With yet another deep Tasman  low we wonder whether this pattern will break
down.

Over the past three summer seasons we experienced La Nina conditions and
usually you would expect enhanced tropical cyclone activity with
possibility of a severe TC east coast landfall.

However the synoptic patterns over these three seasons were quite different
to La Nina Patterns.

Attached is the Jan to March 500 and 200 hPa height anomalies for the 1999,
2000 and 2001 composite: attachment
Notice the composite low SW of Noumea which brought tended to bring strong
SW to W vertical  wind shear into the Coral Sea resulting in an absence of TCs.

These composites differ from the  La Nina 200 and 500hPa composites
(attached) which show deeep anomalous easterly flow across the Coral Sea
which helps steer TCs towards Australia.

From John McBride

Thanks Jeff for the composite charts...for the last three years versus the
long-term La nina conditions.

It is an interesting approach.... but, I find it a little hard to
interpret.  It would be worth following this line of thought further
though.... E.g.  Take the years with many east coast cyclones, those with
very few/none, and do actual three month averages of the flow for the 200
hPa and 500 hPa levels.  Composites and anomalies are good... but they
need to be looked at in conjunction with the full field and with the set
of individual years.
 

While we are on this theme, I have often wondered what limits the
southernmost extent of Tropical cyclones off our east coast... Why don't
we get them in Brisbane, or even in Sydney, and could this change under a
greenhouse scenario?.... Have any of you blokes and blokesses given much
thought to this?
 

And another one on cyclones.... I sort of missed the development of Bernie
as it was over the New-year break and when I did come in, I was looking at
the NSW fire synoptics.  Does anyone up north have a succinct conceptual
summary of what triggered off Bernie?  Having a quick look at the charts,
it seems to be related to the trailing-front vorticity line we have been
discussion between the Coral Sea trades and the westerlies associated with
the higher latitude lows.  Any comments?

From David Thomas
Hi John et el,
Brisbane can get cyclones. One passed nearby when I was working there in
1981/82. Perhaps why we see such a large difference between west coast and
east coast southern extent of TCs is the fact the upper troughs that capture
them steer them southeast. Hence the bombs that hit Perth and NZ.

cheers
Dave
 

From Jeff Callaghan

John
It is interesting that a composite made up of years where a severe TC makes
landfall along the east coast has an anomaly very much like the most common
synoptic pattern associated with such an occurrence . Namely a slow moving
deep layered high in the Tasman with a  stationary relaxing upper trough
over Central Australia.
Ben Annells, a Met 1 here, is doing a project on this data for his ELP.

Re TCs off SEQ and NSW, they happen.
In 1954 a 973 hPa TC made landfall at Coolangatta with hurricane force
winds a large storm surge and rainfall rates of 35 inches in 24 hours (or
just under a metre). 26 people (some say 30) perished in this event.

Dinah passes 100 km east of Brisbane in Jan 1967 with CP around 945 hPa
with a clear circular eye well embedded in a CDO. It was a big system and
generated huge seas and a large storm surge on the Sunshine and Gold Coast.
The trendy Victorian enclave of Hastings Street in Noosa was knee deep in
salt water. Dinah still had a central pressure of 970 hPa east of New Zealand.

Pam passed east of the NSW coast as a severe TC. It was an incredibly large
storm and generated a record storm tide in Moreton Bay while near Lord Howe
Island.

The so called Sydney cyclone in Jan 1950 was an interesting case. It moved
from the Gulf to Sydney (Sydney recorded its lowest ever CP of 988 hPa
which still stands today I think).  10 people were killed between Brisbane
and Sydney. A similar  event in 1898 killed around 30 people with three
vessels wrecked off the NSW coast.  The disastrous 1955 Maitland floods low
originated as a Gulf low also.

The two Brisbane flood events 1893 (25 deaths) and 1974 (18 deaths) were ex
TCs and hybrid systems.

There are other instances of TCs or tropical systems affecting southern
parts and they mostly have occurred during La Nina episodes.

Check the details of these events in the Qld Severe Wx home Page under
Tropical Cyclone Coastal Impacts Project

Jeff

From Blair Trewen

Date: Thu, 10 Jan 2002 10:04:58 +1100
From: Blair Trewin <B.Trewin@BoM.GOV.AU>
Cc: synoptic_discussion@bom.gov.au
Subject: RE: [synoptic_discussion] Re: Tasman Sea lows and tropical cyclon es

>Re TCs off SEQ and NSW, they happen.
>In 1954 a 973 hPa TC made landfall at Coolangatta with hurricane force
>winds a large storm surge and rainfall rates of 35 inches in 24 hours (or
>just under a metre). 26 people (some say 30) perished in this event.

Can someone enlighten me as to why the March 2001 system which affected
northern NSW was not a TC? - was it purely that it originated outside the
tropics or were there other reasons? (I'm certainly not an expert in such
things, but my casual impression is that the Americans seem much more
inclined to call systems tropical than we are).

>Dinah passes 100 km east of Brisbane in Jan 1967 with CP around 945 hPa
>with a clear circular eye well embedded in a CDO. It was a big system and
>generated huge seas and a large storm surge on the Sunshine and Gold Coast.

>The trendy Victorian enclave of Hastings Street in Noosa was knee deep in
>salt water. Dinah still had a central pressure of 970 hPa east of New
Zealand.

IIRC, Dinah's central pressure at 'landfall' (on Fraser Island) was the
lowest at landfall of any Queensland cyclone of at least the last 40 years
(and quite possibly since the 1918 Mackay and Innisfail cyclones). (I notice
on the Queensland RO web page that Kathy (1984) listed as a cat 4 at
landfall, but wasn't that in the NT?).

Blair

From Jeff Callaghan

Blair
There is a report on the March 2001 event on our web page.  There is a
strong argument for treating these systems as TCs or at least giving them
an equivalent warning system which is why we are trying to get a high
profile Severe Weather warning system operating. The problem with these
events is that often the most destructive winds a a long way from the
centre. The point location in TC warnings works well as most of the impact
is near the centre. In the case of the March 2001 low  it made landfall
around 3pm near Ballina where there was little damage. The worst damage
occurred around 7 to 8pm at Yamba which is well to the south. The damage
occurred under a rain band associated with the squeeze between the low and
a building MSL ridge.

As an example TC Wanda (Brissy 74 floods) had no gales before landfall but
generated storm force winds well to the south of the centre after landfall
when all the warnning were curtailed.

The US has the same problem with these systems some are named and some are
not. Gary Padgett is organising  a session at the next AMS conference in
San Diego to discuss this problem.

Another type of system we would like to name which don't fit the Dvorak
models are systems like the June 1994 events which developed between Fiji
and NZ and devastated  a yachting fleet and the May 1999 event which passed
close to Lord Howe Island bringing gusts to 85 knots. A paper discussing
the desirability or not of naming these appears in the proceedings of the
International Conference on tropical weather systems in Darwin 2000. Gordon
Jackson produced an excellent book on the proceedings which should be in
the library.

Most  of these systems from the pre satellite days are in the TC data base.

Kathy made landfall near Centre Island in the NT.

Jeff