Gedday again,

We have already discussed the development of Bonnie at some length in the context of whether or not it was associated with an n=1 equatorially-trapped Rossby wave.  There are a few other interesting aspects of it.

Bonnie developed at approximately 9 S, 127 E close to 0000 UTC 10 April.  As can be seen in the archived discussions on my web-page, we had been watching it since 0000 UTC on the 8th.  Following on from my previous post (on Dianne) I have put the vertical shear patterns for the 8th and the 10th on my web-page.

Considering our model of a monsoon trough as having easterlies on the poleward side and westerlies on the equatorward side, in the shear pattern it shows up as the inverse: i.e. a line of separation between equatorial easterlies and higher latitude westerlies.  Following this definition, we have a nice long monsoon trough on the chart for the 8th stretching from the dateline westwards and ending at about 130E, i.e.. the longitude of genesis.  There are a couple of noteworthy points: a) By this shear-based definition there are twin monsoon troughs, one on either side of the equator, both extending westward from the dateline;
b) Looking at the low level charts of relative vorticity (web-page) There are lines of cyclonic vorticity (blue in southern Hemisphere, yellow-red in northern hemisphere) along these monsoon trough axes; but in this case, at the low-levels, the only sharp line of vorticity maximum (as per the Guinn and Schubert 1993 paper) is the one in the Southern Hemisphere.

Let us now look at the 850 hPa flow (again on web-page).  This clear monsoon trough that shows up on the vertical shear pattern and on the 900 hPa relative vorticity pattern is NOT associated with a separation between low-latitude westerlies and trade/higher-latitude easterlies).  rather, it is associated with a clear equatorial edge to the almost direct easterly trades, the edge being so clear there is a strong -partial U/partial-y vorticity maximum along it.

This does not fit the classical pattern of the monsoon trough (separating monsoon westerlies from trades) nor does it fit our other model of the trade wind trough (convergence of easterlies) as described in Fig. 13 of Bill Gray's classical paper on a Global View of the origin of tropical disturbances and storms (Monthly Weather Review, 1968), or in Fig. 3.4 of my own chapter in the WMO report (McBride, J.L: 1995. Tropical cyclone formation. Chapter 3 of  Global Perspectives on Tropical Cyclones, edited by Russell Elsberry, World Meteorological Organization.)

Rather this is a monsoon trough with no low-latitude westerlies, but just a general area of low wind speeds between the trough axis and the equator.  This is not a one-off... I have noticed it several times this summer, which is the first occasion  I have been following synoptics like this on a day-by-day basis.  I mentioned it in the synoptic discussion of 22 January (see archived discussion on webpage) and the forecasters were unimpressed, reacting as if this was something that is not noteworthy as they see it all the time.

It is clearly important though, as it led to the development of TC Bonnie and as it is not a  classical model that appears in the research literature.  For completeness, I have also put on my web-page the GMS full-disc images for the two dates under discussion.

Cheers

John McBride