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Less ITCZ convection! Why?

By: moonlightcowboy, 5:17 AM GMT on August 05, 2007

PLEASE keep in mind, that this is JMHO from listening, reading and observations. It is NOT professional. You should ALWAYS depend on your local authorities for information and instructions! In our short lull this week, I thought I'd blog on the Itcz. What it is and what's happening with one of the driving forces in tropical development. If you see errors, or disagree, please post your thoughts and facts. I appreciate your input, comments and corrections! Thanks!

itczone-2.jpg picture by moonlightcowboy

The Intertropical Convergence Zone (ITCZ) is an area of low pressure that forms where the Northeast Trade Winds meet the Southeast Trade Winds near the earth's equator. As these winds converge, moist air is forced upward. This causes water vapor to condense, or be "squeezed" out, as the air cools and rises, resulting in a band of heavy precipitation around the globe. This band moves seasonally, always being drawn toward the area of most intense solar heating, or warmest surface temperatures. It moves toward the Southern Hemisphere from September through February and reverses direction in preparation for the Northern Hemisphere's summer.


SURFACE MAP If you click on the above map, and zoom in, it makes it easier to see the location of the Itcz which is the red line that runs across the width of the map. If you'll notice, the Itcz is running lower than 10n in the cAtl which is considerably lower than the mean for this time of year and consequently, there's been less activity. The Itcz has regularly been below 10n with only recent hikes further northward and then, mostly only in the eAtl towards the African coast.

That's unusual for this time of year. It may also be "part" of the reason we haven't seen as much activity. And, I think it could also possibly extend the "peak" of this year's season. Below, in the next few graphic/sat visuals, you can see part of the Itcz is running below 10n. That may also have allowed SAL to be more prevalent and milder eAtl SST's. The Itcz reportedly moves towards the warmer SST's north of the equator around this time of the season and I think that's what we're beginning to see.


If you'll look closely, you can see that what thunderstorm activity there is in the Itcz is mostly running near 10n, especially in the cAtl. Less storm activity in the Itcz consequently also means less favorable development conditions. In my humble opinion, activity has been less than normal, possibly because of the strong B/A High, and maybe partly due to the amount of SAL; or moreover, the stable/dry air. And, especially in the area of the cAtl.

The image “http://www.cpc.ncep.noaa.gov/products/fews/ITCZ/west.gif” cannot be displayed, because it contains errors.

But, according to the chart, the mean position for the first 10 days in August would be running near 19n (which seems high to me). It is running nearly that high closer to the African coastline and is on the continent. There it seems to be presently running at near mean average. But, it is not in the cATL and further west towards SA.


During the period from July 21 - 31, 2007, the African portion of the ITCZ was located near 19.0 degrees north latitude when averaged over the ten day period and from 15W-35E. This compares with a normal position of 18.2N and a position last year of 17.3N. In the west, from 10W-10E, the ITCZ was located near 19.8 degrees north, compared to the long term mean of around 19.3 degrees north, and a position last year of 18.3N. In the east, from 20E-35E, the ITCZ was located near 18.0N, compared with 16.6N for the mean, and 16.5N for last year. The ITCZ moved north rapidly during the last dekad. It is now positioned at, or above normal across most of Africa.


This image clearly shows the Itcz running at a considerably higher latitude than it's presently running.

What Keeps the ITCZ North of the Equator?

It is a long-standing mystery that the ITCZ stays north of the equator over the Atlantic and eastern Pacific Oceans despite that the annual-mean solar radiation at the top of the atmosphere is symmetric with respect to the equator. This article reviews recent progress that has shed new light on this old puzzle.

(excerpt in part)...The ITCZ problem thus involves a circular chicken-and-egg argument. The ITCZ stays north of the equator because SST is higher; and the SST is higher north because the ITCZ stays there. The positive WES feedback is at the center of this circular argument. In a coupled ocean-atmosphere model, the WES feedback destabilizes the symmetric climate, leading to an asymmetric steady state with a single ITCZ on only one side of the equator (Xie and Philander 1994). A condition for this spontaneous development of latitudinal asymmetry is the equatorial upwelling that prevents the ITCZ from forming at the equator. This necessary condition thus explain why climatic asymmetry only develops over the eastern Pacific and Atlantic where the equatorial upwelling is observed.
(Complete article here.)

