Hello,

There is a rather nice double ITCZ in the West Pacific right now as can be seen on the TRMM rainfall accumulation maps for the past 7 days.

The more pronounced ITCZ is the Northern Hemisphere component.  I am particularly interested, however, in the Southern Branch, as it being late November, this build-up is presumably a precursor to the Southern Hemisphere monsoon onset.

 On my web-page (link above) a 25-year average time-latitude section of OLR is shown for the West Pac (This figure is from the forthcoming PhD of Mulyano Prabowo, of the Indonesian Weather Bureau, BMG).   The vertical axis is latitude and the horizontal axis is time. OLR 240 W/m2 or less is shaded (with 15 W/m2 contour intervals).

This is a rather wide longitudinal span to average over, but you can see in the figure that the Southern Branch builds up during November and really gets going in December.

Over the past few days, I noticed an intersting interaction whereby the rainfall associated with an easterly (cut-off) low off the east coast of Australia drifted northwards and eastwards and amalgamated with the Southern ITCZ/SPCZ rainfall, the amalgamation occurring between 170E and the dateline.  The cleanest way to show this sequence is actually on the model forecast fields.   The four panels (figure on web-page) are Surface pressure at 24, 48, 72, 96 hours overlain by precip at 0-24, 24-48, 48-72 and 72-96 hours all from the operatational European Centre model.  The starting date is 1200 UTC 15 November, finishing 1200 UTC 19 November.  On these charts, you can see the rainfall in the Southern Branch of the ITCZ is captured by the model, and the rainfall is increased at the end of the sequence, apparently due to the interaction with the low in the easterlies.
 

Whether this is a common sequence, I don't know; but I'll watch for it in future.

regards

John Mcbride