The air above Antarctica is suddenly getting warmer – here’s what it means for Australia



Antarctic winds have a huge effect on weather in other places.
NASA Goddard Space Flight Center/Flickr, CC BY-SA

Harry Hendon, Australian Bureau of Meteorology; Andrew B. Watkins, Australian Bureau of Meteorology; Eun-Pa Lim, Australian Bureau of Meteorology, and Griffith Young, Australian Bureau of Meteorology

Record warm temperatures above Antarctica over the coming weeks are likely to bring above-average spring temperatures and below-average rainfall across large parts of New South Wales and southern Queensland.

The warming began in the last week of August, when temperatures in the stratosphere high above the South Pole began rapidly heating in a phenomenon called “sudden stratospheric warming”.




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In the coming weeks the warming is forecast to intensify, and its effects will extend downward to Earth’s surface, affecting much of eastern Australia over the coming months.

The Bureau of Meteorology is predicting the strongest Antarctic warming on record, likely to exceed the previous record of September 2002.

(Left) Observation of September 2002 stratospheric warming compared to (right) 2019 forecast for September.
The forecast for 2019 was provided by the Australian Bureau of Meteorology and was initialised on August 30, 2019.

What’s going on?

Every winter, westerly winds – often up to 200km per hour – develop in the stratosphere high above the South Pole and circle the polar region. The winds develop as a result of the difference in temperature over the pole (where there is no sunlight) and the Southern Ocean (where the sun still shines).

As the sun shifts southward during spring, the polar region starts to warm. This warming causes the stratospheric vortex and associated westerly winds to gradually weaken over the period of a few months.

However, in some years this breakdown can happen faster than usual. Waves of air from the lower atmosphere (from large weather systems or flow over mountains) warm the stratosphere above the South Pole, and weaken or “mix” the high-speed westerly winds.

Very rarely, if the waves are strong enough they can rapidly break down the polar vortex, actually reversing the direction of the winds so they become easterly. This is the technical definition of “sudden stratospheric warming.”

Although we have seen plenty of weak or moderate variations in the polar vortex over the past 60 years, the only other true sudden stratospheric warming event in the Southern Hemisphere was in September 2002.

In contrast, their northern counterpart occurs every other year or so during late winter of the Northern Hemisphere because of stronger and more variable tropospheric wave activity.

What can Australia expect?

Impacts from this stratospheric warming are likely to reach Earth’s surface in the next month and possibly extend through to January.

Apart from warming the Antarctic region, the most notable effect will be a shift of the Southern Ocean westerly winds towards the Equator.

For regions directly in the path of the strongest westerlies, which includes western Tasmania, New Zealand’s South Island, and Patagonia in South America, this generally results in more storminess and rainfall, and colder temperatures.

But for subtropical Australia, which largely sits north of the main belt of westerlies, the shift results in reduced rainfall, clearer skies, and warmer temperatures.

Past stratospheric warming events and associated wind changes have had their strongest effects in NSW and southern Queensland, where springtime temperatures increased, rainfall decreased and heatwaves and fire risk rose.

The influence of the stratospheric warming has been captured by the Bureau’s climate outlooks, along with the influence of other major climate drivers such as the current positive Indian Ocean Dipole, leading to a hot and dry outlook for spring.

Anomalous Australian climate conditions during the nine most significant polar vortex weakening years (1979, 1988, 2000, 2002, 2004, 2005, 2012, 2013, 2016) on both maximum and minimum temperatures, and rainfall for October-November, as compared to all other years between 1979-2016.
Bureau of Meteorology

Effects on the ozone hole and Antarctic sea ice

One positive note of sudden stratospheric warming is the reduction – or even absence altogether – of the spring Antarctic ozone hole. This is for two reasons.

First, the rapid rise of temperatures in the upper atmosphere means the super cold polar stratospheric ice clouds, which are vital for the chemical process that destroys ozone, may not even form.

Secondly, the disrupted winds carry more ozone-rich air from the tropics to the polar region, helping repair the ozone hole.

We also expect an enhanced decline in Antarctic sea ice between October and January, particularly in the eastern Ross Sea and western Amundsen Sea, as more warm water moves towards the poles due to the weaker westerly winds.




