Wednesday, May 31, 2006

Ready for Stage 2?

Repeating (one more time) from the May 17th posting, our latest weather-climate discussion for the ESRL/PSD MJO international web site has been posted dated May 17, 2006. Please see

to view it.

Also, the following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

To get the most from these postings, I kindly recommend that at least some perusal of our paper be given. The gist is from taking into consideration the interactions of 4 different subseasonal time scales, a sequence depicting a coherent set of repeatable events has been derived. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM.

With the exception of a cool region just off the coast of South America, SSTs across most of the central and east equatorial Pacific remain slightly above average, with anomalies ~ plus .5C to even 1.0C around the date line. SSTs range from ~22C at the South American coast (anomalies roughly minus 3C) to around 31C just south of the equator at 160E (recall that we use SSTs of 29C as a threshold for supporting persistent tropical convection). SSTs of 29C and higher also extend from the southwest Pacific into the IO. At depth, as was true about 2-3 weeks ago, anomalies around plus 1-2C extend from 50-250m east-west along the equatorial cold tongue, meaning a slightly deeper than normal oceanic thermocline. SSTAs from the IO into the west Pacific are at least plus .5-1C, with the South Pacific horseshoe experiencing ~ plus 1-2C. The Caribbean into much of the north tropical and equatorial Atlantic Ocean basin also has SSTAs ~ plus 1-2C. Finally, plus 1-2C SSTAs are also present along the southwest coast of Africa and from the east coast of Africa into much of the South IO. Latest prediction from CPC expects ENSO-neutral conditions to prevail during the next 3-6 months (see latest TAO data here, ESRL/PSD data here ). The following are links to ENSO discussions.

The signal from the MJO remains weak. In general, the global character of the tropical convective forcing has become somewhat unclear during the last few days. Per time-filtered Hovmoller plots of OLR and OLRA (see, other variations of tropical forcing have been important during the last few weeks. For instance, while recent enhanced convection along both the East Pacific and Atlantic Ocean ITCZs appears to be linked to a convectively coupled Kw, a convectively coupled Rossby mode has lead to above average rainfall across most of the Arabian Sea (weekly averaged OLRAs ~minus 70-90 W/m**2 in the latter area). This Rossby mode not only may have contributed to the formation Typhoon Chanchu a few weeks ago, but perhaps to the early onset of the active phases for the Indian and Southeast Asian Monsoons. Monitoring tools such as Hovmollers of velocity potential (CHI) and animations of CHI and OLRA weakly support the notion of the tropical convective forcing still being in the EH (see and )

Real-time satellite imagery does suggest convection is beginning to increase across portions of western Indonesia and even the central Pacific Ocean especially just north of the Polynesian Islands (due to warm SSTs). Tropical convection remains quite intense from Southeast Asia into the Bay of Bengal and the Arabian Sea. Interestingly, much of interior India as well as the equatorial Indian Ocean remain suppressed. Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa; other imagery here; tropical cyclone statements; the latest 3-day averages of OLR total and anomalies, and other data can be found here.

Statistical and numerical models of the MJO (see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools) are generally inconclusive given the weak MJO signal. However, empirical forecasts from the time filtered coherent modes technique do suggest an eastward propagating signal from the North IO through much of Indonesia through week 2.

My feeling is that we are seeing, in some sense, the tropical convective forcing reorganizing in the presence of the seasonal cycle (including the onset of the EH monsoon systems). I think it is only a matter of short time before the very warm SSTs from Indonesia into the southwest Pacific force tropical convection which will have consequences for the global circulation. Should the equatorial regions from the Maritime Continent (MC) into the western Pacific become the dominant area of tropical convective forcing during week 2, that would represent an eastward (and southward) shift from what is now present. That is, we may see a shift from the loose area currently centered ~15N/90E to an intense concentrated region centered ~5N/130E, lending some truth to the empirical method. The west Pacific convection may combine with the Indonesian thunderstorm clusters while the Southeast Asian Monsoon continues. A MJO signal may evolve out of this possible enhanced region of ~MC tropical forcing.

The interplay between SSTs, tropical forcing and circulation response-feedbacks is ongoing. Zonal mean upper tropospheric anomalous westerly flow has propagated off the equator (mainly into the Southern Hemisphere (SH) due to the seasonal cycle) and is being replaced by easterlies as I type. Per time-latitude sections of 200mb zonal mean zonal wind anomalies, westerly anomalies ~15m/s have propagated into the SH extratropics with ~5 m/s or so moving into the NH. Roughly 5-10 m/s zonal mean easterly anomalies are now present throughout the global tropical and subtropical atmospheres. Plots of recent daily mean 150mb and 250mb vector wind anomalies still present a signal of twin subtropical anticyclones centered ~ 40 deg. east longitude, where twin cyclones were a few weeks ago. These IO twin anticyclones have been linked to rapid wave energy dispersions affecting the PNA sector for about a week. One consequence has been an anticyclonic wave breaking event of a large blocking structure near Alaska, helping to flux AAM out of the subtropics. For the USA, this complexity lead to the western states trough of the past several days.

