Sudden Stratospheric Warming Event (SH) 2019

[Cyclone Tracy]

2 cents worth. A parade of 3 upper lows are now separating and cutting off on the latest water vapour image with the hint of a strong long wave trough behind them. First 2 to affect NZ.

1st will spawn the Tasman surface low as part of the SI west coast rain event which also will have a NI convergence zone side effect later in the week. 2nd will move and merge into the northern Tasman next week spawning a new Tasman surface low affecting the NI, EC and GFS now aligning on this. The upper polar trough SE of Africa should be watched by the SI folks later next week, as a Rossby wave moving into the region is supported by the displaced polar vortex that moves under Africa over the Antarctic shelf.

Water vapour 1130am 0810.jpg

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[TonyT]

Extended range models have a very changeable six weeks ahead with high pressure in the north/central Tasman and plenty of sub-polar troughs ploughing north from the southern Tasman and Southern Ocean. Rainfall likely to be above normal for all parts of the country.

[Awhituobs]

i think you mean 6 weeks ahead after mid october?

[spwill]

With west/southwesterlies we can end up with a lot of ridging this end of NZ from mid October with the central tasman high having a lot of influence and a drier than normal pattern.

[Awhituobs]

I agree, polar originated air masses "dry out"quickly as the head north this time of year (relatively, due to the relative low dew points where they originate from)
and so I cant see how rain will be above normal in all parts of the country
however showery W to SW flows are a different story for western places , but you need air masses originating from more temperature latitudes and not polar origin

[TonyT]

With west/southwesterlies we can end up with a lot of ridging this end of NZ from mid October with the central tasman high having a lot of influence and a drier than normal pattern.

Yes, but its a fine balance. At the moment modelling is suggesting it will tip slightly onto the wetter side for the north of the country and significantly wetter for the west and south of the South Island.

[TonyT]

and so I cant see how rain will be above normal in all parts of the country

Modelling does see it that way currently. The SSW event is still on-going but propagating lower into the Antarctic troposphere and as Mr Tracy notes there are signs of impacts closer to mid latitudes. You can't shake up the atmosphere to the extent we have seen and not have some fall-out further afield. The longer it goes on the closer we get to having sub-tropical air masses mixing in which complicates the rainfall outcomes. MJO is stuck in phase 1, which is the southwest phase for NZ and looks like it will stay there and possibly decay away. For now its all about the balance in the southwest flows - the models seem to think the current troughing and blocking in the Tasman is temporary, and we will go back to west/southwest from mid next week.

[Awhituobs]

but SW flows don't bring in subtropical air masses

[Razor]

but SW flows don't bring in subtropical air masses

But isn't it fun when they interact with one? That's the point.

[spwill]

but SW flows don't bring in subtropical air masses

If the air orignates from well south they dont but SW flows do tend to be stable from now.
Moist air can come down from the north Tasman/ Queensland in westerly pattern but there is less cold upper air over northern NZ from now.

[Cyclone Tracy]

A quick air chemistry check from NASA is giving a nice view of where the weakened and displaced vortex is currently positioned compared to the same time last year on 6th October. Ozone hole is around half the size.

Ozone 6 Oct 2019.jpg

[Chris W]

The stratospheric vortex continues to bounce back towards the Weddell Sea, looks like it is staying over between there and nearer to South Africa. While it’s nearer to the Weddell though, I believe that means a return to colder southwesterlies later in the month as we get more of that cross-polar flow at 250 down to 850mb and SW jet.

https://www.metcheck.com/SEASONAL/strat ... ecasts.asp

Meanwhile, its Arctic cousin is starting to form.

What is really interesting now is the effect the displacement is having on the stratospheric equatorial winds (jet?) at 10hPa, above the inter-tropical convergence zone (ITCZ). You can see here that the vortex is causing a narrowing of the equatorial jet over Central America, but what the consequences are I don’t know:

5B604CED-4B7C-49FD-BAD8-52D277044936.png

[snow101]

Some long range modelling in agreement for quite the cold spell (when comparing to normal), with enhanced rainfall and snowfall coming to parts of New Zealand to round out the month, and start of November. Still a while away, but could be an interesting period of weather.

