Since I began this synoptic discussion a few months ago, I have mentioned blocking often; as a common feature of the upper level charts over our longitudes has been the split jet pattern. When we call this split jet a "block", I am not sure of at the moment, as the word also carries implications of stationarity (if there is such a thing). Anyway, there is no doubt the current situation is a block; because when one looks at the week of MSLP analyses, you see that the large high pressure system formed south of the Bight, at about (45S, 150E) on the morning of Friday 21 May, and has stayed rock-solid stationary ever since.
There was a special issue
of the Australian Met Mag on blocking, back in 1983. It contained
an interesting set of papers: a review of the climatology contrasting the
northern and southern hemispheres by Mike Coughlan, a set of NWP experiments
by Peter Noar and surveys of the relevant theoretical mechanisms by Jorgen
Frederiksen, by David Karoly and by Peter Baines. For the definition
of blocking and the climatology in the Australian region all five papers
relied on a 1974 Bureau Tech Note by David Wright. There is an interesting
summary of David's results in the Baines paper. According to Baines,
David's definition states that blocking satisfies the following characteristics:
a) The basic westerly current
splits into two branches
b) The 5-day mean 500 mb
ridge at 45S (defining the longitude of the block) has a rate of progression
of less than 20 degrees of longitude per week and progresses no more than
30 degrees of longitude during the entire blocking occurrence
C) The ridge of high pressure
at the longitude of blocking is at least 7 degrees south of the normal
position of the sub-tropical high pressure belt and is maintained with
recognisable continuity
d) The occurrence lasts
for at least 6 days.
Going on, from summarising
from David Wright, Peter Baines states "the following salient features
emerge from Wright's study:
a) The durations are generally
shorter than for the northern hemisphere.
b) The most common form
of blocking is that where two or more highs occupy the same region sequentially,
rather than one high for the whole duration which is the northern hemisphere
experience.
c) Blocking is most common
in
the Tasman Sea, but also occurs in the Western Pacific, Great Australian
Bight and eastern Indian Ocean.
d) For the region 110E -
150 W there are two pronounced seasonal maxima in the frequency of occurrence
of blocking - one in the summer and one in the winter.
e) Compared with the northern
hemisphere, blocking in the Australian region is a smaller scale phenomenon
spanning ~ 40 degrees longitude, with a correspondingly smaller sphere
of influence.
f) The splitting of the
basic westerly current usually implies a cyclone-anticyclone pair oriented
approximately north-south with the high poleward. The high is usually
larger and more conspicuous.
I am interested in how blocks
form; so also on my web-page under today's date I have put a set of thumbnails
of the hemispheric 500hPa flow for the entire month of May. The split
jet pattern has existed over Australia for most of the month. As
I said above, whether we would call the pattern a block early in the month,
I'm, not sure yet.
The feature that is different
about the last seven days (i.e since the 20th) is that the polar front
jet, the southern member of the pair, has poked way south. This results
in a "square on its side" - type pattern in the jet, with one corner forming
an upper level trough over low latitude and the opposite corner forming
a high latitude ridge.
I also looked at the potential
vorticity patterns but found them a little unispirational. Looking
at the IPV at 350K for the past week, it has been marked by a "trough"
of high PV air sitting up over the Australian continent, with high PV/Stratospheric
air extending right up to Darwin and Cape York. I must confess I
don't understand this at all as the tropopause is still high (120 hPa or
so) at Darwin and other places far north).
IPV350: 20/05
21/05 22/05
23/05 24/05
25/05 26/05
The occurrence of the two jets implies an interesting thermal structure. By thermal wind we have to have cold air poleward and warm air equatorward. There is also a tropopause discontinuity across a jetstream. It would be nice to be able to see this structure. I haven't found time yet to call Phil Davill and download Kenny on to my PC -- so, if any Kenny users out there volunteer to do a north-south cross-section through the block, I'd like to see it.
This is turning into a long note.... apologies for that... but I'll ramble on for just a few minutes more.
Why is blocking interesting? One aspect is that persistent blocking can bring about droughts. E.g: a couple of blocks during a season can have an enormous negative impact on the total rainfall for the season; so they're interesting in the context of droughts and of extreme weather events.
An aspect I am interested in is that of "predictability". The current ensemble forecasting strategy is based on the fact that we believe mid-latitude weather is inherently chaotic. However, intuitively one would think that in a block, the weather pattern remains stationary; so the sensitivity to initial conditions is very much smaller.
Does anyone out there have figures as to whether the models have been more predictable than usual during the last 7 days of this blocking situation? Have the spaghetti diagrams stayed closer together than usual... Has the truth (analysis) stayed within the spaghetti longer than usual? Have the range of skill scores and anomaly correlations from the range of operational models been better (showing more skill) than usual? Time permitting, I'll have a look at some of these things myself.
cheers
John McB
P.S. Concerning the
relevant literature, there have also been some observational papers on
Southern Hemisphere blocking by a chap named Mark Sinclair fron New Zealand.
They are in Jnl of Climate and Monthly Weather Review in the mid 90's.
I haven't read them yet.
500 hPa analyses for the month
