Gedday,

I won't apologise for being so quiet lately... Most likely you all need a rest from my ramblings. I'm still busy as I am hosting two scientific visitors from O/S.

However, there are a couple of interesting things going on today, which I shall point out:

a)  Equatorially Trapped Rossby Wave:

If you look at the current 850 hPa TLAPS analysis (which, of course is on my web-page) you will see a nice westerly burst right on the equator between 140 and 160 E, with twin vortices on either side: one at about 8 N, 153E and the other at about 12S, 161E.

Both vortices have moved a little and were much more symmetric to one another about the equator a couple of days ago (see 900 vorticity chart for two days ago on my web page).  These show up as equatorially trapped Rossby Waves on BW's diagnostic (also on my web-page -- second panel down on the diagnostic is the n=1 Rossby)).  That diagnostic is based on filtered OLR.  Last time we had one of these, someone on this discussion asked how we recognise them in the wind fields, without looking at the filtered OLR.

Well... its easy... like today:  you see a westerly burst, right on the equator, and a cylonic vorticity maximum on either side.  Look at the classic Gill 1980 Quartely Journal paper, solution for heating symmetric on the equator (QJ, 1980, p.447-462)... Gill, Fig. 1.  This solution consists of the Kelvin wave, which goes from the heating eastwards, and the n=1 planetary or "Rossby" wave extending from the heating westwards.  In the zone of the heating, you can't see the separate waves on Gill's figure as they are interfering with one another; but most of what you see in the heating longitudes is n=1 Rossby.  The main features are the westerly equatorial maximum and the twin vortices.  Other features are  the symmetry characteristics:  symmetric about the equator wrt the u field antisymmetric in the v field.

Another nice feature about today's Rossby wave is that it shows up as a first internal mode feature: i.e you can see the mirror of it at 250 hPa with a easterly max on the equator and twin anticylones.  Interestingly the scale is larger at 250 hPa, for reasons I can go into when I have time...  but basically I believe it is because the dissipation is smaller at upper levels. (The model is not perfect as the upper level anticyclones are too far eastward.... though the accompanying mass of equatorial convection also  was about 10 degrees longitude further east a few days ago when the system developed))

b)  The monsoon low over WA.

        This is an interesting system.... Structurally, I notice that it still appears as a separate cut-off weak trough at 500 hPa.  It is interesting that the ones that never actually made it to tropical cyclone status (viz this one) do not do much when they travel around as a wet low over the continment, whereas those that were cyclones often interact with the higher latitudes and become major rain systems over Victoria and western New South Wales.

c)  The monsoon

    Historically I have never been much of a fan of the MJO. However, the recent monsoon burst was very clearly an MJO event, and now the MJO has moved eastward, our busrt has finished.  So...what happens now?  Its still only February, the two (northern and southern hemisphere) jet stream axes are still at their southward locations.  I suspect we still have another burst to come.

d)  The South east Oz cool changes.

    We still have the changes coming through.  Synoptically they look pretty similar to what we were having a month ago.  However, as the continent cools down and the tropics begin to dry, the basic configuration over the theta maps and the thetae maps will change.  Eventually we will lose the precursor  warm continent (that looks like a front on the theta charts) and the precursor tropical east-west oriented thetae front (possibly the same feature as what forecasters have traditionally called the dry-line); so when the higher latitude troughs come through they will not have these continent-scale frontal-type features to interact with.

I have been meaning to look at the sequences of theta and thetae charts from a month back and see if they look strikingly different to the current sequences... but haven't had a chance to do it yet.

Does anyone else out there monitor the change in seasons, e.g a continuing measure of strength of fronts?

Thats enough... back to the visitors

John McB