Gedday,

I have been busy the last few days, hence have been quiet.  I wasn’t TOO busy though as I took the day off and went to the tennis on Friday.  Anyway, there are a few interesting things happening at the moment. I’ll start with the tropics, ITCZ, monsoon trough, and go on to the others in subsequent emails.

A) Current Monsoon Trough

The Australian monsoon has been very quiet/almost dormant over the past couple of weeks, as can be seen via Matt’s time series of convection over north Australia as a deviation from the mean seasonal cycle  or through looking at the maps of 850-500 deep layer mean for today  or two week’s ago.  Looking at the deep-layer-mean charts, you will see the deep flow over Darwin and westward is actually easterly.  Whereas, going by our AMEX experience and from Wasyl Drosdowsky’s monsoon interannual variability paper (Jnl of Climate, January 1996 – se his Fig.7), you need deep westerlies up to 500 hPa and higher for a decent monsoon burst.

Despite that, there is still an interesting feature of the broad-scale flow to the extent that you can see a clear monsoon trough in the vorticity pattern stretching across the chart: See web page: 900 hPa vorticity today, and four days ago.  The monsoon trough is the blue cyclonic vorticity line stretching across about 10S and dipping down over the continent in the Oz longitudes.  Over the western half of the domain, it also shows up a classical monsoon shear-line in the 200-850 vertical shear pattern for today.

The monsoon trough in the shear pattern is largely accounted for by the upper-level equatorial easterlies that we discussed at length last week. The structure at the surface is interesting though.  The very clear monsoon trough-line of cyclonic vorticity at 9oohPa is not associated with the classical -partial dU/dy between trade easterlies and monsoon westerlies.  Rather it is a partial dU/dy associated with a very abrupt edge to the trade wind flow at about 10S.
850 wind charts:  today,  four days ago

I have never heard of this before, but it has been a clear feature of our monsoon or equatorial trough over the past week.

B) Pick-Up of low-latitude convective activity

The other interesting thing is that the convection over our longitudes/over Indonesia has suddenly picked up over the past two days, so that if you squint your eyes, you can see a clear southern Hemisphere ITCZ stretching across the page (sat-pic on web page).

There has also been an interesting development in the last two days whereby a mass of tropical deep convection has blown up between 160 and 180 E.  There are a couple of interesting points on this.
*    It is tempting to hypothesise this as being a response to the SST anomaly lying in those longitudes as a result of the Kelvin wave generated by the westerly wind bursts of several weeks ago, as discussed by Jeff Callaghan (see discussion of 15 January on my web-page).

*    This enormous convective mass does not show up yet on Boy-Wonder’s diagnostics of current convection in terms of MJO’s Kelvin Waves and n=1 Rossby Waves.  This is because, so far it is only a two-day event and so will not appear in his filtered time series. (Sequence of sat pics:)

                       17 th                                                               18th                                                        20th
 

                       21 st                                                                22nd                                                            23 rd

John McB

Andrew Tupper

>See web page: 900 hPa
>vorticity today, and four days ago.  The monsoon trough is the blue
>cyclonic vorticity line stretching across about 10S and dipping down over
>the continent in the Oz longitudes.  Over the western half of the domain,
>it also shows up a classical monsoon shear-line in the 200-850 vertical
>shear pattern for today (Web page).

If you look at the 900 vorticity anal, you'll see a trough lies across the
Arafura Sea north of the continent.  Darwin RSMC / NMOC began analysing
this as the 'Tropical Trough' (avoids arguments about what constitutes a
monsoon trough) yesterday.  There hasn't been much connection between this
trough and the continental trough(s). (Normally you'd see this on the
manual Gradient Wind Anal as well as the NMOC chart at
http://www.bom.gov.au/weather/national/charts/, but our scanner's on the
blink).  Anyway, it's nice to see that something is happening at last.

Actually, TLAPS has been analysing and forecasting a 900 cyclonic vorticity
line through the Arafura Sea for the past week...  I'm not sure if it was
prescient or optimistic.

>The monsoon trough in the shear pattern is largely accounted for by the
>upper-level equatorial easterlies that we discussed at length last week.
>The structure at the surface is interesting though.  The very clear
>monsoon trough-line of cyclonic vorticity at 9oohPa is not associated with
>the classical -partial dU/dy between trade easterlies and monsoon
>westerlies.  Rather it is a partial dU/dy associated with a very abrupt
>edge to the trade wind flow at about 10S.

John, can you expand on this a bit more?  By my (probably wrong) reading of
this, you're saying that light and variable winds north of a weak
near-equatorial/monsoon/tropical trough are unusual?

>**      This enormous convective mass does not show up yet on Boy-Wonder's
>diagnostics of current convection in terms of MJO's Kelvin Waves and n=1
>Rossby Waves (see figure on web-page).  This is because, so far it is only
>a two-day event and so will not appear in his filtered time series.

There has persistent upper level troughing over the eastern coast of
Australia since about the 15th, and a quite diffluent pattern in the region
where the convection has blown up.  Is it fair to say that that's partly
responsible for the blow up, or can you argue that a wave creates an
appropriate upper pattern, or that the upper pattern is generated by a low
level wave that caused convection, or.....  how do you argue cause and
effect?  Is there any way that we can tell that something is caused by a
Kelvin wave without seeing it in BW's filtered product?

cheers

Andrew Tupper
NTRO

John McBride

> John, can you expand on this a bit more?  By my (probably wrong) reading of
> this, you're saying that light and variable winds north of a weak
> near-equatorial/monsoon/tropical trough are unusual?
>
 

I didn't mean that.... What I am speculating as being unusual is a clear
line of cyclonic vorticity maximum, but being entirely associated with a
sharp edge to the trade easterlies.

I'm  not sure how unusual it is, but its not in the commonly written about
models of monsoon trough structure (e.g. by people like Sadler, Gray and
me).   Possibly the problem is none of the people who write about these
things spend anytime in a forecast office... but now with the wonders of
the web, we can follow the day by day charts anyway.
 

I agree the light and variable winds equatorward of a weak tropical trough
is the general state for non convective conditions.  I would not have
expected that configuration to have been associated with a long line of
cyclonic vorticity in the trough.
 

> There has persistent upper level troughing over the eastern coast of
> Australia since about the 15th, and a quite diffluent pattern in the region
> where the convection has blown up.  Is it fair to say that that's partly
> responsible for the blow up, or can you argue that a wave creates an
> appropriate upper pattern, or that the upper pattern is generated by a low
> level wave that caused convection, or.....  how do you argue cause and
> effect?  Is there any way that we can tell that something is caused by a
> Kelvin wave without seeing it in BW's filtered product?
>
 

Impressive as this mass of tropical convection is, it does not have any of
the Gill-Matsuno-Webster structure about it: I.e westerlies to the west,
twin vortices across the equator (both associated with the n = 1 Rossby)
and easterlies to the east (associated with the Kelvin wave)......
So... its interesting: a mass of tropical, near-equatorial convection, now
on its fourth day; but no Gill-pattern in the winds, and consistent with
that, no reflection in BW's filtered diagnostics..

So.... the answer is yes: you can tell if you look for the characteristic
structure in the wind fields.
 

Michael Foley
Hi there,

If you're interested in comparing vorticity evolution for various global
models (and the constituent winds), take a look at the display I've
developed on
http://serva.nt.bom.gov.au/rgn/docs/ntchartdiscussion/ at the
'MSLP/Vorticity' link under the 'long term' menu.  (Any suggested
improvements to these displays are welcome!)

Regards,

Michael Foley
a/PO2 Meteorologist
Darwin RFC/RSMC