Matt Wheeler

A week ago, Mark Willimas, Bill Wright and I were discussing the
possibility of a NW cloudband resulting from the mass of convection
over the Indian Ocean (from the MJO) that was occuring back then.
(you can follow the thread on John's web-page)

If you look on the current GMS images, a NW cloudband has finally
arrived, streaming down into Western Australia, largely the left-overs
from the southern TC of the second pair. The question is then, why
did it come later than we thought it should? My feeling is that this
behaviour varies quite a bit from one MJO event to the next.

-Matt.

Mark Williams

Well we seem to be getting a NW cloud band!

As Matt said the NW cloud band seems to be originating from the remains of
the southern cyclone of the second pair which formed in the Indian Ocean.
As this has moved south it has become rather a large vortex and it seems
to have broken through the STR at 700 hPa and below.  This appears to have
allowed the tropical moisture to break through into the mid-latitudes and
feed across southern WA.  This is quite different from what has been
occurring till now, as the STR has been strong with easterlies across the
eastern Indian Ocean in the vicinity of Cocos Island which clearly showed
in the analyses at the time.  But now the wind has gone to northerly at
Cocos (see http://serva.sa.bom.gov.au/data/skewt/map.html and click on
Cocos).

My guess is that this MJO was unusual with the strong vortex generation
that occurred.  Also at the time we had the blocking across Oz and
easterlies penetrating across the Indian Ocean from north Australia and
the whole MJO seemed to be tightly constrained to the tropics.  In a more
normal event the convection does not seem to be so well organised and my
perception is that the flow into the mid-latitudes from the Indian ocean
can occur much earlier in the event, or perhaps even without an event
occurring at all!

Mark