Mark Lander (to Tropical Storms List)
Date: Sun, 6 Oct 2002 17:21:39 +1000 (GMT+1000)
From: Mark A. Lander
Subject: [Tropical-storms] Big Typhoon Brewing in WPAC and perhaps a twin

TC Group,

    A huge monsoon depression has formed in eastern Micronesia.  It
appears to also have a southern twin with it.  Maybe we are back to the
season of N=1 Rossby mode again.  What are the size limitations
on the N=1 Equatorial Rossby?  John McBride noted that some of these waves
that occurred last spring were quite small by his reckoning.  This
time around the pattern is enormous!  Numerical guidance is go! (NOGAPS
generates a large typhoon near Guam in a few days, the AVN typhoon goes a
bit further to the north).
     I was on the "Island hopper" flight from Majuro to Guam yesterday, and
there is no doubt that the Westpac is ready to go crazy!  The SLP at
Majuro fell to 1002 mb a couple of days ago, and yesterday, each island
along the way was blowing strong from the west.  At Kosrae (6N 162E) it
was blowing a near gale from the west, and our plane, landing on runway
23, got a good sideways shove from the cross-wind component as we landed
(the biggest I've ever experienced on a landing!).
 

     Regards,  Mark L.

Matt Wheeler

Mark,

Regarding the size limitation on n=1 Equatorial Rossby waves, theoretically [by shallow water theory dating back to Matsuno (1966)], their size in the meridional direction is limited by the Equatorial Rossby Radius given
by       Re = sqrt(sqrt(gh)/Beta)
where g is gravity, Beta is df/dy at the equator, and h is the equivalent depth. For the n=1 ER wave, the divergence/convergence centres should be centered at a position a little poleward of 1 Rossby radius, and
for reasonable values of h for the atmosphere, this should be from about 7 to 15 degrees of laitude.
In the zonal direction there is no real limitation on their size. That is, they can occur for any zonal wavenumbers, or group of wavenumbers.
This theory, of course, is for "dry" waves, and how they should be limited in size when interacting with moisture and convection is anyone's guess!

I'll attach a couple of figures of the current situation to go along with your e-mail.

-Matt Wheeler.

 Roger Edson

Ok, Matthew,

Can you please explain if the 'blue'/and or 'cold' area is supposed to be
'up' and yellow 'down', how does this match the attached imagery?

Please note that there are two (2) westerly wind bursts currently going on
(with accompanying trough/circulations in the NH and SH centered at around
85E and 165E (shouldn't the 'down' portion be centered right over 120E--over
the PI?.

Are you missing one event? Maybe N=1 is not appropriate?

Roger

 Matt Wheeler

> Can you please explain if the 'blue'/and or 'cold' area is supposed to be
> 'up' and yellow 'down', how does this match the attached imagery?

Blue indicates negative OLR anomalies. This is the 'cold', or 'up'.

> Please note that there are two (2) westerly wind bursts currently going on
> (with accompanying trough/circulations in the NH and SH centered at around
> 85E and 165E (shouldn't the 'down' portion be centered right over 120E--over
> the PI?.

Yes, looking at your figure, it would appear that the orange "down" portion in my n=1 ER de-construction is too far to the west for the 6th of October. Part of the problem is that my 2-d filtering approach is attributing the 85E convection to the MJO. As more OLR data comes in (the current last day of data is the 5th of Oct (UTC)), we will be able to see whether the filtering changes its mind about what is MJO and what is n=1 ER.
Given the current El Nino state, it would also appear unlikely that the n=1 ER convection will propagate into the Maritime Continent region, like it is forecasting. Note that I am not using any El Nino information in the
filtering.

Just treat this as a guide only - certainly not a be-all and end-all.
-Matt.

John McBride

    Equatorially-trapped n = 1 Rossby Wave??? Did someone say n = 1 Rossby Wave??/ Quick, run into John McBride's office... turn on the light switch... shake him a  bit to wake hime up... yell "Rossby Wave" in his ear...

Yeh... This is a nice one.... It is so good I have put the GMS full-disc image for 0000UTC 7 October on my web-page (listed with Mark, Matt and Roger's discussion on the 06 October entry).  You can see the by-now familiar cloud signature very clearly with well-formed vortex orientation of the deep convection in each hemisphere and the east-west oriented squall line along the equator and displaced westward from the two vortices.

I have also put up a few charts:
the 850 hPa flow,                                             the 900 hPa vorticity

and the 200 hPa vorticity.

You can see the clear equatorial westerly jet at 850 hPa extending from about 150 E to the date line, the clear twin vortices at 160E in the 900 hPa vorticity.  Interestingly there is no clear feature of the opposite sign at 200 hPa.  However, this clear n=1 Rossby structure does show up in the first tropospheric internal mode, as mapped via the 850-200 hPa vertical shear map, also shown on my web-page.

Looking at the Boy wonder's hovmoller diagram of OLR anomalies as per his recent email (and reproduced above on my web-page), you can see the strong El-Nino related cold anomaly has now sat over the dateline virtually as a steady-state feature for the last two months.  By Gill theory one would expect this to generate a steady-state equatorial Rossby wave to the west.  I have placed the NOAA CDC surface wind anomlay map averaged for the last 30 days on my web-page; and in fact you can see that.  A strong westerly wind anomaly on the equator with twin vortices on either side.  There should also be a Kelvin wave surface easterly anomaly to the east of the convection.  It IS there on the anomaly plot; but its nowhere mnear as strong as the westerly anomaly.

While we are on the topic of ER waves, there is also one currently in the Indian Ocean.  I have put last night's Eumetsat full disc image on the web-page. It is not as beautiful as the one in the Pacific but the twin vortices and the westward displaced equatorial squall line are clear.

 

I have also put up the 850 and 200 hpa wind-plots for the Indian Ocean; and once again you can see the clear equatorial westerly jet and twin vortices at about 70E in the low-level flow.

 

Going by the very large number of these n=1 ER's that we had last
April-May and the fact that we have one in each ocean currently, it seems
they are common at the two transistion seasons  as the sun
crosses the equator.

cheers

John McB