Howdy,

I would just like to summarise and to add a few points to the discussion.

a) n = 1 Rossby Waves and winter-time North west Pacific TC formation.

So far over the past month we have had three Tropical Storms or depressions form in the North west Pacific In all three cases the formation came occurred as a development from the northern of the twin vortices of an n=1 equatorially-trapped Rossby wave.   These were discussed on both these lists (Australian synoptic discussion and international tropical_storms) as they occurred.  The  n =1 Rossby wave structure was quite clear cut, having a jet on the equator at 850 hPa and two very clear cyclonic vorticity maxima on either side.  It varied from case to case as to whether there was a clear first internal mode vertical structure as would be refelected by a inverse image of the structure at upper tropospheric levels. The real-time discussion of these developments is on my web-page (link below).. follow the links to synoptic discussion, and then to the discussions to the first system: 28 February and 1 March 2002,  the second system :19 March, and the third : 5 April

b)  The Development of TC Bonnie near Indonesia

    Mark Lander wrote saying this devlopemnt clearly did not come from the twin vortex of an n = 1 Rossby.  I totally agree with him in terms of the wind-structure of the developing system.  We discussed this back on the 8 April on my sysnoptic discussion when there was the precursor system, and the satellite picture and relevant charts are on that web page.  Clearly there was no localised jet near the equator.  Rather the equatorial winds were quite weak, and were not westerly in those longitudes.

However, Matt Wheelr has pointed out that his real-time OLRdiagnostics DID give an n=1 Rossby wave in exactly that position and time... the Rossby wave being the same one that gave the development about a week earlier (5 April) back at longitude 145 E.  On Matt's Hovmollers, this Rossby wave moved westward and is located over the location of development of Southern Hemisphere midget TC Bonnie, which occurred on 10 April.

Thus, though there was clearly not the appropriate twin-vortex wind structure, if one uses Matt's diagnostics as the measure of presence of the equatorially-trapped Rossby wave you would have to count Bonnie as having been a development from the mass of convection in the southern of the two twin convective areas making up the satellite signature of the wave.

c)  The divergence pattern of the waves is not necessarily what we see in the atmosphere

Paul Roundy of Penn State pointed out that that wave forms come from the "dry theory" of the equatorial Beta-plane... the OLR imagery we see comes from the interaction of these divergence patterns with moist thermodynamics; hence the signal we see in the atmosphere can be quite different.

Relevant as Paul's comments may be as to why we get a TC out of one of the twin vortices, but not the other, it does not help us in the case of Bonnie where we apaprently obtained the OLR signal, but not the wind signal.