Mark Lander

Subject: [Tropical-storms] Massive ITCZ
TC Group,

    The ITCZ has grown to impressive proportions stretching from the IO into the EPAC.  There is a TC (01B) in the bay of Bengal, and the NHC had been interested in an area of convection south of Mexico.      With the death of EL Nino, there is mostly easterly winds throughout the region.  Transition monsoonal westerlies are in the IO and may extend eastward within the convection out as far as to the south of Palau.

    Each day, something sort of stands out in a general search of the global geostationary imagery, and this very active ITCZ jumped out today. Not sure any TCs will come of it, with low-level winds predominantly easterly overlain by upper westerlies.

    Regards,  Mark Lander

John McBride
> Not sure any TCs will come of it, with low-level winds predominantly
> easterly overlain by upper westerlies.
>

It's possible something may develop in the system near Mexico; but further west in the Western Pacific, my guess would be no, because all the vorticity is in the ITCZ itself, and the latitude is fairly low, around 5N.  Sure the system developed in the Souther Bay of Bengal; but that had an equatorial westerly surge equatorward of it.

John McB

Roger Edson

Hi John and all,

I decided to have a little fun with this, and also show how wonderful the scatterometer data are for
identifying flow in the tropics (where no data use to be available).

I have included a group of scatterometer passes from 11 May to 14 May in the area of the 'ITCZ' between
130-140E and from the Eq to about 6N or so.  I was curious about what both you and Mark had said about
there being no westerly winds near the equator and all of the 'vorticity' (assuming you were referring to
horizontal shear) being located in the ITCZ.  Well, this analysis indicates that there are (were) indeed
westerlies near the equator (possible instigated by the westerly surge in the Bay of Bengal) and these
westerlies seemed to have propaged from around 130E on the 11th to east of 140E by the 14th.  In so doing,
you can see the development of small (transitory?) circulations that develop within the trough axis during this period.  If you look at the satellite imagery during this period, you will find that most of the convection lies in and slightly north of this
axis;however it seems to show me that all it takes is a little relaxation of shear and you quickly get a 'wrap' of convection around any of thesecirculations....and 'wha-la' (sp??) out comes a tropical depression....

I've indicated for reference the 0 and 4N parallel and also the 130 and/or 140E longitude, depending upon
where the swath fit over the area during this period. The analysis was conducted with a larger scale (with
ambiguities) and then reduced to fit into this demo.

Comments are welcomed.
Sincerely,
Roger

John McBride

Roger,

 That's a pretty amazing sequence.  I like the way the westerly surge comes in, from the west; the surge stays on the equator and undercuts the trade easterlies to the north, creating -partialU/partial y shear vorticity and so spinning up small discrete vortices.

 Sound familiar?  We've seen it before.,,, but later in the season when the WestPac ITCZ was better developed.  Go to my web-page/synoptic discussion..... Go right down to the bottom to the archived discussions classified by subject matter.  Look under the "n=1 Rossby wave" heading (I know... that's Roger's favourite wave)...and in that classification read the entry for 8 July 2002,   Development of Rammasun and Chatann .  It is a pretty  similar sequence.

Cheers

John McB

Mark Lander
John,
    Do the westerlies really surge eastward and undercut the trades or
does some other large-scale process lower the pressure and weaken the
ambient easterly flow so that the westerlies develop in-situ and
appear ever more and more to the east? Or even make what appear to be
large jumps to the east, say from the equtorial IO into the western North
Pacific?

    Mark L.
John McBride
Short answer is I don't know.  If the equatorial westerly surge is an equatorially trapped Kelvin wave; then I would say it is propagating eastwards under its own steam (or following gravity wave propagation dynamics) and so IS undercutting the pre-existing easterlies.

I am writing this at home at 11 pm ,accompanied by the glass of Whiskey and in front of the TV; so  am not in a position to look at any diagnostics, to see whether equatorial surge has Kelvin characteristics.

If someone doesn't beat me to it, I'll have a look tomorrow.

John McBride

Well, I arrived at work this morning and stopped by Matt's office.  He didn't follow my argument about the Kelvin wave, as he thought when I used the word "surge" to describe the westerly winds on the equator that I was implying advection was involved.... he gained that impression from the context of the "cold surges" in the South China Sea.

 I see his point.... maybe my terminology was a bit loose; so, I'll start again.

 Mark asked if the eastward propagation of the equatorial westerly anomaly ( a westerly "burst"?) was really the westerlies "propagating"  eastward and undercutting the trades or does some other large-scale process lower the pressure and weaken the ambient easterly flow so that the westerlies develop in-situ and appear ever more and more to the east?

What I meant by my response was as follows:

The only equatorially trapped wave we know of with eastward propagation is the Kelvin wave. If you take a slice of a Kelvin wave along a parallel of latitude, it appears as a pure gravity wave, in which case the eastward movement is governed by the internal rearrangements of mass and wind as governed by gravity wave dynamics. So.. The answer would be YES.. The
westerly anomaly is propagating eastward under its own steam.

I hope that's clearer.

I had a quick look at Matt's diagnostics and there is a Kelvin wave, as diagnosed from the OLR spectra) in the right place and time. The real-time link is
http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/f.1.kelvin.html

The data for the current situation is here
The Kelvin wave is also embedded in the active part of an MJO event (current diagnostic here) , however; so there is more than simple Kelvin wave dynamics going on.

cheers

John McB

Roger Edson
Hi,

One more example for today...

