Okay... so here is a question for Mark Williams, Matt Wheeler and other lovers of equatorially trapped, n=1, first tropospheric internal mode Rossby Waves.
Right now, we have a quite good upper level equatorial easterly flow in our longitudes, as seen in today’s 250 hPa TLAPS analysis.
However, there is no tropical convection to speak off in those longitudes or to the east (Rossby Waves having westward phase speed) — see today’s GMS full-disk and Matt Wheeler’s diagnosis of today’s OLR in terms of MJO, Rossby wave etc images.
So.. The question: Given the lack of convection, what is maintaining the upper level equatorial easterlies.
Mark Williams
I've been looking at the
charts myself and have asked myself exactly the
same question. The
upper easterlies are quite strong to the east of the
convection, north of a well
developed STR. I can't see how they can be
generated by the convection.
Mark
From:
Jeff Callaghan <j.callaghan@BoM.GOV.AU>
To: synoptic_discussion@bom.gov.au
Subject: Fwd: Re: [synoptic_discussion]
Upper level equatorial easterlies
Its like the upper anti-version
of Twin cyclones. On the TLAPS anals over
the last four days an upper
anticyclone strengthened east of the
Phillipines as well as the
upper anticyclone building over Northern
OZ with the easterlies
in between
Jeff
From: Matthew Wheeler <
John and others,
Sorry, but I can't explain
it either, although note that if you
were to look at an anomaly
plot like this one:
250 wind anomaly - today's
image (General link: http://www.cdc.noaa.gov/map/images/fnl/250wnd_01a.fnl.gif)
then you would see that
the strongest near-equatorial wind
anomalies occuring right
now aren't in our longitude domain,
but are out over the western
Indian ocean (easterly anomalies).
These easterlies (anomalies)
no doubt *are* generated by the
convection that is going
on in the central Indian Ocean. For the
OLR of the last 3 days,
and the anomaly, see current image
(general link http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR/m.3d.html)
And as an aside, note that
this organized convection in the Indian
Ocean is projecting quite
nicely onto what my filtering diagnostics
say is the active phase
of the MJO. See current situation
(General link =
http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/h.6.MJO.EQ.html
)
...and if the last active
phase is anything to go by, we should
be getting more strong surface
westerlies over the W. Pacific (important
for El Nino) in another
2-4 weeks.........
-Matt.
From:
Noel Davidson
Hello Folks,
I guess I'll poke my nose in here and say something stupid.
Being a long time believer
in the influence of mid-latitude
circulation changes on the
flow in the tropics, (and particularly
the dry dynamics) I guess
I will suggest that the enhanced
upper level tropical easterlies
are associated with the large
amplitude trough (and jet
structure), and downstream
ridge DEVELOPMENT that has
occurred over the eastern IO, WA and the NT
(and possibly in the northern
hemisphere as well).
As part of the (dry) development
of the (downstream) ridge the tropical
easterlies should accelerate
- shouldn't they?
The question then is : should
there be a response in the convection?
I would say not necessarily
(maybe the low level changes are
not optimal - whatever that
means). But if there were to be a response
perhaps it should be over
the equatorial Pacific. It has looked a
little more active over
there.
--------------------------------------------------
It must be that damn MJO
that does everything. Do forecasters
see large scale tropical
convective outbreaks that are not associated
with the MJO?
I think I'l go home before some-one else around here gets grumpy.
Mark Williams
Noel
I'm prepared to subscribe
to your theory. That is downstream development
in the mid latitudes possibly
strengthening the STR over Nth Australia
thus
leading to low latitude
easterlies.
Mark
Matt
Wheeler
> Being a long time believer
in the influence of mid-latitude
> circulation changes on
the flow in the tropics, (and particularly
> the dry dynamics) I guess
I will suggest that the enhanced
> upper level tropical easterlies
are associated with the large
> amplitude trough (and
jet structure), and downstream
> ridge DEVELOPMENT that
has occurred over the eastern IO, WA and the NT
> (and possibly in the northern
hemisphere as well).
> As part of the (dry) development
of the (downstream) ridge the tropical
> easterlies should accelerate
- shouldn't they?
Could be. The only inconsistency
I see with this is that 2-4 days ago the
easterlies were stonger
than they are now (in the 140E to 160E domain).
So they aren't accelerating.
> The question then is :
should there be a response in the convection?
> I would say not necessarily
(maybe the low level changes are
> not optimal - whatever
that means). But if there were to be a response
> perhaps it should be over
the equatorial Pacific. It has looked a
> little more active over
there.
We'll see! My bet is for
it to remain active (on the large scale) over
the equatorial Indian Ocean
(rather than the Pacific) for the next week
or two.
> It must be that damn MJO
that does everything. Do forecasters
> see large scale tropical
convective outbreaks that are not associated
> with the MJO?
Noel, Noel, Noel, of course
the MJO isn't everything, or even all that
much! It is just one thing
that we DO know about that has some
consistent behaviour on
the large scale. There are many examples of
large-scale convective outbreaks
that are not associated with the MJO.
After-all, for some years
the MJO isn't active at all. As an example,
the time-series in the attached
plot
is an indication of the strength
of the signal of the MJO
since 1979. Late 1999 to early 2000 is an
example of a period with
hardly any MJO signal. 1996-97 is an example
of a period with very strong
MJO signals.
