Sudden Stratospheric Warming Event (SH) 2019

[Cyclone Tracy]

The BoM and models didn't see this coming this morning for NSW and Canberra.... and so it begins

https://www.abc.net.au/news/2019-09-17/ ... y/11519280

[Awhituobs]

but that snow is not related to the SSW I would suggest
but instead to a falling barometer with a surface low pressure

[Cyclone Tracy]

but that snow is not related to the SSW I would suggest
but instead to a falling barometer with a surface low pressure

Canberra air pressure went from 1022 hPa to 1024 hPa when the snow was falling was overnight . http://www.bom.gov.au/products/IDN60903 ... 4926.shtml

It was an upper low cut off from the polarJet that came from the first phase of the SSW that reached the troposphere in the last 14 days.

SSW GPH zonal.jpg

[TonyT]

but that snow is not related to the SSW I would suggest
but instead to a falling barometer with a surface low pressure

One of the likely consequences of the SSW is relatively rapidly intensifying and decaying short wave weather systems which the models may not latch on to until quite late. Expect a reduction in model accuracy while the SSW is on the prowl.

[hozza95]

It has been pretty wintery here today as well with a couple of snow flurries earlier this morning to just 150m when MS were originally forecasting it to 400

[Awhituobs]

you can see the cut off low forming on the satellite images now
these things occur during spring under jet streams and can occur with out much warning, seen it happen before with no SSW event
but maybe there is a cause and effect going on

[Cyclone Tracy]

you can see the cut off low forming on the satellite images now

That's the surface low that has cut off from the upper low into northern NSW and the Tasman. The upper low triggered the rainfall and snowfall overnight at the same time the surface high pressure was rising 1022 to 1024 hPa as seen above on the BoM readings.

[Awhituobs]

the models did show that all was going to happen (this new low) and a week ago or so there was talk about a cold snap for southern australia with air coming up and around the new south australia bight high coming in
so not sure it was all that much of a surprise?

[Chris W]

The 10hPa and 70hPa vortex continue their journey to the Weddell Sea. The strongest warming attack seems to be over the pole at 10hPa but further north over the East Antarctic ice sheet at 70hPa. So the vortex at 70hPa and jet at 250hPa are dragging their heels there a wee bit. Metcheck has the centre of the vortex forced completely off the continent in the next few days. If the vortex follows I wonder if that opens us up to something of a cross-polar weak southerly flow/remnant jet through October?

BC5EF193-A66E-49C3-9B34-2E73D51E6A96.png

[Cyclone Tracy]

- Easterlies now running @ 2 hPa and 5 hPa 60°S
- Displacement of the vortex centre has now been equatorward of 66°S for more than 7 days
- Parts of the polar region exceeded a 40°C change in one week

Ticking plenty of the official SSW boxes. It's now starting to get volatile in parts of the troposphere

[snow101]

I notice some climate models bringing colder then normal temperatures, including SST through spring, with some carrying it through into summer. How does a SSW event affect weather down at lower levels in Summer? Could we be on the verge of a colder then normal summer for NZ?

[TonyT]

I notice some climate models bringing colder then normal temperatures, including SST through spring, with some carrying it through into summer. How does a SSW event affect weather down at lower levels in Summer? Could we be on the verge of a colder then normal summer for NZ?

Yes.

[Chris W]

Have a look through the Antarctic strat forecast at Metcheck:
https://www.metcheck.com/SEASONAL/strat ... ecasts.asp#

The vortex rebounds towards the pole, then back out towards South Africa! This might not be over soon. I don't know how accurate this model is, but I can't find a comparison and it seems to have had a decent handle on things so far.

Also, check out the 10hPa wind field now!

2019-09-20 11_12_19-Window.png

[Cyclone Tracy]

The BoM's weather balloon readings from the Antarctic shelf suggests the tropopause is currently lowering. The irregular position of the polar vortex will support an aggressive tropospheric upper trough into the NZ region next week.

Did a September thread post on that subject. https://www.weatherforum.nz/phpBB3/view ... 69#p185369

[Awhituobs]

next week does look to be another (relatively short lived) cold outbreak
which might just be enough to make it the first colder than normal month in like 2 and half years or so?

[Cyclone Tracy]

NASA ozone maps comparing 2002 to 2019, clearly showing this year's event has occurred much earlier on the air chemistry impact. 2002 at this stage was at the beginning of the warming.

SSW 2002 2019 compare ozone.jpg

[Cyclone Tracy]

Checking in on surface temps, it's still brutal around the South Pole. The sun will rise at the South Pole on the equinox. It's going to be interesting to watch how thermometers react.

[Cyclone Tracy]

Record temperature change is now underway above the tropics in the stratosphere, especially at 30 hPa. This did not occur in 2002. The latest observations suggest this is not just a Southern Hemispheric event but a global air circulation change is now underway. Air chemistry is also changing along with the temperatures over the tropics. No one will know what this means, as it hasn’t been observed before at this time of year in recorded times (records date back to 1979) .

Tropics stratosphere 2019 to 2002 compare.jpg

[Awhituobs]

But that is colder than normal temperature instead of warmer than normal
But the current readings do occur at certain times of the year anyway ,ie so it's much earlier than normal. What does it mean?

[Cyclone Tracy]

As you can see in the chart, the tropics has a fairly minor and stable temperature range in the stratosphere. The lowest point of the year is associated with the northern hemispheric winter, which is obviously the opposite to what we are now observing. There seems to be fundamental short term behavioural change with the Hadley cell circulation. I can't find any historical evidence to compare this to at the moment. I'm trying to process what this could mean myself...… but anything involving the Hadley cell is major.

[Awhituobs]

Yes, disruptions to that could change north/south exchanges of hot and cold from equator to pole and vice versa and changes to the sub tropical ridge positition and strength... Seeing highs further south than normal currently for time of year.. Niwa suggest related to north Australia / Indo being a bit colder than normal currently but that is easing

[Awhituobs]

could the changes in the tropics mean CB's can reach higher altitudes before hitting the warming stratosphere?
i.e more intense CB's?
I see a big area of cb's south of the equator has errupted..a new MJO developing? (although that usually propagates from indo...not much been happening there)

[TonyT]

Looks like downstream impacts are arriving right on cue!

[Bradley]

Looks like downstream impacts are arriving right on cue!

In your opinion what are the characteristics of this cold change today that is different to the norm Tony? The fact the models didn't pick it up until very late?

[TonyT]

Looks like downstream impacts are arriving right on cue!

In your opinion what are the characteristics of this cold change today that is different to the norm Tony? The fact the models didn't pick it up until very late?

Surely you can think of a few! This southwest change is as normal as I am!

- very cold for the time of year
- snow to low levels in late September
- thunder risk in late September 2 days in a row
- risk of thunder/hail initially, followed by risk of snow
- possible snow confined to the Plains not the mountains
- predicted sudden clearance and frost with consequent ice risk
- prolonged period of changeable west to southwest airflow
- models not seeing it till late
- timing of impact after SSW event

The doubters will say its just part of the normal ebb and flow, but I'm calling it as unusual.