Wednesday, August 02, 2006

Waiting for Emergence???

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

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. 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. Ideally it would be advantageous to post our weather-climate discussions (link at the bottom) with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

While Western Hemisphere tropical SSTs remain generally above normal, there has been ~ .5-1.0C of cooling from the North Indian Ocean/Bay of Bengal east-southeast into the west central Pacific. The latter is a response from persistent tropical thunderstorm activity (discussed below). However, anomalies of ~plus 1.0C have returned into the Indian Ocean just north of the equator, with actual SSTs in excess of 29C.

During the past couple of weeks, linked to generally persistent surface westerly wind anomalies and even actual westerlies on the equator from ~150E-180, another downwelling oceanic Kelvin wave has developed. Based on the latest five-day averaged TAO data (link below), this feature was located at approximately 170W accompanied with SSTAs ~plus 1-2C down to at least 150m. This is the third oceanic Kelvin wave since mid-March, and appears to be the strongest (but still fairly weak) in the series. Additional global SST information can be obtained from latest TAO data here, ESRL/PSD data here, CPC data,

and BMRC at .

From continuous monitoring and various diagnostic and dynamical tools, there continues to be growing evidence to me that the weather-climate system is tilting toward a warm event. In fact, a weak warm event may have already emerged. However, the magnitude of any possible warm event and global impacts are unclear. The following are links to ENSO discussions.

Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and remember that official USA information on anything related to ENSO comes from CPC.

While there is still difficulty identifying coherent signals of tropical convective forcing, I think there are at least 3. As discussed in my last posting, there is a dynamically convectively coupled signal moving through the Eastern Hemisphere, centered ~10-15N/110E (with a convective flare-up) per coherent modes Hovmollers and other plots, etc. (links below). Enhancement of the monsoon systems has been a response. This appears to be another ~30 day mode of organized eastward propagating enhanced tropical rainfall (with at least early interactions from the Southern Hemisphere) having 3-day OLRAs ~minus 50 W/m**2. I think this is about the 8th such kind of event since December 2005, and my “back of the envelope” calculation gives this feature a phase speed of ~10 m/s, or about 8 deg long/day eastward movement. If that is the case, tropical convection may become very intense across the Tropical Northwest Pacific (TNWP) by the end of week 2 (more said below).

Downstream the SST boundary forced convection continues, centered ~ 5N/150E. This region has intensified (again) during the past week (and may represent a global warming as well as an ENSO signal given the general persistence of convection across the west central Pacific for the past several years). In fact, per OLRA Hovmollers one could ague a recent tropical convective flare-up of ~minus 30-50 W/m**2 along the equator at 160E. Meteosat satellite imagery supports this, with convective clusters both north and SOUTH of the equator. There are, in fact, twin low-level cyclonic circulation anomalies (~ 5m/s) with negative sea level pressure anomalies ~ minus 3-5mb straddling the equator in this region.

Between the 2 above regions of tropical convective forcing, tropical cyclone activity has been slowly increasing during the last few days. For the latest information please see for details. My own feeling would be to see some consolidation of the above discussed 2 regions perhaps around 10-15N/150-160E by the end of week 2 (am I too slow???). This evolution may contribute to an increased risk of tropical cyclone activity above what is now going on (as well as above climatology) and could affect locations such as the Philippines, Southeast Asia and even Japan during weeks 2-3.

The third reasonably coherent signal of tropical convective forcing is moving across the Western Hemisphere. For the last 2-3 weeks there has been enhancement of tropical rainfall centered on about 90-100W affecting locations from the Amazon Rainforest through Central America into the Desert Southwest of the USA. A convective coupled Kelvin wave has apparently evolved from this activity, and is rapidly approaching Africa. The near equatorial coherent modes Hovmollers offer some support to this notion; however, given the weak signal I am sure other (defensible???) interpretations can be offered. This feature contributed to the recent development of Tropical Storm Chris, and current satellite imagery indicates enhancement of tropical convection from central Africa into the eastern North Tropical Atlantic Ocean.