Heavier precipitation occurring over warmer waters


In recent days the Itcz seems to be moving further north, despite it's day to day flucuations n and s. Presently, it has dipped down again in the cAtl at around or below 10n. There's been less activity and with the amount of dry air that we've seen, all of the waves thus far have been dependent on moisture from the Itcz. That's why (IMO) there's been less Atlantic activity. As it moves further north for the Cape Verde season, we can expect more activity and more opportunities for tropical waves to organize and produce spinning storms. A more northerly Itcz makes it easier for developing storms to coriolis. And, more Itcz moisture further north also weakens SAL and dry/stable air, making conditions more favorable.

Look here on the Latest Surface Map for the Itcz location, waves, and other surface features.



[GOES-12 14 km WV]
RAMSDIS 14 km Water Vapor
Dust is not really a factor in the cATL or the Caribbean, but dry air still is. There's a new pocket of dry air that has made its way down into the cAtl at the sfc level and some dry air also still lingers at the sfc level in the central and north Caribbean. But, that can change fairly quickly as 99L demonstrated that a developing storm can survive despite dry air entrainment. Again, the Itcz moving northwards should also relax the dry air making it less of a prohibiter.

TCHP These "boiler" ingredients could create a dangerous situation if a storms tracks through these deep, hot waters which are more serious than the 2005 season.

***NEW FEATURE***Current GOM Loop Eddys Click to enlarge.



There are several links to SAL, but EUMETSAT is my favorite. For me it shows a more true, actual view of the dust which is in pink. These links also show airmass and fog. Views appear to be "dimensional" and while SAL mostly occurs at the surface or low levels, if you'll take a look, it also shows orange and red, depicting a look at mid and upper level convection. For me this is a good first way to look at the layers of any developing CV storm.

EUMETSAT (dust angle 1)
EUMETSAT (dust angle 2)
EUMETSAT (dust angle 3)

REAL-TIME SAL satellite imagery for tracking can be found here, too.

The National Buoy Center


GFS 850 Vorticity
GFS 850-200mb Shear
24hr shear tendency


Latest Infrared Shortwave in the eATL

Latest TWO
Latest NHC Tropical Weather Discussion

"IMHO Tropical Summary"
- All the factors are beginning to shape up for something to develop soon: warmer eAtl sst's, low shear, less dry air and dust to prohibit organized t'storm activity. With the Itcz moving further north, there should be more activity; and, it should supply more moisture for waves to develop and make it easier for storms to produce the coriolis effect.

If you look at the graphic below, you'll see that a number of storms develop this time of year in the area just east of the southern islands. That's where the last few invests (except for 98) have tried to get their acts together and I think that's where we'll also see 90L develop from. There is also a chance that we could see something develop in the Caribbean and the GOM itself, depending on ridging highs and fronts that drop down.

The chance (percentage) of a named tropical cyclone in August

Tropical Cyclone Probabilities - Named Storm in August
This is the chance at any particular location that a tropical storm or hurricane will affect the area sometime during August. Based on years 1944 to 1999 in the analysis and counted hits when a storm or hurricane was within about 100 miles (165 km). (Figure by Todd Kimberlain.)

Lastly, we've already had three named storms and have closely followed two invests recently. As each wave approaches, the odds are that at least "one" will develop soon and become the first hurricane of the season. The thing that concerns me most is the position of the Bermuda/Azores high that just dominates the Atlantic.

That factor complimented by other highs extending the ridge westward with less troughs creating weaknesses, steering currents, high SST's/TCHP in the Caribbean and GOM, low shear and just general odds, points to the possibility of a serious storm developing and finding its way towards CA or the GOM.

Conditions can change, but for the time being they seem to be pointing to an intense season. If not in frequency, at least in potential intensity. The forecast for the next few days is forecast to be relatively quiet. Fortunately, that gives us more time for awareness and planning. Hopefully, we'll be better prepared in the event something does come our way!

The tropics are definitely heating up! We may get lucky, but the odds don't lend favor. The "Dean" of storms may just be in the next patch of wind!!!

"Hold down the fort and keep the gates closed!"


Your comments and suggestions are welcome. Thanks in advance!
Have a good one!


PS: Don't forget to check out Tropical Lagniappe for some great links to other blogs and websites from fellow WU members! There's some great info here and I always learn something each time I visit them!

