly
influenced by the Solar Cycle in some instances, again whether singly or in tandem with other influences
as mentioned earlier. The attached graph shows the monsoonal summer rains, generally maximized between
November and April expressed as a percentage of the average of this 6 month period against the Solar Cycle
each year in January, back to reliable record gathering in 1872. For comparison is a similar graph for Brisbane
summer rain ( Dec to Mar ). Brisbane summer rainfall is more influenced by southern patterns then Townsville
therefore the similarities are not as well defined. One interesting parallel with the two sites is the very
long time it took to cast off the influence of the Federation Drought ( Brisbane more so than Townsville )
with the summer rainfall peaks moving generally upward roughly in line with the upwards movement of the
solar peaks from the Federation Drought period up to about 1960. The 2 graphs follow…..’

 

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Troposphere height

Links , pictures, research, information.

troposphere rheemoclineatmosphere temperature layers with,height

In no specific order.

Theory. Height of the troposphere

http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html

extract

‘The height of the tropopause depends on the location, notably the latitude, as shown in the figure on the right (which shows annual mean conditions). It also depends on the season (1, 2). Thus, it is about 16 km high over Australia at year-end, and between 12 – 16 km at midyear, being lower at the higher latitudes. At latitudes above 60� , the tropopause is less than 9 -10 km above sea level; the lowest is less than 8 km high, above Antarctica and above Siberia and northern Canada in winter. The highest average tropopause is over the oceanic warm pool of the western equatorial Pacific, about 17.5 km high, and over Southeast Asia, during the summer monsoon, the tropopause occasionally peaks above 18 km. In other words, cold conditions lead to a lower tropopause, obviously because of less convection.

Deep convection (thunderstorms) in the Intertropical Convergence Zone, or over mid-latitude continents in summer, continuously push the tropopause upwards and as such deepen the troposphere. This is because thunderstorms mix the tropospheric air at a moist adiabatic lapse rate. In the upper troposphere, this lapse rate is essentially the same as the dry adiabatic rate of 10K/km. So a deepening by 1 km reduces the tropopause temperature by 10K. Therefore, in areas where (or at times when) the tropopause is exceptionally high, the tropopause temperature is also very low, sometimes below -80� C. Such low temperatures are not found anywhere else in the Earth’s atmosphere, at any level, except in the winter stratosphere over Antarctica.

On the other hand, colder regions have a lower tropopause, obviously because convective overturning is limited there, due to the negative radiation balance at the surface. In fact, convection is very rare in polar regions; most of the tropospheric mixing at middle and high latitudes is forced by frontal systems in which uplift is forced rather than spontaneous (convective). This explains the paradox that tropopause temperatures are lowest where the surface temperatures are highest.

The tropopause height does not gradually drop from low to high latitudes. Rather, it drops rapidly in the area of the subtropical and polar front jets (STJ and PFJ respectively in the Figure on the left), as shown in the Palmen-Newton model of the general circulation (Fig 12.16 or Fig on left). Especially when the jet is strong and the associated front at low levels intense, then the tropopause height drops suddenly across the jet stream. Sometimes the tropopause actually folds down to 500 hPa (5.5 km) and even lower, just behind a well-defined cold front. The subsided stratospheric air within such a tropopause fold (or in the less pronounced tropopause dip) is much warmer than the tropospheric air it replaces, at the same level, and this warm advection aloft (around 300 hPa) largely explains the movement of the frontal low (at the surface) into the cold airmass, a process called occlusion (Section 13.3) (4).

 

 

………

Google search.. ‘pictures tropopause height’

https://www.google.com/search?q=picture+tropopause+height&tbm=isch&source=univ&client=firefox-b-d&sa=X&ved=2ahUKEwjlnsDf-bDjAhVDfX0KHcEUAu0QsAR6BAgEEAE&biw=1025&bih=491

………..

 

 

 

 

 

 

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Australian weather research

Have you ever gone on to a blog and wanted to quote about a research paper you read and couldn’t find it to quote it.

Here l would like to do myself a favor and record their links and title and points of interest. Feel free to do the same in the comment section below.

Click title if they don’t load

Thanks to ‘colin maitland’ of Australian weather forum

https://www.researchgate.net/profile/Blair_Trewin

for providing this link to Blair Trewin ‘s research gate page with a plethora of research to read

Here is a link to my favorite moon cycle physicist. and the amazing links to Earths climate he has found. BTW Ian Wilson is not an  astrologer

https://tallbloke.wordpress.com/2014/06/23/ian-wilson-18-6-year-lunar-cycle-in-high-rainfall-years-in-victoria/

Ian Wilsons’ home page

http://astroclimateconnection.blogspot.co.uk/

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Obviously this thread is in progress.Go to comments section for further research links. They are not A to Z but just posted randomly