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Thanks to improvements in modelling and the Bureau’s new supercomputer, these types of events can be forecast better than ever before. Compared to 2002, when we didn’t know much about the event until after it had happened, this time we’ve had almost three weeks’ notice that a very strong warming event was coming. We also know much more about the process that has been set in train, that will affect our weather over the next one to four months.The Conversation

Harry Hendon, Senior Principal Research Scientist, Australian Bureau of Meteorology; Andrew B. Watkins, Manager of Long-range Forecast Services, Australian Bureau of Meteorology; Eun-Pa Lim, Senior research scientist, Australian Bureau of Meteorology, and Griffith Young, Senior IT Officer, Australian Bureau of Meteorology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Why warmer storms could lead to more flooding than expected


Conrad Wasko, UNSW Australia and Ashish Sharma, UNSW Australia

As the climate changes, we can expect more frequent and more extreme weather events, which will put pressure on our current infrastructure. It has been suggested that increasing temperatures will intensify rainfall, indicating that we are likely to endure bigger storms and more dangerous flooding in a future warmer climate.

Our study, published today in the journal Nature Geoscience, shows that this intensification in flooding may be even greater than expected. This is because of changes to the distribution of rainfall within storms – something known as the “temporal pattern”.

This study is the first to show that temperature changes are disrupting temporal rainfall patterns within storms themselves. When it comes to flash flooding, this is just as important, if not more so, than the total volume of rainfall that a given storm delivers.

If this trend continues with future climate warming, more destructive flooding across Australia’s major urban centres is likely. Because our findings were true across every Australian climate zone, ranging from tropical and arid to temperate, we can expect similar risks throughout the country, and conceivably elsewhere in the world too.

Looking to the past

Whether it is in politics, science or engineering, the past can be a good indicator of the future. Historical records of rainfall have long been examined for patterns to help us make sense of how the climate might change in the future.

By linking existing observations of rainfall intensity and temperature it has been found, in general, that we can expect more rainfall when temperatures are higher. This observation is founded in thermodynamics and underwritten by the Clausius-Clapeyron relationship, which states that for each degree Centigrade increase in temperature, 7% more moisture will accumulate in the atmosphere. It is not a large step to surmise from this that rainfall volumes will be 7% greater.

However, historical observations do not necessarily confirm this rate of increase – at least, not in a uniform way. Some places have experienced rainfall increases of more than 7%, while others have seen less than 7%.

This discrepancy is important. It suggests that changes in overall storm intensity are not the only change in rainfall a warmer climate may bring. There are other, more subtle disruptions we need to look for.

More flooding ahead?
Kate Wall/Newzulu/AAP Image

Finding the unexpected

In our study, we used historical data from 79 different locations around Australia, collected by the Bureau of Meteorology. This includes sites at each of the major capital cities, as well as regional areas in all states and territories. At each location we isolated storm events and then split each storm event into five segments, to determine the percentage of rain that fell in each. So, for example, a one-hour storm would be divided into five 12-minute segments.

By comparing the amounts of rainfall in each of these fractions to the average daily temperature at that location, we were able to check if there was any systematic relationship between the rainfall fractions within the storm and the ambient temperature.

Our results were unexpected. At every location, we saw that higher temperatures were linked to an increase in the largest fraction and a corresponding decrease in the smallest fraction. In other words, the storm pattern was less uniform and more erratic when the temperature was higher. Moreover, we found that these changes would increase flood peaks even if the storm volume remained unchanged, because more of the rainfall was concentrated into intense bursts.

Factor in the changes in overall storm volumes, which are also likely to increase with warming, and this is a recipe for more flood danger in areas including Australia’s urban centres.

Informing flood guidelines

So why is this important? Engineers Australia is in the process of rewriting the Australian Rainfall and Runoff guidelines, which dictate how we estimate potential flooding when designing infrastructure. Every structure, whether it be a roadside gutter, a bridge, or an office block, is built to withstand a flood of a given size and risk of occurrence. But if rainfall is changing, we need to plan for how we will design and build these structures to withstand the possibility of more destructive floods.

We need to make sure our infrastructure can handle the strain.
AAP Image/David Moir

Although history doesn’t necessarily have to repeat itself, the increase in non-uniformity linked to higher temperatures suggests that if temperatures increase we may see more increases in the destructive force of floods in the future. Planners need to consider whether the existing infrastructure that we take for granted every day needs added fortification to withstand the impacts of climate change.

The Conversation

Conrad Wasko is PhD Candidate in Civil Engineering at UNSW Australia.
Ashish Sharma is Professor, School of Civil and Environmental Engineering at UNSW Australia.

This article was originally published on The Conversation.
Read the original article.