Following my own advice, I had a look at the AMS site, and came up
with this which I found useful:-

http://ams.confex.com/ams/17Fluid15M...per_153709.htm

[ Unfortunately it needs to load some additional software to read it,
but it's worth persevering! [ I note, BTW, that the TWO board has been
discussing this, so it may not be new to some here. ]

[Summary]
... Conference held in June 2009 (and discussed the event of early
2009/February 16th on)

... Two categories of stratospheric polar vortex anomaly: displacement
(single centre/shunted to one side), and split (two centres, one
usually dominant).

... The split vortex is more likely to propagate influence to
troposphere.

... Both events/SSW - +ve ht anoms polar regions (c.f. -ve AO)

... However, split events (such as the one discussed, and the one we
have currently) have effect over greater number of defined longitude
bands - greatest impact Europe/Asia.

... With the specific event (start~16th Jan, 2009) rapid switch
from -ve temp/ht anoms to +ve temp/ht anoms, propagating downwards
through lower stratosphere/upper troposphere within period of 24-48
hrs, but delayed to lower troposphere to around +96/+120hr.

... Then effect weakened again (at low levels).

And at the end of the presentation, the following slide concluded:-

1. Not all Sudden Stratospheric Warming events descend into the
troposphere.
2. The implied switch (or enhancement) of a -ve Arctic Oscillation
wasn't global or persistent (not sure what 'persistent' meant in this
context).
3. Many other factors were involved!

My reading of it all was that this was another 'tool' in the climate
forecasting armoury, but it wasn't a 'silver bullet' by any means.

For interest, I had a look at the events leading up to our recent
change of type, from the perspective of the 10 mbar level: charts
available on the University of Wyoming site
(http://weather.uwyo.edu/upperair/uamap.html)

And very roughly the sequence at this level was ...

+ Up to the end November '09:
Single vortex / hugging north 'shore' of Asian Russia/Siberia -
drifting slowly east under influence strong jet on southern flank.
High cell western Canada, initially strong, declining. Polar
temperatures this level MS75C or lower, small areas MS80C.

+ Early days December (to ~ 5th)
Single vortex / moving over and/or straddling polar region (heading
for Canada .. see below). Major Aleutian high formed. Strong (at least
190 knots) jet high-arctic Canada from west or NW. Temps broadly
similar across poles.

+ 6th - 9th December
The single vortex drifting across NE Greenland then Baffin Island then
to far N. Hudson Bay BUT, strong trough developing towards Novaya
Zemlya (NZ). Temperatures polar region MS85 or a shade colder -
possibly the coldest of this sequence.

+ 24hr up to end 10th
Split vortex achieved: main centre NW Canada, secondary NZ. Warming
across 'neck' between centres ~+5C
( It was, *very roughly* during this period that our 'local' pattern
changed from an 'Atlantic'/CW or CSW type to an A type, with a
meridional pattern in the central North Atlantic. )

+ Current (13/00Z)
Strongly split vortex / temperature change across pole (relative to
just before split) ~+10C. Marked ridging NE Atlantic region with
temperature change across the Iceland region (at this level) ~+15C.

These increases in temperature (and associated height changes) are
quite modest BTW, and certainly wouldn't be classified as 'major':
cases have been documented where temperatures at this level (10mbar)
go from -80C odd, to around 0C in a very short time!

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023