For those who follow the equatorial waves, there is something bearing a similarity to the Dickinson and Molinari Tropical cyclogenesis from mixed R-G waves process, occurring currently in the southern Indian Ocean at about (15S, 80E). (See   Dickinson, M. J., and J. Molinari, 2002: Mixed Rossby-gravity waves and Western Pacific Tropical Cyclogenesis Part I: Synoptic Evolution. J. Atmos. Sci. 59, 2183-2196.... see also synoptic discussion for 11 October 2002 on my web-page).

On the 850 hPa GASP chart for a couple of nights ago ((1200 UTC 19 January -- chart is on my webpage under today's date),  there was a clear mixed R-G wave structure sitting on the equator between about 75E and 100E.  Similar though this is to a mixed R-G, it had not propagated in from the east (see the 700 hPa and 850 hPa hovmollers of meridional wind also on my web-page); and when it first appeared about a day earlier, it was already linked to cyclonic vortex immediately to the South.

850 hPa chart 1200 UTC 19 Jan

Hovmollers of meridional wind at equator:
                    700 hPa                                                            850 hPa

Nevertheless, the structure looks very similar to the Northern hemisphere case studies described by Mike and John; and I notice on the Eumetsat images,  a circulation is showing up in that location... see the latest image, again on my web-page:

The process has occurred over the past one to three days; so is fast:  To see a process like this, Mike and John were running a 6-10 day bandpass filter..... So, the time scale is different; the history is different (no westward propagation); but synoptically it looks incredibly similar to their process of an equatorial-wave changing structure.  It will be interesting to see what happens.

Copyright Eumetsat
 
 
 

While I have you here, the monsoon depression we were talking about over Northern Australia several weeks ago (see my discussion for 7 January) has sat over the "top end" for two weeks now and has been a major rain-bearing system.  It has finally begun to drift westward and is producing considerable MCC (mesoscale convective complex) type activity over the ocean west of Western Australia.

If time permits, I'll post a sequence of TLAPS analyses for the history of this system tomorrow.  In the meantime, however, I have put a few "current enhanced GMS image closeups on my web-page) to show the activity occurring now.

John Molinari

Hi John,

         That certainly looks like an MRG wave at 850 mb on the 19th.  Did it propagate westward between then and now?  I don't have easy access to  maps from your part of the world!
         If the process were very similar to that of Dickinson and Molinari, there should be an active MJO in the region, and the clockwise  gyre on the equator at about 92E should grow and turn poleward, and a tropical cyclone should form within that.  Not having the maps, I am not  sure how the cyclone at about -15,75 (on the 19th) evolved previously, but I would guess that it is not related to the MRG waves, at least not directly.
         One question we did not address in our study is whether a Southern Hemisphere counterpart to our northwest Pacific example often occurs, either in the South Pacific or Indian Oceans.  On the basis of the Wheeler and Kiladis (1999) charts (their Figure 7i), the Indian Ocean, with its much weaker MRG activity, seems less promising than the southwest acific.          With Mike's graduation, we have not done a climatology of such events, but it certainly needs to be done!

         Cheers,
         John

Michael J. Dickinson

John and John

The Hovmoller diagrams of the total (unfiltered) wind on John's web site did not show any obvious westward propagation from upstream. It is possible that the  distruabnce was of small amplitude and time filtering needs to be done to better see the propagation. The next question is what mechanism is responsible for its intensification. In checking out Matt Wheeler's OLR modes web site, there does not appear to be an active MJO. In fact if I interpret the shading correctly, this is occuring during a supressed phase
(http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/JA.Ma.30to0.htm

l). It looks like the MJO was active in the Indian Ocean from late December to  early January. We would need to do the appropriate time filtering.

Liebmann and Hendon (1990) found phase (westward ~5-10 m/s) and group (eastward ~2 m/s) speeds in the Indian Ocean similar to what we found in the 1987 case. Perhaps filtering in the 6-10 day range would bring something out. That said
there is no a lot of power in either the 3-6 or 6-10day bands in the Indian Ocean.

>         One question we did not address in our study is whether a Southern
>Hemisphere counterpart to our northwest Pacific example often occurs,
>either in the South Pacific or Indian Oceans.

Great question. Since we (John and I) were looking during NH summer the  off-equatorial movement into the Northern Hemisphere followed the monsoon trough. This "wave track" (if you will) shows up rather clearly in long term  climatology studies of Takayabu and Nitta (1993) and Nitta and Takayabu (1985,1985a) (among others).

No one has really looked at the SH summer. The 2-6 day and 6-10 day meridonal  wind variance composites I did for my dissertation focused on the Australian region and did not extend eastward in the Pacific. I have the data, it might be
worth another look. The South Pacific convergence zone is not oriented properly to facilitate off-equatorial movement.

>With Mike's graduation, we have not done a climatology of
>such events, but it certainly needs to be done!
>

I haven't left yet ;-)  Perhaps a grant proposal is in order.

___________________________________________________________________
Dr. Michael Dickinson: Postdoctoral Research Assistant
                       Department of Earth and Atmospheric Sciences
                       University at Albany, SUNY
                       Albany, NY 12222