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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

Nicely Done Blog & Spot-on Observations.
Keep Up The Good Work.

ps--the desert island & cboy hat, very fitting logo for yourself :o)
Thanks, the main blog is a great place for learning and some great posters there. Here, I just want to put some quick links, and simple obs for quick access. It's slow and a work in progess. Hey, but thanks for the stop by and comments.
good work there, did the guy´s at afirme got in touch with ya? drop me a line if not, i could probably help you with translations.
Thanks, Gabrielaca. You've got mail.
Hey MLC!

I think we have to watch the all the waves, that come off Africa, from now, because the models are showing some development in the CATL this week.
I hear ya, Ryang! Things are definitely heating up! It's gonna be a bunch to keep up with for sure. Hey, you're on the point down there...keep a watchful eye, stay safe and keep us posted! You bring much to the blog, thanks!
Just checking in this morning before heading out to meet the demands of {insert dramatic, suspenseful music} Back-to-School shopping!!!!
Hope YOUR day is pleasant and stress-free. : )
Me…..well….I’m going to eat some chocolate every chance I get!
Photo Sharing and Video Hosting at Photobucket
yes indeed, it came over us went pass thru and still keep on going, sounds like a batery advertise.
Evening MLC....

Just stopping by to say hey, and repeat (I think)my tnx for the link to the WU Evac discussion I posted.

....trying to beat the heat here tonight by having a tall "cuba libre"... and I hoist it in a salute to the MLC.... although I still have problems picturing palm trees and a cowboy in MS... a failure of my imagination which I hope the rhum will rectify.....

LOL, Gabby, the "EnergizerCane"...haha, a new term. Glad you're safe.

--CRS, you're quite welcome. Awareness and safety has got to be what this is all about! And, I too, salute you, friend! Many kudos! Imagination? Well, a couple of those usually helps me! lol

Stay safe!
This map seems to suggest that the ITCZ is further north than average; I say this because you will notice that it has positive rainfall anomalies north of negative rainfall anomalies, which is what you would expect if it was further north than average (the positive anomalies however are much stronger than the negative anomalies, indicating an overall stronger ITCZ).

Thanks, STL. I knew you'd bring a smart addition to the topic. I may be confused on the subject...lol. I've just been watching the sats, the sfc maps and the overall convection, really; and, it made me want to blog the topic. These are anomalies, right? And over the past 30 days? Maybe, there's been a n/s fluctuation in the last week or so? TIA, again! I always appreciate your interpretations and posts!
MLC GREAT blog. Nicley done and keep up that good work!!
Thanks, Hurricanegeek. Things may be about to get busy!
Yes I agree MLC!!
Has the GFS ever predicted a storm for so many runs in a row earlier this season?
I think they are called runs.
Hgeek, models as I understand them are all about consistency. Each are different in their criteria and the variables they employ. Some are better for different storms, some better for cyclogenesis, and others for tracking. The GFDL is a respected track model, but it has a hard time with initializing. It gets confused in the cyclo phase. As far as the recent GFS consistency, I'm sure that has happened, but not to that degree this season. So, it must smell something I think. lol, JMHO.
Hello MLC!

Great topic... I always thought the ITCZ never came about 10N, Well until the high moves futher east, these waves won't have very favourable conditions, to form!
Yeah, maybe so, ryang. I just keep thinking about all this energy that has to go somewhere. I think STL is right, there's been little Pacific, especially ePac, and that typically should mean a more active Atl season. It may be slow popping, but when it does, my guess is, that it's really going to pop.
Well, if the atlantic, doesn't get going, we'll see another 1977.
lol, ryang, that's just about exactly what we've seen so far in 2007; but, I don't think it's gonna stay that way!
Doc Master's blog today, speaks to how inactive the Itcz is right now, too.
MLC: Great blog. I really enjoyed reading and learnt a few things that i didn't know. I enjoy coming to your blog and a few others. I have learned so much through you guys on here and I want to say Thank You & Keep up the great work, I know it takes time to get all this together and it's nice that you take the time to do it.
KUDOS to you.
Thanks, Sheri! It's definitely interesting following the tropics. WU is a good place. Awareness, preparedness and safety is the name of the game! Thanks for stopping by and commenting!