Wednesday, August 09, 2006

You Tell Me!!!

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.

Western Hemisphere tropical SSTs remain generally above normal, with weekly mean anomalies ~ .5-1.0C. As a response to persistent tropical thunderstorm activity, SSTs have cooled ~120-140E along and north of the equator to the Southeast coast of Asia, with roughly minus 0.5C anomalies. There are still regions of warmth from the central Indian Ocean to western Indonesia as well as west of the date line. Actual tropical SSTs of 29C and warmer still cover most of the Eastern Hemisphere and also from the East Pacific into the Caribbean.

The downwelling oceanic Kelvin wave discussed in my last posting was located at about 170W based on the latest five-day averaged TAO data (link below). This feature was accompanied with SSTAs ~plus 1-2C down to at least 150m, and is the third oceanic Kelvin wave since mid-March (also fairly weak). A trade wind surge has developed across the central and eastern equatorial Pacific over the last 3-5 days, with easterly anomalies ~3 m/s just east of the date line (discussed below). These easterlies have led to some reduction of the anomalous warmth across the central Pacific. The westerly anomalies that had been present along the equator at the date line have propagated northwest (associated with tropical cyclones).

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 evidence that the weather-climate system is tilting toward a warm event. If a weak warm event has already emerged, the current trade wind surge discussed above will tend to dampen the positive SSTAs associated with it. 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.

Understanding the tropical convective forcing continues to be problematic. The 30-day mode of variability did come back around into the Eastern Hemisphere about 2-3 weeks ago and significantly enhanced the SST boundary forcing activity across the TNWP (a consolidation). Latest 3-day averages of OLR anomalies are ~ minus 70-90 W/m**2 centered ~10N/140E, with magnitudes of minus 30 W/m**2 and lower extending from north of New Zealand to the coast of South East Asia and into Japan. There are at least 3 tropical cyclones occurring across the TNWP at this time, including Super Typhoon Saomai (see for details). This consolidation of tropical forcing has led to the trade wind surge mentioned above. Westerly wind anomalies of roughly 2-4m/s are still occurring along the equator from ~140-170E.

There is also the weak convectively coupled Kelvin wave propagating through central Africa into the central Indian Ocean. A local baroclinic response appears to be occurring with the feature, with, for instance, easterly wind anomalies of ~1-3m/s from 60-90E. Coherent modes Hovmollers suggest the TNWP convection to shift west as a Rossby mode (and this is already occurring) and consolidate with the convectively coupled Kelvin wave around 110E in about 3-5 days. Satellite pictures suggest this process may be already starting.

My own thought is for the Kelvin wave to consolidate with the activity downstream “anytime”, and maintain the intensity already present across the TNWP. I would also expect the Monsoon systems to regain their robustness. During weeks 2-3 at least some of the forcing should propagate into the Western Hemisphere (will this be much sooner??? -- satellite pictures already show the ITCZ convection is increasing).

Across the Western Hemisphere, there is some suppression with OLR anomalies ~ plus 30-50W/m**2 from the East Pacific into the Caribbean. The strong tropical wave approaching the Lesser Antilles is working against this suppression (please see for details of this feature and other tropical cyclone concerns).

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. These tools generally rely on a moderate to strong MJO signal, which is nearly non-existent at this time. However, I think it is worthwhile to note that the Wheeler index has had a ~1 sigma signal for at least the last 2 days, centered on Southeast Asia and the TNWP (see link), not inconsistent with the above and tropical wind fields (discussed below). As already stated, an eastward propagating signal into the Western Hemisphere is likely to evolve, but should not project onto a MJO (recall my “wild card” scenario from a week ago).

Time-latitude sections of 200mb zonal mean zonal wind anomalies show anomalous westerly flow has increased from around 15S-30N, with magnitudes ~5m/s. Anomalous easterly flow replaced again by westerly flow (and so on) extends poleward into both hemispheres, with propagation (generate plots at and see for yourself some of the details).

This distribution of zonal mean anomalies still has some symmetry to it, implying there is (and has been) important tropical forcing (even though there has been difficulty to diagnose the generally weak signals among the components). The recent increase of zonal mean anomalous westerly flow throughout the tropical and subtropical atmospheres discussed above can be partly attributed to the recent flare-up and consolidation of convective forcing across the TNWP.

If the interested reader does a time-latitude section of 200mb zonal averaged zonal wind anomalies from at least boreal spring, a relatively coherent poleward propagation of westerly anomalies starting about mid-June will be seen (see previous postings for details). Starting around mid-July into the present, zonal mean westerly anomalies of ~5-10m/s have been present from ~40-55N, north of easterly anomalies of similar magnitudes (part of the meridional symmetry). One can also think of this as poleward movement of atmospheric angular momentum. From that understanding, dynamically there would be an expectation of stronger than climatology subtropical ridges across the midlatitudes. This notion is consistent with recent observations, and has contributed to heat wave episodes across the USA and Europe.

