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The SMH journalist writes about a cold snap hitting the East Coast of Australia in July 2015:

Proof of the big chill will be the extent of regions across Tasmania, Victoria and NSW experiencing zero temperatures, the Bureau of Meteorology said.

A belt of low pressure systems is a fairly permanent feature of Antarctica. One of those lows - at a latitude of about 55-60 south - is currently to the south of Tasmania and further from the Antarctic coast than usual, Dr Trewin said.

What would cause this low pressure zone to extend further from the Coast of the Antarctic than usual? Could it be a large chunk of ice melting?

My question is: Are Southern Hemisphere 'Polar Blasts' caused by global warming?

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Southern Hemisphere Jet stream 10 July 2015 (C) squall.sfsu.edu/ Looking at the southern hemisphere polar jet, there is a large trough (meander) in the jet stream bringing cold air a long way north from the Antarctic to south eastern Australia at present. The relevance of the position and shape of the jet stream is thought to be important for keeping cold polar air in place above the antarctic/arctic, with a strong jet stream keeping cold air hemmed in above the poles while a weak jet stream meanders allowing cold waves/blasts to escape into the midlatitudes. This previous Stack Exchange question sets out how the theory of arctic/polar amplification which is thought to affect the position and shape of the jet stream is being studied in connection to climate change (Generally it has been proposed that global warming will make the jet stream more wobbly/meandering, and so increase instances of cold weather in the mid latitudes).

Single weather events such as this are not caused by global warming, but it has been hypothesised that their frequency of occurrence may change due to warming, though as you can read in the linked stack exchange this theory is not universally accepted.

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In general, climate is the statistics of weather over a long period of time. In general, an individual weather event cannot be attributed to global change / global warming. An increase in the frequency of such events could be attributed to climate change. But considering how noisy weather and climate are, it takes a very long time series to measure an increase in frequency, in particular for rare events.

That being said, if an event occurs that is extremely unlikely under the null hypothesis of a stationary climate, but only moderately unlikely under a changed or changing climate, one could consider that this event likely means that climate has changed. But one needs to be very careful with such an analysis, because even unlikely events do happen, and on a global scale, the probability that this year there is a one-in-a-million-years event somewhere is far larger than one in a million. And we'd still need to be sure that we understand the present climate correctly — we could be underestimating the likelihood of an event in the prior climate regime, in particular if we have no prior experience with those events.

So to summarise: for a single event, we cannot say if this polar blast is caused by global warming. If the frequency of those increases along with global warming, then we may be able to establish a causal relationship. Or not.

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