Gedday,

I'll make this real quick as I have come into the office on the weekend for a task more urgent (though less important).

On my web-page I have put up a sequence of real-time GASP analyses for the tropical eastern hemisphere, in reverse order, so that you will see last night's (1200 UTC).  As per usual the background shading is relative vorticity.

Looking at last night's analysis, you will see that after our ex-mixed RG wave has undergone  transition to a TD, it has attached itself to the eastern end of the monsoon trough.  There looks like there has been some westward energy dispersion along the trough as there are now a series of four distinct vortices stretching along the trough from Haishen at the eastern end to a near equatorial vortex at the western end.  The picture is remarkable in a number of ways:

a) The sequence of a spun-up trough of distinct vortices is exactly what we saw in the previous example we discussed (see my synoptic discussion for 11 October)

b)  When the precursor vortex was on the equator, it was an MRG which has group velocity eastwards.  However, now it is  in the monsoon trough, it apparently has some form of Rossby wave character.  For Rossby waves of this short a wavelength the dispersion is westward, hence the phenomenon we see of the monsoon trough to the west spinning up.
(Correction, added later:  I got this wrong: -- The group velocity of short-wavelength equatorially-trapped Rossby waves is, of course, eastward).  ---- so the question is: How does/did the energy apparently disperse to the west?  Any ideas?

c)  I have put up a sequence of three charts so you can see the spin-up.  There is an interesting appearance of another classical MRG structure on the first day of the sequence (20 November) between 80 E and 100 E.  This has a complete wavelength. As per Matt's discussion the associated divergence field is that that there is convergence at the poleward end of the away-from-the-equator flow.  Thus the convective signal is such that there should have been convection at about 5N 80E (over Sri lanka) and at about 5S, 90E (lines up with the vertex of the Bay of Bengal).  I have put up the satellite image for that time (close to 1200 UTC 20 November); and you'll see the satellite signal of this "short-lived) MRG is quite weak.

d)  Michael in his email commented on how remarkable it is that we can see the MRG waves in unfiltered data.  Not only that, but we can also see the eastward group/energy dispersion.  Have a look at the 700 hPa hovmoller on my discussion for the 22 November. There is a clear example around the 10th of October.  There is also an example in the current event, the MRG's at 80E having high amplitude on about 14 November and this being dispersed to high amplitude at 130E on 21 November.

The reason, of course, for being able to see the structures so easily in unfiltered data is that at this time of year along the equator the magnitude of the background flow is so weak.  Also, guided by the research results of the Wonder Boy, Mike-and-John, Hendon-Liebmann, and of course Yukari Takayabu, we know what to look for.

e)  Referring back to the most recent 850 hPa analysis (1200 UTC 22 Nov), the vortex at the westernmost end of the monsoon trough, around 60E is the northern pair of a set of n=1 Rossby-wave twin vortices.  This location immediately east of the African coast  has been seen before (see my discussion for 4 November) but this may be pure coincidence.

f) A new full-wavelength mixed Rossby-gravity wave has appeared along the equator between 130 and 160 E, as if to "replace" the wave that had undergone transition.  Thus the large scale ITCZ structure has a northern hemisphere monsoon trough stretching northeastwards from about (5, 60) to (15, 140), then there is a jump to the south with a single member MRG between 130E and 160E.  Eastwards of that we have a zone of easterly trades.

Back to the "urgent" task.

John McB

Satellite image for last member of the above sequence (close to 1200 UTC 20 November), [copyright 2002 EUMETSAT ]