Gedday,
I went to the VRO chart discussion this morning and heard that for Melbourne the January just completed was the coldest for 12 years. In meteorology 12 years is not a long time; but the noticeable aspect was the absence of hot days. As the fronts came through we did not get the strong northerlies ahead of them.
Just naively I would think
this would be related to a lack of a blocking high in the Tasman.
I had a look at the NOAA page
http://www.cdc.noaa.gov/map/clim/glbcir.shtml
where you can construct
mean and anomaly charts for the last 30 days. On my webpage I have
placed the mean and anomaly chart for the past 30 days.
It sort of fits, there is a low pressure anomaly over Victora-Tasmania and the Tasman high is displaced eastward over New Zealand.
Any ideas or comments?
Following this reasoning,
one would think that bad January fire weather in Victoria would correspond
to when there was a strong high in the southern Tasman, which would correspond
to cold sea surface temperatures in that location (following the logic
that a high follows the cold underlying surface). I went to a different
NOAA site
http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl
and constructed an msl
pressure anomaly map for February 1983 (the Ash Wednesday fires).
The pressure theory seems
to work in that there is a 2-3 hPa anomaly in the Tasman. However,
looking at the SST anomaly, there
is actually a warm anomaly; so the LOCAL sea
surface temperature idea does not work.
Just a few thoughts.
John McB
Blair Trewin
Gedday,
I
went to the VRO chart discussion this morning and heard that for
Melbourne
the January just completed was the coldest for 12 years.
****
This statistic says more
about the last 11 years than it does about 2004 - the January mean
maximum ranks at = 36th
lowest out of 149 years, below normal but certainly not
exceptionally so. (The most
abnormal anomalies occurred in western Victoria and SA from
Adelaide southwards - Mount
Gambier AP had its lowest January mean max in ~60 years of
records).
****
In
meteorology 12 years is not a long time; but the noticeable aspect
was
the
absence of hot days. As the fronts came through we did not
get the
strong
northerlies ahead of them.
****
There was only one day over
32 for the month, something that has occurred in only 2
previous Januarys. (Conversely,
there were no really cool days either - the lowest maximum for the month,
19.9, is the equal 9th highest
on record).
Blair
Robin Hicks
My thoughts on the issue
are that in January this year there has been a persistent trough to the
east of Victorian / Tasmanian longitudes, keeping the region either in
cooler south westerlies or in an area that with other
triggers favours cyclogenesis.
This contrasts with years such as 1983 (Ash Wednesday fires) and 2003 where
the extreme weather events occurred with a trough forming just to the west
of Victorian longitudes. From the days of the Cold Front Research Project,
Garrett put forward the model of two types of summertime frontal structure,
the type 1, or deeper structure that was predominant in spring and early
summer, and the type 2 or shallower structure that often manifests itself
as the weak sea breeze type change during the latter part of summer. The
transition from type 1 to type 2 changes varies from year to year. I can
recall several years in the early 1990's when there was a period between
Christmas and the second week of
January when Victoria remained
cool under persistent SW'ly winds, with perhaps an occasional warm to hot
day ahead of a weak type 1 change. In 1983 and 2003 high fire danger events
resulted from strong type 1 changes occurring later during the summer.
In both cases the strength of the change was increased consequential to
the drought and strong surface heating over inland Victoria and NSW. Without
checking I suspect that there should be negative MSLP anomalies for these
seasons over inland NSW.
Robin Hicks
Acting Supervisor Fire and
Air Quality Services
Weather and Ocean Services
Policy Branch
Bureau of Meteorology
Blair Trewin
"My
thoughts on the issue are that in January this year there has been a
persistent
trough to the east of Victorian / Tasmanian longitudes, keeping
the
region either in cooler south westerlies or in an area that with other
triggers
favours cyclogenesis. This contrasts with years such as 1983 (Ash
Wednesday
fires) and 2003 where the extreme weather events occurred with a
trough
forming just to the west of Victorian longitudes."
****
....although in neither
1983 nor 2003 were monthly mean temperatures in southern Victoria
exceptional (indeed January
1983 was cooler than 2004, and the December-January period went
close to setting records
for the most days under 20 in Melbourne in such a period).
Mean summer maximum temperatures
in Melbourne are actually weakly negatively correlated
with ENSO (i.e. there's
a slight tendency to cooler summers in El Nino years), but the
frequency of extreme high
maxima (above the 95th percentile) is strongly positively
correlated with ENSO. This
may seem an apparently contradictory result but isn't - what's
happening is that the region
where El Nino is associated with high extreme temperatures
extends somewhat further
south than the region where El Nino is associated with high mean
temperatures (Queensland
and much of NSW), due to advection of hot air to the south on days
favourable for extreme maxima.
There is also a marked tendency towards a reduced frequency
of prolonged heatwaves in
Melbourne (as measured by indices such as frequency of 4/5/6
consecutive 30+ days) in
El Nino years.
(The extreme temperatures
material has, so far, only been published in my Ph.D thesis,
which explains why no-one
else has seen much of it yet).
In a lot of ways 1983 and
2003 were classic 'El Nino' summers in southern Victoria, with a
number of spectacular 1-2
day heatwaves interspersed with extended cool spells.
Blair
John McBride
Gedday,
A few days ago Noel Davidson wrote
"(I personally do not think that the discussion on the location of
Highs and lows is very informative
in understanding the cold Jan in ML.
We know that northerlies
(southerlies) are needed for warm (cold) temps in
ML (and SA?). A more fundamental
question is why these subtropical weather
systems established and
persisted in the way they did."
I disagree with your first statement, but agree with the second.
. As Blair Trewin pointed out, the distinguishing feature of this January was the lack of extreme hot days and as we living in good old Vic are well aware, extreme hot days are isomorphically associated with situations whereby you have northerly advection.
Perhaps I didn't spell out
my argument well enough as to the importance of the high in the Tasman.
The reasoning was that in summer you are going to get westerly troughs
and/or cool changes coming through about once weekly,
If there is a high in the
Tasman, then the front will come up against the high, causing a strong
northerly gradient and so the extreme warm days.
As to the cause of the high
being there, I had proposed local sea surface temperatures, but that didn't
work. What seems to happen is that during an El Nino, there is a
high pressure anomaly lying over our longitudes. The high anomaly avoids/skirts
the Australian continent, however, presumably due to the heating of the
continent. Thus, as shown on the February 1983 pressure anomaly chart
on my web-page
http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/04Feb02b.htm
the anomalous high pressure
arches around the western and eastern coastlines, and so effectively brings
about a strengthening of the high in the southern Tasman, thus bringing
about the situation where the fronts
will have strong northerlies
ahead of them.
John McB
