Gedday,

Since 29 June there have been six tropical cyclone developments in the Northwest Pacific.  Four of these I have already discussed:  Chatann and Rammasun on approximately 00UTC 29 June and Halong and Nakri, both around 00 UTC 8 July.

Since then we have had two more:  Fengshen: 1800 14 July at 12.4 N, 170.2 E
                                                         No-name:   0000 19 July at 11.7 N, 125    E

The relationship between these developments and the large-scale structure of the ITCZ is fairly interesting.  I have put a bunch of low-level (850 hPa) charts up on my web-page, under today's date, to illustrate the following points.

Starting with the chart for 29 June 2002, approximately the time of development of Rammasun and Chatann:  The ITCZ across the Indian Ocean is marked by the jet-like westerly monsoon current.  Over the Pacific the ITCZ  is associated with the trade easterly current.  They meet or converge in the location of the western Pacific monsoon trough, which on this chart (and climatologically) has a northwest-southeast orientation; and that is where these first two developments occurred.  As we all know, a (the?) dynamical precursor for TC development is the presence of high values of ambient relative vorticity.  On the chart on my web-page northern hemisphere cyclonic vorticity is shaded yellow. The westerly/Indian-Ocean current has the cyclonic (partial U/y) vorticity along its poleward edge, while the Pacific easterly flow has the cyclonic vorticity along its equatorward edge.  In the West-Pacifc (sloped trough) region, both effects are present; and hence this trough is the preferred genesis location.

29 June:  Rammasun (135E) and  Chatann (155E)

 

Moving on to  8 July, the time of development of Halong and Nakri:  These systems also developed in the location of the Northwest-southeast oriented western Pacific monsoon trough.  Virtually all the vorticity at this time is associated with the poleward side of the westerly monsoon current/jet.  At this stage it is a fair way north (poleward of 10 degrees) so there doesn't seem to be any influence of the n =1 Rossby wave action we saw earlier in the year when the ITCZ was close to the equator; nor does there seem to be any MJO influence on these two developments.  Whether the mixed-Rossby gravity wave mechanisms of Molinari and Dickinson are involved, I don't know as I find them a bit hard to see on daily charts (without producing some derived products... but I'll let you know in due course).

The westerly current has a fairly low latitude near Sri Lanka or western Bay of Bengal; so in that location there is cyclonic vorticity over the tropical ocean.  I have put up the contemporaneous Indian Ocean (Eumetsat) image; and it seems there was a weak depression in the southern Bay of Bengal; though it doesn't appear on any of the official storm tracks.
 

8 July  Halong (155 E) and Nakri (118.5 E)

Moving on now to 14 July, the development of Fengshen:  This development occurred closer to the central Pacific, at the very eastern end of the monsoon trough, or possibly with the easterly trade current.  There the cyclonic vorticity is on the equatorward side of the trade current.  For reasons I don't understand the trades have their maximum along about 15 degrees north so the band of cyclonic vorticity is slightly equatorward, e.g at about 10N.  This pattern during the past month has continued right across the Pacific (as per the western hemisphere map I have also put  on my webpage).
 

14 July   Fengshen  170.2 E

Turning now to the most recent development: 0000 UTC on 19 July at 11.7 N, 125 E.  A couple of days ago the monsoon trough in the west Pacific lost its normal orientation.  Looking at the chart I have put up for two days ago, you'll see the Indian Ocean westerly monsoon jet crosses IndoChina and skirts along the China coast; i.e it is further west and northwest than normal.   Thus it does not converge with the central Pacific trade easterlies, but rather there is a northeast-southwest oriented trough in the normal location of the monsoon trough.  This presumably is an example of Mark Lander's inverted monsoon trough.  What maintains the inverted trough, I do not know... but that is where the recent development occurred near the Phillipines, within this inverted trough.  The chart for two days ago (1200UTC 17 July is also interesting over the northern Bay of Bengal where there is once again a concentration of vorticity over the poleward side of the monsoon jet...This looks like a clear circulation to me, but once again no named system as far as I know.
 

19 July  No-name  125 E

So... summary of it all:  It seems to me at this time of year, in the Indian Ocean and Northwest Pacific Tropical Cyclogenesis is dominated by the meanderings of the two ITCZ's: the Indian Ocean far-west Pacific westerly jet, with its band of vorticity on the poleward side, and the central Pacific trade easterly maximum, with it band of vorticity on the equatorward side.

The meanderings of these two elongated strings of concentrated vorticity may well be governed by other dynamical mechanisms, e.g equatorial westerly bursts, MJO's and Dickinson-Molinari mixed waves.  Whether or not this is so, it is still clear that the concentration of the ambient vorticity in these large-scale climatological features (viz westerly monsoon current, easterly trade maximum) constitutes a lot of the trick to understanding genesis in this region.

cheers

John McB