Gedday,

 After approximately a year's intense activity monitoring the atmosphere in real-time and maintaining my web-page, I am on the point of calling it a day, and going back to what I used to do: study climate change, write research papers, etc.  This is not my farewell blurb.... I'll write something in a day or so to try and maintain some sort of synoptic discussion that doesn't require as much effort on my part..

Anyway, despite that, occasionally the atmosphere does something so impressive, one just has to drop everything and send out an email.  Last night (overnight Australian time), that was what happened across virtually the entire western hemisphere.

Remember the Guinn and Schubert ITCZ rolling-up-into balls (McBride terminology) or vortex sheet roll-up (mathematician terminology) that we have discussed through last year  (see for example discussion of October 24)?

Well have a look at the satellite images for the Indian Ocean and the West Pacific for 1200 UTC on 5 February (last night).  You'll see the ITCZ has suddenly snapped and produced vortices all along its length.  There is a huge elongated vortex in the western end of the Indian ocean, one at around 90E, another northwest of Australia (about 110 E). The continent looks cloud-free, but a couple of vortices are there over  land in the wind-shear north of the easterly trades, and than a major vortex/forming-cyclone  across on the dateline.

On my web page I have put the GASP western hemisphere tropical analyses for two days ago (12 UTC 4 February and for last night 12 UTC 5 February).  The ITCZ across the Indian Ocean has been an impressive elongated line of vorticity (blue shading for negative relative vorticity, which of course is cyclonic in the southern hemisphere) for over a week now.   Watching it, one could get a feeling of it being stretched and loosened and I was wondering when it was going to suddenly roll-up like this.

One interesting difference, from the  Guinn and Schubert model and the north eastern equatorial Pacific events, is that in those classical configurations the pre-roll-up ITCZ is an elongated singular line of cyclonic vorticity.  The current Southern Hemisphere Indian Ocean ITCZ has the elongated line of cyclonic (blue) vorticity but it also has an elongated line of anticylonic vorticity which is weaker but of comparable magnitude immediately to the north (equatorward side).  How, I wonder does this affect the vortex sheet dynamics?

Even more interesting, what maintains the anticylonic vorticity?  the easy answer is advection of cyclonic vorticity from the other side of the equator.  Vorticity doesn't normally have a two-phase structure: cyclonic vorticity seems to be able to lie around undisturbed for some time; but anticylonic vorticity of any appreciable magnitude doesn't hang around for very long.  This can be partly understood from a potential vorticity viewpoint, to the extent that if the potential vorticity is anticylonic the state is inertially unstable, and so is removed by some form of horizontal eddy overturning.

cheers

John McB
 

copyright 2003 EUMETSAT

western hemisphere analysis two nights ago

 

Western Hemisphere analysis last night
Wesrt

Jim Kossin

John McBride wrote:
>
> One interesting difference, from the  Guinn and Schubert model and the
> north eastern equatorial Pacific events, is that in those classical
> configurations the pre-roll-up ITCZ is an elongated  singular line of
> cyclonic vorticity.  The current Southern Hemisphere Indian Ocean ITCZ has
> the elongated line of cyclonic (blue)  vorticity but it also has an
> elongated line of anticylonic vorticity which is weaker but of comparable
> magnitude  immediately to the north (equatorward side).  How, I
> wonder does this affect the vortex sheet dynamics?
>

John-- in pure 2D vortex dynamics, I don't think the presence of negative vorticity is a big player. If you define the meridional
vorticity structure and then simply translate everywhere by subtracting some fixed vorticity, the dynamics should be the same. This can also be argued through linear stability analyses. The non-interacting Rossby waves are dependent on the local angular velocities, but the interacting Rossby waves (the ones that can phaselock and cause roll-up) depend only on the vorticity gradients on which they propagate.

Hope to see you at Long Beach.
--Jim
--
James P. Kossin
AOSS/SSEC/CIMSS
1225 West Dayton Street
University of Wisconsin - Madison

Dave Nolan
Jim -

I find myself in the unusual position of disagreeing with you! If the vorticity profile were equivalent to adding a
constant amount of negative vorticity everywhere, then this would be true (approximately). But what was described was a
line of negative vorticity near a line of positive vorticity. This suggests siginficant changes in the vorticity gradient
profiles, with the likelihood of new waves on the gradients of the negative vorticity line, and also interactions between
the waves on either side of the two lines.

I also say approximately because we shouldn't forget about the planetary vorticity gradient - which is largest near the equator
as beta goes like cos(phi).

Dave Nolan

Jim Kossin
Dave-- I agree. My description was hypothetical. I would guess however that the fact that the anticyclonic strip appears to be meridionally broader (and perhaps slightly weaker) than the cyclonic strip would suggest that the cyclonic strip would undergo Kelvin-Helmholtz instability first. In that case, the other waves on the additional ivorticity gradients would not play an active role in the dynamics.
--Jim
 

John McBride:  A week later (Feb 13)

I know I promised I was going to stop doing this; but the downstream (in time) consequence of the vortex-sheet rollup we were discussing last week for the Southern Indian Ocean is quite beautiful.  On my web-page at the bottom of last week's discussion (February 6 entry) I have put an 850 hPa vector wind and relative vorticity chart, and the EUMETSAT IR full-disc image for last night: 1200 UTC 12 February.  There are now four fully developed tropical cyclones across the Southern Indian Ocean.  From left to right they are:  Gerry (approximately 56E), Hope (approx 70E),  Noname/18S (approx 83E), Fiona (91E, 21S).  There are also well-developed vortices west of Madagascar and north of Australia, either of which could kick off at any time.

cheers
John McB
COPYRIGHT 2003 EUMETSAT