Hello,

Its time we got this discussion up and running again; so as a contribution I shall just point out a few features of the current state of the large-scale tropics.  As always, I'll put up some charts and satellite images, they being the operational GASP analysis at 850 hPa  for the eastern hemisphere and two satellite images covering the same region: (see below)

In terms of monsoonal structure, this is very much the transition season with the Southern hemisphere low level westerlies having pretty much disappeared; but the northern hemisphere westerlies not having developed yet.  Looking at the satellite images, the ITCZ is still sitting in the Southern Hemisphere across the Western Pacific; but it dips up north of the equator over the longitude span 60E to 120E (say from Central Arabian Sea to just west of Borneo).  All the way across the domain the latitude of the ITCZ is small, it being always between 10N and 10S).

Now for a couple of interesting phenomena that currently exist over the domain:

a) There is a major outbreak of convection at about 10S over the dateline.  This has been a persistent feature over about the past month as can be seen in the 30 day anomaly charts of OLR at  http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR/m.lm.html       (copy current at time of this message ---here)

b)  The line of organised convection on the Indian Ocean stretching across the equator with a south-west to northeast orientation is interesting. The two ends of those organisation are at approximately (-10, 60) and (+10, 80).  This has some aspects of the structure of a mixed Rossby-gravity wave; but such waves are not usually seen at these longitudes. Rather they are further east in the West Pacific as per the climatology of Wheeler and Kladis 1999 Jnl of the Atmospheric Sciences.  See their Fig. 7i.
Interestingly this area of organised convection in the central Indian Ocean has been diagnosed as belonging to the MJO by Matt Wheeler's real-time Monitoring and Prediction of Modes of Coherent Tropical Variability web-page.  According to Matt's diagnostic, there is an MJO convective region at about 60-80 E over his Southern tropical band: 2.5 to 17.5 S. See his diagnostic at http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/h.6.ALL.S.html   (copy current at time of this message ---here)

c)  As mentioned above, it is is the southern summer to northern summer transition season. There are a number of meteorological events occur at this time of year. Some of the southern equatorial regions facing  the south east trades have copious rainfall at this time of year (e.g the southeast facing coastal regions of Timor and Bali).  One of the most well known events is the "long rains" of eastern equatorial Africa, which occur at this time of year during the switch of the ITCZ from the southern to northern hemisphere (e.g see Okoola : International Journal of Climatology, 1999  Vol. 19, no. 2, pp. 143-168.,  or  Ntale, Gan and Mwale, J. Climate, 2003, pages 2105–2112.)  According to these papers, we should be in the middle of the long-rains season along the east African coast right now.  The satellite image looks dry, however, as does last 30-days OLR average.  If we have anyone from east Africa on this list, perhaps they could comment??

d)  Last comment:  looking over today's satellite images and charts to assess the current situation, I was looking at the vertical sounding data at the Australian tropical stations.  The vertical structure of the tropical atmopshere in this "transition season" is interesting in that it seems to be very much in "layers", rather than it having a simple barotropic or first internal mode structure as implied in much of the theroretical discussion.  E.g looking at the soundings for this morning for Weipa and for Darwin, both along Australia's north coast, you get an impression of a deep layer of easterly trades between the surface and 700 hPa with a very sharp and sudden lid at about 700 hPa.  Above that is a layer of westelies from about 600hPa to 450 hPa, then a layer of easterlies at Darwin (southerlies at Weipa) between 450 and 250, and another layer of flow above that from 250 to the tropoause at about 150 hPa.  Thus we have four distinct layers stacked above one another with relatively sharp transistions of shear regions across the interfaces.  Presumably all the operational people on this list see this all the time and think nothing of it  My question is :does anyone know of any theoretical work addressing or incorporating this layered structure of the tropical atmosphere (indeed the same is true for the mid-latitude and subtropical atmosphere... so the question applies there as well).
 

 As always, I talked for too long; but hopefully someone will take up on one of these four points and we can have some discussion.

cheers

John McBride

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