Hello all..... another short
summary. As I said last time, it would be good if others would chip
in, comment, disagree, add, etc.
As always, there is so much
going on, I could talk forever; so I have just chosen a few aspects to
discuss: The charts/figures I refer to are on my web-page.
a) Active-break cycle
The monsoon active period looks like it may have run its course: Looking at the two time latitude plots from the NOAA CDC page, the upper plot is for 60-90 E, and you can see the cold OLR region retreated about 3 days ago to south of the equator . The lower section is for the expanded region 60-120E. That region still is experiencing an active monsoon; but the activity is restricted to equatorward of 15N.
Looking at a plan view of
OLR anomalies for the past seven days, there is still the cold OLR
anomaly on and south of the equator in the western Indian Ocean.
The area over India and Indochina is pretty clear of convection; and from
the long east-west positive (orange) anomaly on the OLR plan-view, you
would have to say we are now in a "break period" .
b) Monsoon trough, absolute vorticity analysis
Referring back to the OLR
anomaly plan view, the major convective activity over the past seven days
of the activity is in the South China Sea and West Pacific between the
equator and about 15 N. That zone has been the location of two tropical
cyclones, Koni now located over land at about 20N 105E and Imbudo now over
the South China Sea at about 19N, 116E. These two cyclones are within
the western Pacific/Indian Ocean monsoon trough; and as usually happens
as they move westwards, they take the eastern edge of the trough with them
so that you have monsoon westerlies up to the longitude of the cyclones
and easterlies eastward from that point. This can be seen on an 850
hPa GASP analysis for last night that I have put on my web-page
On this analysis the wind vectors have their strength designated by the length of the arrow. You can see the monsoon westerlies between the equator and 15N all the way across the Indian Ocean to about 118E, the longitude of the eastern-most tropical cyclone.
The shading or colouring on this analysis is absolute vorticity (zeta + f). I asked one of our operational people (William Wang from our NMOC) to prepare these daily for a while so I can see what is going on in a absolute vorticity framework. The contours at the lower range are set to f values. Thus 25.3 corresponds to f at 10N, 49.9 to f at 20N, 72.9 to f at 30N, 93.7 to f at 40 N, 112 to 50N) The two cyclones stand out as dark red blobs at 105E and 1119 E. Interestingly there is a third blob over the northern tip of the Bay of Bengal, presumably corresponding to a monsoon depression. This line of high absolute vorticity joining the three blobs denotes the monsoon trough; and there is another line of absolute vorticity (with f or potential-latitude values between 20N and 30N) further south along the core of the westerly current.
Also on this chart you can see there is well developed low-level cross-equatorial flow across the monsoon longitudes. Interestingly the absolute vorticity values go along with the Tomas-Webster hypothesis of negative/anticyclonic absolute vorticity. The region of enhanced OLR anomaly on the map for the past seven days corresponds to this negative absolute vorticity (thus intertially unstable) region; so there may be something in the Tomas-Webster story. It is interesting as it is a sort of "anti-CISK" or negative-CISK hypothesis, the convective activity in the original Chareny-Eliassen-Ooyama CISK theory being proportional to the magnitude of the low-level cyclonic vorticity.
c) The MJO
Moving on, the state of the MJO is fairly quiet. From the MJO diagnostic of OLR on the Matt Wheeler web-page ( http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR_modes/h.6.ALL.EQ.html copy of the current chart here) one can see the convective region in the north-west Pacific is diagnosed as an MJO; but eastward movement is fairly weak. Matt's page also has equatorial zonal wind anomalies (http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/NCEP+GASP/hov.last6m.an.EQ.u850.gif copy of current conditions here) . Referring to that chart the easterly anomalies across the Indian Ocean have died out and in recent days an equatorial westerly anomaly has developed in the Indian Ocean between 40 and 80 E. There is still an easterly anomaly at 90E. This has been there for some time and so has caused upwelling along the south and west coasts of Sumatra and Java thus supressing convection in those regions.
d) ENSO
Moving up in scale again, the El Nino-Southern Oscillation story is fairly interesting. With the cold anomaly that existed in the eastern Indian Ocean in May-June there was speculation of a La Nina developing. Now, however, there is quite a strong subsurface positive anomaly across the eastern Pacific as seen on the cross-section on my webpage which was taken from the TAO/TRITON display site (http://www.pmel.noaa.gov/tao/jsdisplay/) .
TAO/TRITON 5-day temperature
anomalies, 2S to 2N average
The SOI (or the atmospheric
surface pressure based index) has remained negative throughout the the
year so far; so if one looks at this index it looks like last year's El
Nino event has lingered on. The positive anomalies at Darwin
and negative at Tahiti have both disappered over the past few weeks;
so the SOI must be drifting back towards neutral conditions. Countered
against this though is the fact we now have positive equatorial subsurface
anomalies all the way across the Pacific.