a) The Oz monsoon:
 

From the Darwin office weekly tropical climate note of last Tuesday, it seems the Oz monsoon onset had occurred back then (i.e. before 11 December).

Looking at Today's 850 hPa chart, you would have to agree and put the monsoon trough along about 20N across Australian longitudes. It is quite a weak monsoon flow, however. One of the other characteristics of monsoon onset is the jump of the two jet streams (northen hemisphere and southern hemisphere) southwards. Looking at today's jet-stream map (below:pinched off a NOAA site) I guess it has happened with the northen hemisphere jet; but our Southern Hemisphere model is a bit sluggish in responding.

B) The tropical low in the coral sea. This is a relatively weak system and I notice the Queensland office has said it is not expected to develop. An interesting aspect of this is that it seems to be related to a cut-off wave-break from higher latitude potential vorticity see the sequence of 350 IPV maps below:
 
 

c) The westerly wind burst at about 160 E.  This is a beautiful looking system with a mass of tropical cloud and cyclonic vorticity maxima at about 5 N and 5 S, reminiscent of the twin-vortex model, equatorially trapped n=1 Rossby mode model. (See the current 900 hPa vorticity map on the coloured tropical LAPS page of NMOC). Interestingly the system is virtually stationary, and if anything has retrogressed slightly westwards. See the attached satellite map from about 3 days ago, and compare with today.

           A few days ago                                                                                                                    Today