AAM anomalies are ~ minus 1 standard deviation below the 1968-1997 climatology, with anomalous zonal mean easterly flow across the subtropics and midlatitudes of both hemispheres (particularly the SH). AAM tendency has returned to near zero based on the reanalysis data, which lags 3 days from the current time. The operational data plot shows that AAM has decreased to 2 standard deviations below the 1979-1998 climatology (see, and I would expect the current AAM tendency to once again be negative (see reanalysis AAM tendency plot ).

In addition to the tropical forcing getting better established across the EH, other components within our GSDM framework contributing to the decline in AAM include the mountain and frictional torques (see plot for mountain torque and plot for the frictional torque; see for all AAM plots). A general global mountain/friction torque index cycle has been going on since early April (loosely with mountain torque going up and frictional torque going down) while global mountain torque has been undergoing ~ 25 day variations. The latter include contributions from north-south mountain ranges such as the Andes and those present across East Asia. I would expect the East Asia mountain torque to soon become negative given the anomalously low mean sea level pressure in that region. The point is there is still westerly flow being removed from the atmosphere by the earth both in the zonal mean and global sense.

GSDM Stage 1 still best describes the global circulation at this time. The tropical convective forcing returning into the EH and consolidating ~ 0/100E (see around a week ago helped with establishing Stage 1. Now, I think the circulation working with the warm SSTs is going to re-establish more robust EH tropical convective forcing (as discussed above). Again, one must always think in terms of forcing, response and feedbacks with some understanding of the dynamics.

I also think there is a possibility for the Indonesian convection to start shifting toward the western Pacific during later week 2 into week 3, possibly as a MJO. Within the GSDM framework, a transition to Stage 2 would be probable if this occurs. I have discussed this possibility during at least my last 2 Blogs, and I am going to stay with it. Most ensemble prediction systems (EPS) do not show this since models cannot predict tropical convection very well after about 4-5 days. As I stated in my previous posting, forecast uncertainty is higher than has been true recently. Sources include tropical convective flare-ups from other ocean basins such as the North Atlantic and the roughly 25-day variations of the global mountain torque, especially from the Andes Mountains this time of year.

In the outlooks that follow, I am keying on a possible intensification of the tropical convection ~ the MC during week 2, possibly shifting toward the west Pacific warm SSTs as a MJO during week 3. I am also considering the mountain/frictional torque index cycle and ~ 25 day variations of at least the east Asian mountain torque. All of these components may be increasing by week 3 (seasonal cycle and other considerations understood).

Week 1 (1-7 June 2006): GSDM Stage 1 is most probable. I think we are seeing a summer time rendition of this Stage given shorter wavelengths. As all models generally show with low ensemble spreads, the pattern of an east Pacific trough, Rockies to Great Plains ridge and East Coast trough is likely. There will be at least a couple of short-wave troughs moving along the relatively strong westerly flow just north of the ridge. This pattern translates to above normal temperatures and below normal precipitation for about the western 2/3 rds of the country with the opposite for the East. Excessive heat may become a concern for portions of the Intermountain West and Plains (understanding dewpoints will be generally low) with heavy rainfall for portions of the East and Northeast. Finally, some severe local storm and MCS activity is likely for especially the northern Great Plains into the Great Lakes/Ohio Valley states.

Week 2 (8-15 June 2006): The beginning of this period may be similar to week 1. However, as discussed above, a transition to GSDM Stage 2 may be probable. Like was observed during especially the summer of 2004, that situation tends to favor cool/wet across the central part of the country with warmth/dryness along the west coast and southeastern USA (please see for example).

This type of pattern can be a prolific producer of heavy rainfall and severe local storms from roughly the eastern Rocky Mountain States of Colorado-Montana through the Central/Northern Plains, Mid/Upper Mississippi Valley into the Great Lakes and Ohio Valley regions.

Week 3 (16-22 June 2006): GSDM Stage 2 continuing and/or emerging would be most probable.

Specifically for Southwest Kansas, we had better hope that there is some truth to my outlook for weeks 2-3, or we may remain “hot and dry forever”. Well above average temperatures and little precipitation is a good bet through most of next week. Yes, hopefully there will be an opportunity for rainfall tonight (5/31) and other things can occur this time of year to get a few storms. If we transition to GSDM Stage 2, our rain chances will increase and temperatures will cool down. That transition may start next weekend.

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from Storm Prediction Center not only for severe storms, but also fire weather concerns. Finally, the CPC USA Hazards Assessment for offers additional insights not only for possible week 1 high impact weather, but week 2 as well.

I will attempt another posting sometime this upcoming weekend.
Ed Berry

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