Taking this thread one step farther, an argument may be offered that the above average 200mb westerly flow throughout the northern extratropics may be the result of enhanced north-south temperatures differences. If the interested reader also does a time-latitude section of zonal mean 500mb temperature anomalies (for example), loosely, warmer than average temperatures will generally be seen across polar latitudes with the reverse across the subtropics, with ~3-5C anomalies. Whatever the case may be (and however anybody wishes to do this calculation), a thermal wind justification cannot be made for this enhanced westerly flow. I would offer this additional westerly flow had a contribution from complex interactions involving recent well defined ~50-day mountain/frictional torque index cycle along with an intense version of the 30-day tropical forcing signal moving into the western Pacific, at that time. How much of a contribution that was and the details which were involved would be speculation. Some attention to this behavior may be given in the weather-climate discussion we are working on.

Once this additional westerly flow was able to interact with properly phased baroclinic wave packets (whatever that means), strong subtropical ridges became dominate across locations such as the USA and Europe leading to the intense heat. Was all this predictable??? The models did offer useful information and through monitoring I also captured some of this. However, that is as far as I will go.

Tropospheric global relative AAM is only slightly below normal based on the 1968-1997 reanalysis climatology through August 6th and the global torques are weak. The operational plot based on the 1979-1998 climatology shows AAM ~.5 sigma below normal. The AAM tendency was not available today; however, I think it is weakly negative since the global mountain torque is becoming negative. Lowering pressures along the east slopes of both the Andes and East Asian topography are likely contributing to a negative global mountain torque. I would expect the frictional torque to become positive soon given the increasing trades throughout Pacific and Indian Oceans (per above).

In a region of upper tropospheric divergence centered ~10N/130E (per animations of daily mean 150mb and 250mb vector wind anomalies), distorted twin subtropical anticyclones have been appearing in that area. These anticyclones are interacting with baroclinic wave packets from both hemispheres, and continue to properly phase these packets to support additional Pacific Northwest troughs and maintenance of a large subtropical anticyclone across the USA. Equatorial westerlies continue from about the date line into at least northern South America, with anomalies ~15-25m/s. These have become less coherent during the last couple of days. Why is that? Are these westerlies going to work around into the Eastern Hemisphere at some point, while propagating poleward and downward? Easterly anomalies continue throughout tropical Africa.

To summarize, I think we have a weather-climate situation consisting of 1) a consolidation of the 30-day tropical convective mode with the SST boundary forced tropical rainfall, 2) the weak convectively coupled Kelvin wave about to merge with (1) as the latter shifts west as a convectively coupled Rossby mode, 3) the mountain-frictional torque index cycle (loosely), 4) the rapid extratropical Rossby wave energy dispersions/baroclinic wave packets, 5) weak or non-existent MJO, 6) possible ENSO and/or global warming signal, and 7) the possibility of a convectively coupled signal propagating into the Western Hemisphere during ~weeks 1-3. We also need to start thinking about the role of the seasonal cycle (more) as transition to autumn approaches. I can list a lot of components within our GSDM framework; however, it still all adds up to a weak (if any) predictive signal.

GSDM Stage 1-2 best describes the current global circulation/weather-climate situation. Unlike what I have been thinking during the last couple of weeks, GSDM Stage 1-2 may persist roughly a week longer thanks to the Eastern Hemisphere tropical forcing shifting back to the west. Thus any retrogression of a ridge back to near the USA west coast looks to at least be delayed, perhaps unlikely. This goes back to the “wild card” scenario I was concerned about, minus the MJO. At least I was able to remain open minded about that possibility.

About the only signal I can go with is for the Eastern Hemisphere forcing to starting propagating into the Western Hemisphere as another 30-day mode, perhaps consisting of several Kelvin waves. A mature GSDM Stage 2 may be a reasonable option for weeks 2-3. Uncertainty remains tremendously high.

Week 1 (10-16 August 2006): GSDM Stage 1-2, seems probable. Per the NCEP GFS and other models, there look to be at least a few more mobile troughs to dig along the USA west coast, then move inland through the northern states. This would suggest additional episodes of heat to expand from the Central Rockies into much of the Central/Southern 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. Other than the strong tropical wave cited above, the window of opportunity for North Atlantic tropical cyclone development may be closed. Please see for the latest tropical cyclone information.

Week 2 (17-23 August 2006): A continuation of week 1. However, even though the NCEP GFS model keeps the trough along or just off the USA west coast through week 2 (and the CDC ensemble is “different” – see links below), I think there is some possibility the trough will shift inland into the Rockies and eventually the Plains later this period or week 3. As already stated, this is different then my previous thoughts and I would not consider it statistically useful, only something to keep in mind. I am offering this based on a late summer going into fall rendition of GSDM Stage 2. This situation would bring cooler and wetter conditions to much of the Rockies and Plains, while the East and Deep South remain warm. Tropical cyclone development across the North Atlantic may become further suppressed, while the East Pacific may become active.

Week 3 (24-30 August 2006): See week 2.

Week 1 looks like a continuation of the same old warm/dry weather situation for Southwest Kansas. However, there look to be at least 2-3 energetic mobile troughs which will pass by to our north, dropping fronts into this region. As we have seen this summer, at least diurnally dependant storms will be scattered along these synoptic boundaries. It is just a matter of who gets the rain. If there is any truth about my week 2-3 options, cooler and wetter weather would be more probable here. Again, my forecast confidence is as low as it gets. Maybe an oracle or a wizard can help us see the weather future.

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 this month (pushed back).

Ed Berry

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