Gedday,

DOUBLE ITCZ

There is an interesting double ITCZ structure in the western and central Pacific.  I have put the GMS image and relevant charts on my webpage, under today's date.  The double ITCZ runs along approximately 10N and 10 S.  The southern member is marked by a line of cloud clusters.  The northern member is actually quite suppressed; but regular in space, quite small (about 100 km diameter) MCC's still exist right along the 70 degrees longitude length of it.

There are a couple of interesting aspects of this.  Both ITCZ's are marked by an east-west line (along 10N and 10S respectively) of cyclonic vorticity at low levels, as seen on the TLAPS operation 900 hPa vorticity analysis (Blue is cyclonic in the southern hemisphere; green is cyclonic in the northern hemisphere).  Looking at the vector wind analysis for 850 hPa, both ITCZ's are associated with -partial-U/partial-y cyclonic vorticity, associated with  a sharp edge to the easterly trades between 10 and 20 degrees North and South.

So.... what has happened to the Gill-model of the large scale structure of the tropics??  According to Gill, the trades are the Kelvin wave response to the maritime-continent heat-source; and as such would have maximum value on the equator.... which, clearly is not the case today.  The maintenance of today's trades, seems to be associated with higher latitude/sub-tropical high mechanisms... whatever they are.  Don't get me wrong -- I am actually a great fan of Gill; ...and Mark Williams and I are in the process of writing a paper demonstrating that Gill's four main waves (the long Kelvin, n=0 mixed, n=1 and 2 Rossby) can account for much of the interannual variability in large scale structure...... But, from today's example, clearly there are also other mechanisms operating.

The second interesting aspect of today's double ITCZ is that the southern component being dominant is unusual for late May.  On my web-page, I have put a link to some time-latitude sections for various longitude strips (from another work in progress.. this one with Prabowo).  The time-latitude sections are means over the 25 years of OLR data; and you can see for the relevant strips (140-180E; 180 - 160W) by May-June the northern ITCZ has usually taken over.

Tropical Upper Tropospheric Trough:

The other interesting aspect of today's photo is the really nice, clear example of the TUTT (Tropical Upper Tropospheric Trough) that Jim Sadler used to talk about and publish on in the context of North West Pacific Tropical cyclogenesis.  (Sadler, Monthly Weather Review, August 1978).  The TUTT is convergent, and therefore is consistent with subsidence in the upper troposphere; so it stands out as a cloud-free area on the GMS image.  This example extends from about 10N, 120 E (east coast of Borneo) to about 20N, 145E.  It can bee seen clearly as an easterly trough on the 200hPa chart for 12UTC last night and as a long filament of cyclonic vorticity on the derived upper Tropospheric vorticity analysis.

Cheers

John McB

Mark Lander

Hi John,

    Two strong ITCZ cloud bands are an interesting sight.  Not sure how
they might have come into existence (maybe the SST pattern would help:
cool on the equator and warm on both sides, Attached fig.).  I wonder if
Gill's analytical solution for steady state heating in the cloudy regions
would not produced just the wind pattern that is seen (second attached
Fig.)?  It would seem that the eruption of convection along both ITCZs
would try to add a westerly component to the wind between them, but not
quite enough to overcome the Kelvin easterlies produced by the heating to
the west.

    Regards,  Mark Lander