We have just had two more developments in the Northwest Pacific:  Halong (0718UTC, 9.3N, 155E) and Nakri (0818UTC, 22.6N, 118.5E).

    I don't know yet what caused these developments.  Looking again at the 850 hPa display for last night they both formed right on the axis of the monsoon trough.  Halong is at the easternmost tip of it, where it transforms into an easterly-convergence or trade-wind type trough. There was no new equatorial disturbance that  I could discern to trigger it.

I have put on my web-page for today a sequence of 900hPa vorticity analyses for the last 11 days.  The images are small, but the features stand out clearly.  You can see two cyclonic vorticity bulls-eyes on the monsoon trough axis on 29 June signifying the developments of Rammasun and Chataan.   The western-most of these (Ram) was fairly mobile; but the vorticity centre associated with Chataan remained fixed at the easternmost extent of the monsoon trough way up until  4 July.  When it finally got moving, it shedded a residual vortex in its wake, which remained in that same location (10N, 160E) as Chataan moved on towards the northwest.  Looking at the vorticity trend for the last couple of days, the vortex has intensified only slightly and has drifted westward, and has become Tropical cyclone Halong.

As often happens when we see something new and interesting, there is an old Bill Frank paper talking about the process.  In this case he wrote a paper in Monthly Weather Review of June 1982 titled "Large Scale chharacteristics of tropical Cyclones", whereby he demonstrated from rawinsonde composites that the wake area of westerly moving typhoons and hurricanes exhibit large scale vorticity patterns that are favourable for tropical-cyclogenesis.  I haven't seen before, though, any mention of a residual vortex as appears on the last week's sequence of vorticity analyses.

28 June                                                            29 June

30 June                                                                              1 July

2 July                                                                                    3 July

4 July                                                                                   5 July

6 July                                                                                  7 July

8 July

 

The other development, Nakri, occurred on the north-western end of the sloped monsoon trough.  It was perched on the northern periphery of the band of strong low-level monsoon westerlies that lie across the Indian Ocean and the South China Sea -- see last night's 850 hPa analysis on my web-page.  I don't have any particular insight on this development, as I don't understand the dynamics of that westerly band at all.  However, there was a clear TUTT-type interaction with a higher-latitude westerly trough that extended down into that location.  See the 200 hPa vorticity chart for 00UTC 8 July on my web-page.  You can see the line of cyclonic vorticity extending down from the higher latitudes to 20, 120, which is the genesis location approximately... thus there was some sort of apparent upper-trough triggering going on.

850 hPa                                                                                200 hPa vorticity