-Matt.
John McBride
Noel’s theory: The upper level equatorial easterlies are caused by intensification of the high over Oz, as a downstream energy dispersion response to the trough amplification in the eastern Indian Ocean:
It looks good:
Look at the 500 hPa analyses for a few days ago (1200
Utc 13 Jan ) compared with that for today (1200
UTC 16 Jan). You can see the energy dispersion/downstream
development beautifully....
The problem is though, as Matt the boy-wonder, pointed out: the upper level easterlies were actually stronger a few days ago: viz the 250 hPa tropical charts for 1200 13 Jan, compared with 1200 UTC 16 Jan (both on my web site).
So... over to you Noel
As an alternative view, I tried playing around with the Harry Hendon idea (Bergman and Hendon, Journal of Atmospheric Science, 2000) of negative heating. If we interpret the Gill model as a perturbation model for the deviations from the seasonal-mean/background mean state. Then, in the current situation, we have a negative latent heat anomaly over our longitudes (or a positive OLR anomaly). Thus, the whole calculation being linear, this would amount to the negative or opposite response to a heat source, and so could give us upper level Kelvin wave easterlies in response to a subsidence region to the west).... Appealing though the idea is, I played around with it by looking at the current charts, and decided that by combining negative and positive heating areas over the chart, I could get just about any tropical response I want.
Jeff Callaghan
Re the strong easterlies
over PNG at 12Z 13th, TLAPS shows a 200 hPa low
develop over the N coast
of Irian Jaya between 12Z 11 and 12Z 12 which
developed a strong near
equatorial contour gradient between Honiara and
Irian Jaya.
This low has persisted and
seems to have peaked (heights <1242 dam) at 12Z
15th when a strong
contour gradient existed between the Coral Sea and
Irian Jaya and hence Ely
winds over PNG.
To see these features you
need to plot 1 dam contours using GOATS
TLAPS: 8th
00, 0812, 0912,
1012, 1112,
1212, 1312,
1412, 1512,
1612
What caused this low development
and hence the development of the E jet?
Jeff
John McBride
I actually don't get much
out of the Isentropic Potential Vorticity maps;
but... much as I hate to
say it, this upper low may be traceable back to a
shearing off from the circumpolar
cyclonic potential vorticity vortex.
If you go to the Comms Home
page, GASP colour mapos, 350K level PV, you
can see a Java loop of the
past 10 days Potential vorticity. This shows
a shearing off and a little
wisp of cyclonic vorticity drifting up to New
Guinea on the days leading
up to the development of the Callaghan low.
IPV 350 charts: 11UTC
11th
23
11
11 UTC 12th
23
12
11
13
23
13
11
14
It is such a small scale
feature, and we see so many similar features that
don't lead to anything,
I am not sure how useful this information is.
JMcB
P.s. I use the GASP
hemispheric IPV maps as I am trying to see the sort
of developments described
in the famous Hoskins et al paper from a decade
or more back. For
details over Australia, I actually find the TLAPS 350K
IPV charts more useful on
Gordon Jackson's (Comms home page -- TLAP
(colour) site.
However, that doesn't have an archive of IPV charts from
the past 10 days, so we
can't go back and easily look without a bit of
effort. This is NOT
a complaint, though... as Gordon as heaps of other
useful "past 7 days" charts.
Noel Davidson
Hello Folks,
I am not sure I want to pursue
these tropical easterlies
further, but a perusal of
the 200 maps from ~ 7 Jan onwrads
suggests (as noted by Jeff
C in earlier correspondence) that
the whole episode may have
started in the NHEM. A full-latitude
trough (one of those beauties
that eventually extend
across the equator) amplified
around
100E from ~ 9 Jan onwards.
The tropical easterlies seemed to accelerate
~ 12 Jan. So the same downstream
process may have occurred during this
period but in the NHEM.
Maybe then as the NHEM midlat circulations
relaxed the tropical easterlies
were further maintained by
similar events/processes
occurring in the SHEM.
Well its just as plausible
as -ve heating? What d'ya reckon?
My comment on the damn MJO
being everything, was rather tongue-in-cheek.
(It is difficult to impart
sarcasm on an email.)
My possibly-wrong perception
was that the MJO was
becoming an explanation
for too many things. I am pleased Matt
corrected that.
Wouldn't it be also good
to look at Matt's non-MJO periods to
see if some common factors
were present during those major
convective events. Not having
the MJO present
would eliminate that process
from consideration and somewhat
simplify the analysis. We
might even find out what short-term processes
modulate the MJO when it
is active.
[For information, we have
been looking at some extreme rain events over
tropical SE Asia - two seemed
to have an MJO influence
and two seemed to be related
to non-MJO influences (trade surges
preceding monsoon trough
development, and overlaying mid-latitude troughs).
Also interesting was that
the short-term forecasts from TLAPS were
all quite good. Apparently
there were these large scale controls on
the events even though the
response (presumably because of the conditional
instability) was on much
smaller scales. Is the same true for storms
in NSW and Q? Sounds like
it.
Also highlights the importance
of mesoscale analysis
and initialisation - but
GM (alias GM) should have this under control soon]
I've said too much. See-ya.
ND
