There has been an equatorially trapped n=1 Rossby wave event over the Indian Ocean during the past week, which is an absolute beauty:

On my web-page at  http://www.bom.gov.au/bmrc/clfor/cfstaff/jmb/0_04May11a.html
I have placed this email plus links to a number of low-level analyses and images:

The Rossby wave developed on about 28-30 April at  (75 E, on the equator), and has grown in size and expanded westwards.  The analyses are such that relative vorticty is shaded (cyclonic being yellow in the northern hemisphere and blue in the southern).  There have been all sorts of interesting aspects of this event:
*    the expansion westwards
*    The northern vortex having given rise to a tropical depression that sat for a while over southern India before moving northwestawrd across the Arabian Sea, while the southern vortex has given rise to an un-named tropical cyclone at about 10S, 70E
*    From about May 6, the symmetric Rossby wave pattern shifted northward slightly and the northern half increased in magnitude, giving rise to the first burst of low-latitude monsoonal westerlies across the Indian Ocean.
It is so interesting, I have put the entire sequence of charts up on my page (though this may make it slow to load).
 

Unfortunately, I didn't have my "eye on the ball" during most of the sequence and did not save the satellite imagery.  I grabbed what was there off the Eumetsat and US Navy web pages yesterday, but they only go back to 5 May (Does anyone else keep an easily accessible archive?):

Anyway, on my page I have put links to the imagery at the following times.  The visible twin vortices are spectacular, expecially in the first few pictures I have put up.

5 May 00
5 May 12
6 May 00
6 May 12
7 May 00
7 May 12
8 May 00
8 May 12
9 May 00
9 May 12
10 May 00
10 May 12
 

Interestingly the Matt Wheeler real-timediagnostics of the  waves based on wave-number frequency filtering of OLR did pick up constant Rossby wave activity throughout the sequence. I have placed a loop for the sequence on my page, the images having been grabbed in real-time from Matt's page: http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/amaps.all.50to20.gif

Despite the consistency in that there is Rossby wave activity in the OLR-diagnostics, they differ from the impression gained from inspection of the wind fields,  in a number of respects:
a) The OLR attrinbutes most of the convection in the Indian Ocean during the period as being attributable to an MJO event, rather than a low-order Rossby wave
b) The OLR diagnostics pick up a westward phase movement in the OLR, which by eye I find hard to see in the unfiltered 850 hPa analyses (though I guess there is the expansion westward of the equatorial westerlies in the early part of the sequence of charts
c) The OLR diagnostics pick up a Kelvin wave travelling rapidly through the region between about 3 May and 9 May, which again I find difficult to see in the unfiltered wind signature.... though it probably IS there... look at the little westerly burst at 150E on the last analysis in the sequence (12 UTC 10 May).  Time permitting, I'll run a hovmoller of zonal winds at the equator and see if I can pick up the Kelvin wave signal).

Anyway, the different perspective gained from the two approaches (Me looking at the charts with my eyes, versus Wheeler's objective wave-frequency OLR-based diagnostics) are interesting.  To me, the  clear equatorial zonal jet and twin vortex structure on the daily charts and the accompanying satellite images are quite spectacular.  Yet we don't see the westward phase speed a Rossby wave should have... why not?

Presumably this is part of the reason why it was picked up as MJO rather than Rossby-wave in the objective technique... Whatever the reason, it may hold part of the clue as to why the region of wave-number frequency space occupied by the MJO does not correspond to any of the waves on the dispersion diagram.  (If that sounds incoherent, it is.. a half-formed thought).

As always, I hope this generates some discussion.  Does anyone have any background on how common it is for a twin-vortex arrangement like this to play a role in the establishment of low-latitude monsoon westerlies at this time of year?

Also, a month or so back I wrote a script to save gif images of the 2-day and 4-day GASP numerical progs for the same domain (and using the same display package) as the 850 hPa charts I showed above.  When I get a chance, I'll report on how GASP did in maintaining this wave and twin-vortex structure.

cheers

John McB

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