Recall the warm equatorial Indian Ocean SSTs discussed above. Given the energetic Southern Hemisphere for at least the past 6 weeks, it is plausible to offer there may be another tropical convective flare-up across the equatorial Indian Ocean by roughly the end of week 2. If that occurs, what, if any impacts, this development would have on the circulation is unclear. Since at least March the MJO signal has been pretty much absent. It is not out of the realm of options this “wild card development” could lead to a real MJO, and begin to dominate the weather-climate system in terms of forcing-response and feedback on the subseasonal time-scale (even though that would be against climatology). Taking this argument to the limit, should tropical convection across the Indian Ocean “take over” as a MJO, there could be a trade wind surge at the date line, the enhanced convection across the TNWP could shift back west as a Rossby mode, and perhaps cut off any development of a warm event (or end any on-going weak one). This chain of events is unlikely (except for a possible Indian Ocean flare-up); however, I put this out just to give readers the kind of uncertainty that exists not only making defensible statistically useful predictions, but even trying to defend something you may really believe is going on. This is why disciplined monitoring with a commitment to learn and understand is important.

Empirical, statistical and numerical prediction tools continue to be inconclusive for useful information about the future evolution of the tropical convection. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and for the details (and draw your own conclusions). These tools generally rely on a moderate to strong MJO signal, which is nearly non-existent at this time. My own thoughts have already been offered.

Time-latitude sections of 200mb zonal mean zonal wind anomalies show there has again been some increase in the westerly flow throughout the equatorial and north tropical atmospheres during ~ last 3 days. Zonal mean westerly anomalies are back to ~5m/s. Anomalous zonal mean easterlies continue around 30N and 25S, with westerlies on their poleward flanks (magnitudes generally 5-10m/s). This distribution of zonal mean anomalies has some symmetry to it, implying there is important tropical forcing (even though it has been recently very difficult to diagnose with weak signals among the components). The recent increase of zonal mean anomalous westerly flow discussed above may be attributable to the current tropical convective flare-up ~160E.

Tropospheric global relative AAM is about minus 10 Hadleys based on the 1968-1997 reanalysis climatology through July 31st. The global (remember) mountain and frictional torques are ~ plus 10 and minus 10 Hadleys, respectively. The AAM tendency was not available today; however, if it is not positive, it should be soon. One contribution to the global mountain torque becoming positive is coming from the East Asian topography. Recent baroclinic wave energy packets/fast Rossby wave energy dispersions interacting with the region of tropical convective forcing across the TNWP have been phased to favor positive mean sea level pressure anomalies along the east slopes of the East Asian north-south mountain barriers. This will help to add additional westerly flow to the atmosphere.

In past Blogs there was discussion of ~10m/s zonal mean anomalous westerly flow throughout the equatorial and subtropical atmospheres during the last part of June (see past postings for details). Above I already discussed the notion of some inter- hemispheric symmetry of 200mb zonal mean zonal wind anomalies. Now I will offer, given recent interactions with all the discussed tropical convective variability and a recent mountain-frictional torque index cycle, some of that anomalous westerly flow has propagated into the extratropics of both hemispheres, ~45-60N for ours. These complex interactions have contributed to the recent and on-going excessive heat episodes across the USA. I hope time (which has become very short) will permit us to demonstrate some of those relationships in our next weather-climate discussion which is in the works.

Twin anomalous (10-20 m/s wind speeds at 250mb) subtropical anticylones have been fairly persistent ~120E for the last 5-7 days, as shown by animations of 150mb and 250mb daily mean vector wind anomalies. Several extratropical wave packets have interacted with these gyres. During the last few days upper tropospheric divergence has increased across the Eastern Hemisphere centered ~120-140E on the equator. This is a direct response to the increased tropospheric heating due to the tropical convection in that region, forcing the twin anticyclones already discussed. While anomalous easterly flow (at especially 150mb) still continues from the Indian Ocean into central Africa, equatorial westerlies with 150mb anomalies of ~20m/s have appeared from 120-160W.

