Noel Davidson

Hello Folks,

As most were expecting, there was a burst in the monsoon late Jan-early Feb.
I have attached a figure which illustrates the analysed and (short-term) forecast
evolution of the large scale flow from 6 Jan to 4 Feb (last 3 days
from GASP forecast).'

Note the build-up in moisture, the rather sudden onset of clouds (CTT=cloud
top temp) and ascent, the accelerating upper level easterlies and lowlevel westerlies.

Being an extratropical-tropical person, I draw attention to the
high latitude cyclogenesis over the southwest Indian ocean (MSLP SWIO index);
and the strengthening and decay of the subtropical jet over the IO (STJ IO index).

It is my belief that the strengthening and decay of the STJ (also
seen as the amplification and fracturing of an equatorward-extending
upper trough) is indicative of the passage of a wave train into the tropics.
We hope to make calculations to confirm (or disprove) this hypothesis.

We also hope to create a web site to monitor the monsoon using these and
other diagnostics. Please let us know if you think this would
be useful, or if anyone has done similar things already, or if
anyone has derived quantities that they would like to see on the site.

It was also my feeling that the models gave rather formidable guidance
on the development of this event. Would be good to know why. Most were
suggesting a scenario similar to that above.
Finally : might we get a tropical weather system or two (or NONE?) out
of this event??

ND

John Nairn

Your discussions have attracted a reasonable amount of interest in SA.

We are particularly interested in how the rebuilding of the Monsoon will
feed back into the sub-tropics.
After a long spell of cooler weather we can clearly see the redevelopment
of a warming phase over SA.

Is this the mid latitudes alone? Or is there a coupling...

Regards

John

Noel Davidson

Hello John and John and others,

Interesting point you raise.
These active large scale tropical events are often accompanied by a southward
movement of the STJ over Australia (eg, Troup, 196?, on the monsoon
onset).
Although it could be argued that this (and the development of the upper
ridge and warming) may be associated with
feedback from enhanced tropical convection, it is my speculation that it is
more to do with an evolving higher latitude circulation.

I attach a figure wich illustrates the evolution of the Aust Region STJ
for the period 4Jan - 4 Feb (last 3 days GASP forecast). You will see
the ~steadiness of the jet for the first ~15 days, then the strengthening
and now (or soon) the rapid decay, as the upper ridge (and warming) develops.
Note the timescales of these changes. (Long period of not much, followed
by rather rapid changes. ie, big changes in the circulation following
major high latitude cyclogenesis and then downstraem development?)

THEN, if you REALLY WANT TO LET YOUR MIND RUN FREE - you could
even tie some of this stuff in with John McB's discussion and John N's
query on the occurrence and non-occurrence of cyclones and anticyclones in the
Tasman to the east of Vic, the cold Jan over ML, the severe WX
up the east coast(?), the trend to warming over SA, and unusual WX events
elsewhere????
(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 believe it may be related
to the number and location of high latitude baroclinic developments, or
the lack there of.)

Said to much. Confused? So am I. See ya.
Let me know where I am misguided - but take it easy on me.
ND