I'm sure it is possible that the strongest 'push' is above the boundary layer...and the westerlies, then
get pushed down to the surface?  If so, perhaps they are not aligned which is why the front edge of the
westerly push (now at 153E) shows very weak easterlies to weak westerlies in the interphase.

So, in other cases, has there been evidence of a pressure change that fits along with these surges?

Roger

John McBride

Stimulated by Roger's question, i went to the NOAA CDC interactive web-site (http://www.cdc.noaa.gov/map/clim/glbcir.shtml)    and made myself a hovmoller along the equator of sea level pressure anomaly for the past month.  It is up on my web-page under the entry for 15 May.
 I can now see the point behind Mark's question.  There is certainly "something else" happening lowering the pressures along the equator further east, say between 150E and 150W.

 This is pretty intriguing.  Does anyone else out there have any comment?
 

Mark Lander

Hi John,

    Thanks for doing the leg work to show the large-scale lowering of SLP from 150E-150W.  It has been my observation that such in-situ large-scale lowerings of SLP precede (or accompany) the change of wind in this region from easterly to westerly.  I've always thought of it as a manifestation of MJO, but not really sure about that.
    On your Hovmoller, the pressure has dropped  4 mb in about one week out there -- quite an impressive change of SLP for such low latitudes. But unlike MJO it doesn't seem to have come from anywhere,
just dropped and dropped in one place.

    Mark L.

 Pat Harr
Hi Roger, Mark, and John, and others,

Let me throw out some possibilities that seem to indicate interaction among several circulation characteristics over a variety of space and time scales.  From the NOAA/CDC web site, I made three jpegs that contain three-day averaged anomalies of 850 v, 200 v, and SLP for  May 7-9, May 10-12, and May 13-15 .

On the 850 v anomaly charts a wave train from the circulation in the Bay of Bengal becomes evident.  For example, during 07 May-09 May here are weak positive v anomalies over Borneo, which become larger during 10 May-12 May.  During this three-day period, negative v anomalies appear east of Borneo.  During the most recent three-day period, the v-anomaly
pattern from Borneo eastward amplifies and also becomes connected with positive v anomalies extending from the midlatitude and subtropical SH over northwest Australia.  Throughout the SH, there is a wavelike pattern defined by the v anomalies.

On the 200 v anomaly charts, the SH wave pattern is more evident.  This seems similar to the analyses of K. Straub and G. Kiladis on SH wintertime forcing of Kelvin waves.

Finally, as Mark, Roger, and John have discussed, the SLP is falling over the equatorial WPAC, which is evident in the attached SLP anomaly charts.  It does appear that the equatorial pressure falls are related to the SH circulation patterns.

This may be an instance of interaction between a trailing wave-train from a developed circulation in the Bay of Bengal, SH midlatitude surge into the tropics, MJO as identified by Matt Wheeler's analysis, and Kelvin wave activity associated with the SH forcing (and/or MJO?).  All of which may be enhanced during this transition season.

The bottom line is whether we expect TC formation or a series of TC formations in the well-developed ITCZ?  Also, which of any (or all) of these mechanisms may be dominating the region or dictating when the TC may form in the generally active period?

The NRL satellite team web page shows some organization to a circulation east of the Philippines.  It seems that the WPAC is set to become active.

Pat

Brian Mapes

Hi Pat, Roger, Mark, and John, and others.

This 8-day animation of surface pressure anomalies gives a useful depiction of the low equatorial surface pressure,
which spilled out of Australia onto the equator and spread eastward very quickly:

In this depiction the Indian Ocean anomalies (the Bay of Bengal cyclone, and something across the equator from it)
don't seem strongly linked to the Aus/Equatorial happenings.

-Brian Mapes
Climate Diagnostics Center

 John Molinari
Pat, John, et al.,

         I have spent the last few years trying to figure out what goes on in the western Pacific, and Pat, your email encapsulates what I think goes  on: everything!  I would argue, however, that some is cause and some is effect.  In the events of May 7-15, first in my view comes the MJO, which shows clearly on the CDC page as coming eastward from Africa at the  beginning of the period.  When it is active, all disturbances (Kelvin, Rossby, mixed Rossby-gravity, tropical cyclone, etc.) are more likely to grow, maybe in part because the accompanying westerly wind burst creates cyclonic vorticity in the subtropics.  The storm that grows in the Bay of Bengal developed within the active MJO.  Then to me there is clear dispersion eastward and equatorward from that disturbance, as Pat noted.  The train of waves that extends equatorward and eastward from 15 or 20N is not unlike that shown by many people in the past: it can come from equatorward dispersion from the north (this case, apparently), or by MRG waves turning away from the equator toward the north (Dickinson and Molinari case).  I would argue that with regard to TC genesis, the 200 mb Southern Hemisphere wavetrain and the apparent Kelvin wave racing off to the east are not factors, although they are interesting in themselves.  Rather, the greatest hope for additional cyclogenesis was in the South China Sea and later east of the Phillipines, where an MRG wave packet excited by the dispersion would move.  The fact that the MJO was dying by yesterday makes that less likely.
         Given that we have so much trouble understanding what happened even after the fact, I think we have a lot of work to do to sort these all out.  We need synoptic case studies of tropical waves, lots of them!

         Cheers,
          John