"Not Enough Coriolis"

[Derecho]

Fortunately haven't seen this myth promulgated much recently as the reason stuff doesn't form south of 10N in the Atlantic - but TD 4W in the Westpac just formed at about 5N, provides a good example of why "not enough Coriolis" is NOT the reason you don't see low-latitude formation in the Atlantic.

[Aric Dunn]

interesting.. i can see why people would claim that. but never knew that there was a myth..

5n thats very unusual though

[alan1961]

Fortunately haven't seen this myth promulgated much recently as the reason stuff doesn't form south of 10N in the Atlantic - but TD 4W in the Westpac just formed at about 5N, provides a good example of why "not enough Coriolis" is NOT the reason you don't see low-latitude formation in the Atlantic.

yes just imagine what this board would be like right now if they saw TD 4W in the Atlantic...Hysteria!!!

[Derek Ortt]

5N is quite common for the WPAC. One in the Indian Ocean formed at ~.5N, with almost half of its circulation on the southern side of the equator.

Strong relative vorticity in a monsoon trough mroe than compensates for low planetary vorticity (and planetary vorticity is not exactly high at 10-15N either)

[Aslkahuna]

Actually that was Vamei which formed in the lower reaches of the Gulf of Siam becoming a 75kt typhoon just east of Singapore before tracking along the Straits of Jahore and then along the north coast of Sumatra inland from the Straits of Molucca into the NIO. Another factor in the ATL is that anything that would form south of 10N would quickly run into the northern portion of South America which extends up to almost 10N

Steve

[Cyclone1]

I think he's talking about Agni, not Vamei.

[Nimbus]

We sometimes see systems forming in the caribbean north of Panama early in the season when the waves are still moving off South America.

[DanKellFla]

5N is quite common for the WPAC. One in the Indian Ocean formed at ~.5N, with almost half of its circulation on the southern side of the equator.

Strong relative vorticity in a monsoon trough mroe than compensates for low planetary vorticity (and planetary vorticity is not exactly high at 10-15N either)

So, is it really a myth regarding the Atlantic?

[Derek Ortt]

if there was a monsoon trough near the equator in the Atlantic like there is in the WPAC, there would be TC formation there

The S Atlantic storm of 1991 only formed a few degrees south of the equator

[MiamiensisWx]

Fortunately haven't seen this myth promulgated much recently as the reason stuff doesn't form south of 10N in the Atlantic - but TD 4W in the Westpac just formed at about 5N, provides a good example of why "not enough Coriolis" is NOT the reason you don't see low-latitude formation in the Atlantic.

That's a good point. I believe there are some primary factors, but the lack of Coriolis Effect is used as an excuse. It is important to remember the upper-air dynamics and variables within the equatorial regions. Instability is not an issue, so some other variables are responsible for the unfavorable TC environment. Low-level convergence is weak, and the mid-level conditions may inhibit vertical structural stacking. Systems remain shallow, thus we do not observe significant organization. Undoubtedly, there are more significant negatives for development. I would appreciate more information... I must refresh my brain!

[wjs3]

OK, let's slow down.

If we're talking about the tropical Atlantic, Coriolis in the typical sort of Cape Verde development regions (let's say aroiund 10 N for argument's sake) is not terribly strong--not really that much differnt than the equator. That's one of the reasons why we need to look for existing sources of vorticity (spin) to get a tropical cyclone going. Becasue coriolis is so weak in the lower tropics, the atmosphere sort of needs some existing cylonic spin to get a tropical cyclone going.

Now, as everyone on this board knows, there are multiple sources of "spin" (vorticity) that we keep an eye out for. One is tropical waves, another is sort of an existing low pressure (let's say one that forms on a front then gets cut off or something). Another
can be the spin that comes along with monsoon troughs.

OK....here's the deal...the ITCZ is really a kind of monsoon trough. And, as such, it's a source of vorticity (which is needed to get ANY tropical cyclone going as I mentioned above).

OK, now the equator...As Derek and others point out, the monsoon trough (sort of the ITCZ) lies nearer the equator in other parts of the world. That means that in that (sometimes very strong) monsoon trough lying near the equator, the background "spin" is there and is sitting near the equator.

So, we get tropical cyclones near the equator because the ITCZ (monsoon trough) that starts tropical cyclones spinning lies there. When a convective cluster comes along, though there's virtually no Coriolis near the equator, the background spin can be enough to get things going. (If anyone wants to desscribe cyclostrophic balance, knock yourself out).

OK, so back to the Atlantic. Why don't tropical cyclones form over the equator? Not because Coriois is "too low" at the equator, but because the other sources of spin--the ITCZ and easterly waves--tend to be further north in the Tropical Atlantic, not on the equator.
Tropical cyclones CAN form near the equator. They typically don't becasue sources of existing spin don't lie near the equator in the Atlantic, not because there's not "enough" Coriolis.

I hope this was clear. Fire away!

WJS3