9 February 2004   Block over SE Australia

Harald Richter

Greetings:

Late January SE Australia was sporting a synoptic setup fitting the
classification of a Rex block (Rex Tellus 1950a,b).  An issue of interest
to me is how this block might have formed.  Two hypotheses:

(1) chance occurrence of the temporary phase-locking of individual troughs
    and ridges in separate branches of the westerlies

(2) reconfiguration of the large-scale flow associated with the passage of a
    slow-moving quasi cutoff cyclone across southern Australia leading
    to high latitude ridging

Are there any other ideas out there, or other thoughts on blocking
situation akin to the one discussed here?

Haraldnotwritingaboutstormsforachange
 

John McBride

Hmmm...... I had a look, and this is indeed interesting. The block over our longitudes was preceded by a block further east between about 160W and 180 W.  What happened was that block collapsed; and a new one formed over our longitudes, between 120 and 160 E.

So..we had a regime change..... A major shift in the location of the long-wave pattern.  Once the shift had  occurred, it persisted for some time, as discussed in an email discussion of some weeks ago (http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/04Feb02c.htm)

It is my understanding that a lot of the purpose of looking at ensembles is so  they can pick up  regime changes.... like this one...  Hmmmm

On my  webpage, I have put a series of analyses through the transition.  The series is of GASP 500 hPa analyses: The left-hand image is the GASP wind analysis for 1200 UTC on the given date.  The right hand image is the starting image for the ensemble spaghetti diagram for the same date.  Thus the first two images are for 1200 UTC 19 January, next two for 1200 UTC 20 January and so on.

So... go to my page; and have a look.  When do you reckon the regime change occurred?
 

Go to the GASP spaghetti ensemble archive (  http://gale.ho.bom.gov.au/bm/internal/daas/ensemble2/mv_spaghetti/index.html)     and run the java loops beginning at each date.

How do you reckon the ensembles went?
 

It is VERY interesting:  The collapse of the eastern block occurred between about 1200 21 and 1200 23 Jan.  If you run the spaghetti loop beginning 1200 21 January, essentially the entire ensemble predicted the collapse of the block; and it looks like a single member picked up the new block 50 degrees longitude to the west seven days into the forecast.
 

If you simple follow the height contour on the spaghetti diagram, the establishment of the block over our longitudes happened sometime between 1200 24 and 1200 26 Jan.   Running the spaghetti loop from 1200 24 january, virtually the entire ensemble missed this development.  But... if you run it a day later, from 1200 25 January, essentially the entire ensemble got it......... Hmmm.......

What does it all mean?????

John McB

500 hPa chart     spaghetti loop beginning 1200 19 Jan
500 hPa chart    spaghetti loop beginning 1200 20 Jan
500 hPa chart      spaghetti loop beginning 1200 21 Jan
500 hPa chart     spaghetti loop beginning 1200  22 Jan
500 hPa chart    spaghetti loop beginning 1200 23 Jan
500 hPa chart    spaghetti loop beginning 1200 24 Jan
500 hPa chart    spaghetti loop beginning 1200 25 Jan
500 hPa chart   spaghetti loop beginning 1200 26 Jan
500 hPa chart    spaghetti loop beginning 1200 27 Jan
500 hPa chart   spaghetti loop beginning 1200 28 Jan