[Bradley]

Some long range modelling in agreement for quite the cold spell (when comparing to normal), with enhanced rainfall and snowfall coming to parts of New Zealand to round out the month, and start of November. Still a while away, but could be an interesting period of weather.

Very interesting snow101, which long range modelling did you use for that forecast?

[Cyclone Tracy]

EC 12Z today splits the lower stratospheric polar vortex into 2 vortices on 22nd October. The new vortex launching equatorward to the SW of Australia and couples with the troposphere. Quite an aggressive development if it verifies.

EC 100 hPa 22 Oct.jpg

[Chris W]

Metcheck not splitting the upper vortex, not sure about the lower, but the upper vortex does get shunted from pillar to post and bent out of shape throughout their model run.

[Cyclone Tracy]

My 2 cents worth on the possible SSW event cause. Strong tropical easterly waves in the Pacific kicked off days before planetary waves started propagating into the upper stratosphere. The central equatorial pacific sea surface temperature changes have been in sync with the eddy heat flux over the entire event. It seems the equatorial pacific has got its finger prints on the climate control knob for the event.

Flow on effects in the troposphere, continue to play out.

Nino 3.4 planetary wave.jpg

[Cyclone Tracy]

SAM (AAO) index showing a strong negative bias trend ahead. Might be a cold end to October coming up.

SAM 1610.jpg

[spwill]

It looks to me like the highs are going to be a bit further west and north of NZ than usual for time of year (going by the modelling next couple of weeks) with more westerly through the central Tasman and less southwesterly than usual and hopefully a better chance of warmer weather coming through at times.

[Chris Raine]

Not in Southland where I am maybe

[Bradley]

The polar vortex at 10hpa showing massive signs of weakening the last day or two, this is the sickest I've seen it since the SSW event started so only a matter of time before the lower levels of the atmosphere start showing the same sign of being attacked...the end of October/start of November should be an interesting time weather wise for everywhere south of about Nelson!

2.png

[Cyclone Tracy]

Also, keep an eye on the jet streams. This afternoon earth nullschool is outputting multiple cross equatorial jets in the eastern Pacific (@ 250 hPa). Not unprecedented but not normal. Gives an indication of a major air circulation disturbance that is effecting both hemisphere's. One of them looks like a polar streamer is displacing the subtropical jet in the SH. The other looks like the Central American gyre is winding up in the NH.

[Awhituobs]

maybe it might finally live up to the hype? (any affects have been hard to distinguish from 'normal' without SSW tinted glasses on (haha)

[Chris W]

Also, keep an eye on the jet streams. This afternoon earth nullschool is outputting multiple cross equatorial jets in the eastern Pacific (@ 250 hPa). Not unprecedented but not normal. Gives an indication of a major air circulation disturbance that is effecting both hemisphere's. One of them looks like a polar streamer is displacing the subtropical jet in the SH. The other looks like the Central American gyre is winding up in the NH.

I'll admit I've never heard of a cross-equatorial jet. Thinking now there's no reason why it's not possible, but just hadn't thought of it. I wonder what the implications are especially for TC/hurricane/typhoon development in either hemisphere.

[Cyclone Tracy]

Also, keep an eye on the jet streams. This afternoon earth nullschool is outputting multiple cross equatorial jets in the eastern Pacific (@ 250 hPa). Not unprecedented but not normal. Gives an indication of a major air circulation disturbance that is effecting both hemisphere's. One of them looks like a polar streamer is displacing the subtropical jet in the SH. The other looks like the Central American gyre is winding up in the NH.

I'll admit I've never heard of a cross-equatorial jet. Thinking now there's no reason why it's not possible, but just hadn't thought of it. I wonder what the implications are especially for TC/hurricane/typhoon development in either hemisphere.

The coriolis effect normally prevents it, so a major shift has occured in the air circulation in both hemispheres. SSW is certainly under suspicion again