To summarize, I think we have a weather-climate situation consisting of (for our purposes): 1) generally persistent SST boundary forced enhanced tropical convection across the western Pacific (global warming/ENSO signal?), 2) emerging dynamical signal of tropical convective forcing moving through the Eastern Hemisphere (part of the ~30 day tropical convective variability), 3) the Western Hemisphere convectively coupled Kelvin wave heading for Africa, 4) a mountain-frictional torque index cycle, 5) rapid extratropical Rossby wave energy dispersions/baroclinic wave packets, and 6) weak or non-existent MJO. Even when all this is combined (linear super-position) any predictive signal is weak at best (discussed below). Is “something” robust going to emerge out of these weak signals???

GSDM Stage 1 best describes the current global circulation/weather-climate situation. The global mountain and frictional torques are not consistent with what would be expected; however, I think this goes back to our mountain-frictional torque index cycle and the role of the SST boundary forced western Pacific tropical convective forcing. My thoughts are we will go into a “positive phase” of the mountain-frictional index cycle (loosely both torques positive) about the time the dominate region of tropical convective forcing shifts into the TNWP. Should this occur, zonal mean anomalous westerly flow would continue to increase across the tropical atmosphere, then propagate poleward and downward into the extratropics. GSDM Stage 2 by later week 2 and 3 may be the most probable outcome given our current situation. However, recall the “wild card” scenario posed above, and I can not dismiss other defensible options other folks might add. Uncertainty continues to be very high

Week 1 (3-9 August 2006): GSDM Stage 1, possibly transitioning to Stage 2, seems probable. There is good model agreement on retrogression of the intense USA ridge from the southeast states back into the central part of the country by early next week. That is consistent within the GSDM framework. I think there will be at least one more mobile trough to dig along the USA west coast, then move inland through the northern states. This would suggest another round of heat to expand from the Rockies into much of the Plains and Deep South. An active severe local storm/MCS track would be possible from the Northern/Central Rockies into the Mid/Upper Mississippi Valley/Great Lakes-Ohio Valley. The window of opportunity for additional North Atlantic tropical cyclone development may continue. Please see for the latest tropical cyclone information.

Week 2 (10-16 August 2006): A transition to GSDM Stage 2 may occur, suggesting a trough may develop across the Great Lakes/ eastern USA while ridge amplification occurs from initially the Great Basin into Alaska. This synoptic pattern may retrograde going into week 3. This would suggest a trend toward cooler/wetter for the Central and North Central states into at least New England, and a return to the intense heat for the western part of the country (depending on the ridge position). Tropical cyclone development across the North Atlantic may become suppressed.

Week 3 (17-23 August 2006): Unclear. During August 2004, before the 2004-05 warm event, GSDM Stage 2 was quite robust (please see our weather-climate discussions). Are we going to see that again??? I would have concerns about the ridge remaining across the Great Basin region, etc.

A few opportunities should exist for some rainfall across Southwest Kansas going into this upcoming weekend. However, this has been a summer featuring stronger than normal subtropical ridges across particularly the western and central part of the country meaning periods of widespread beneficial rainfall are less likely. In fact, stronger than average western USA ridges have been a problem since ~summer 2000, and may be part of a longer-term signal linked to a warming western Pacific warm pool. Thus, it may take the seasonal transition to fall to increase any opportunity for repeated widespread beneficial rainfall, understanding the climatology here (falls are generally dry). Perhaps we could get a window of opportunity between summer and fall for precipitation. Temperatures are likely to remain above normal through week 3.

The time -filtered coherent modes Hovmoller plots of OLR and OLRA are at, velocity potential Hovmollers at , and an animation of velocity potential overlayed on OLRAs are at

Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa, ; other imagery here. Latest tropical cyclone statements can be found from, while the latest 3-day averages of OLR totals and anomalies and other data can be found here (animations of various fields from the operational data) (Global Tropical Hazards Assessment available from this site, along with other useful information) (reanalysis AAM plots) (operational AAM plots)

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts (link to our Weather-
Climate discussions) (model performance; please navigate to others)

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from the 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 try to do another posting this upcoming weekend. Work is also on-going to post a weather-climate discussion on the ESRL/PSD MJO web site hopefully by about the middle of August (pushed back).

Ed Berry

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