Gedday,

 Presumably the circulation and characteristic cloud signature we see in the central Southern Indian Ocean, (approximately 10S, 85E) is the first Southern Hemisphere tropical cyclone for the season. If it is indeed the first, this seems a little late.  However, this is consistent of our discussion on the monsoon list some weeks ago where I noted there are El Nino-type conditions over Indonesia/maritime continent, and accordingly the onset of the Indonesian and the Australian monsoons is late coming.  Going along with this, the development of the monsoon trough stretching from Western Australia westwards across the Indian Ocean seems  also to be late.

The current Eumetsat image showing the circulation is on my web-page.

 

I also show a sequence of the GASP eastern hemisphere 850 hPa analyses for the past few days, where you see a gradual formation of a well defined monsoon trough across the Indian Ocean at about 10S (negative/cyconic vorticity corresponding to blue shading). At the end of the sequence, the well-formed monsoon-trough/line of cyclonic vorticity seems to suddenly lose amplitude along most of its length, the vorticity becoming concentrated in the developing vortex.
 

3-days ago:  Note amorphous line of negative (blue/cyclonic vorticity) across southern Indian Ocean
 

2-days ago: Vorticity line / monsoon trough has sharpened considerably.  There is even a hint of a monsoon trough south of Java, but nothing yet over northern Australia.

1-day ago:  The vorticity centre at about 90E is beginning to spin up.

Last night's analysis, whereby the overall monsoon trougyh seems to have lost amplitude, most of the vorticity having contracted into the developing vortex/cyclone at 85E.

 
 

In recent years, we are used to cyclones in this location developing out of the southern twin vortex of an equatorially trapped lowest order Rossby wave.  This doesn't seem to be the case in this instance.  However, if one takes as the equator the line of zero absolute vorticity, then you could mount an argument in favour of Rossby-wave formation.  Look on my web page at the low level absolute vorticity maps (shading) for 3-days ago and for last night:  Three days ago you can see a weak signature of a pair when the zero line at 90E was on the equator.  As the system has developed, to last night, thr zero line is further south and a small twin vortex (positive vorticity) can be seen on the analysis to the northwest of the developing cyclone

3-days ago

 

last night

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Have a merry Christmas all

John McBride