Wednesday, July 19, 2006

The Earth Giveth..........

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.

There has not been much change to the global tropical SSTs during the past week. However, SST anomalies across the extreme eastern equatorial Pacific have rapidly increased to ~plus 1.5C, with somewhat larger anomalies extending downward to nearly 100m. The latter may be partly from the seasonal cycle since SST climatology favors cooling in this region, and perhaps from the very weak Oceanic Kelvin wave initiated ~2 months ago finally reaching the South American coast. In any case, above average SSTs dominate the eastern Pacific Ocean equatorial cold tongue all the way to the date line, extending at depth to ~200m (deeper than normal thermocline). Latest TAO buoy data still shows surface westerly wind anomalies (~ 5m/s) and even actual westerlies along the equator from the date line and west, with evidence of a downwelling oceanic Kelvin wave crossing into the Western Hemisphere. Actual SSTs in excess of 29C still dominate from the South Pacific into the Indian Ocean, and nearly all of the Tropical Northwest Pacific.

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 is some evidence the weather-climate system is tilting toward a warm event. However, the magnitude of any possible warm event and global impacts are unclear. The following are links to ENSO discussions (recently updated).

Please also see the following CPC link (and others therein) for further ENSO, etc., insights.

Coherent signals of tropical convective forcing have been hard to come by recently. The MJO signal has been essentially non-existent since about March (there is a climatology component to this), while 20-30 day variability has been occurring since at least December 2005 (see ESRL/PSD weather-climate discussions). The latter have exhibited faster eastward propagation including convectively coupled Kelvin waves. Starting early May, northward propagation to the 20-30 day modes started (seasonal cycle), particularly from the equatorial Indian Ocean into the Indian and Southeast Asian Monsoon systems. About a week ago, the third of these events appeared across the central equatorial Indian Ocean. There has not been much movement; however, latest 3-day averages of OLR indicate anomalies have decreased to ~minus 50-70 W/m**2. Finally, the monsoon system enhanced rainfall continues from the Bay of Bengal into Southeast Asia and the Philippines, which is a bit east of normal.

Tropical convection persists across the west central and northwest Pacific, with episodes of tropical cyclone development (see for details). This region of enhanced SST boundary tropical thunderstorm activity has continued for nearly 2 months, slowly drifting northwest with the seasonal cycle while having intensity variations from the 20-30 day modes per above. At least 2-3 equatorial surface westerly wind events have come out to the date line with this activity. Given the recent increase in the intensity of the equatorial Indian Ocean rainfall, I would expect another convectively coupled Kelvin wave to enhance the SST boundary forced convection farther east during the next couple of weeks. In fact, full disk satellite imagery already suggests a very loose consolidation of the Eastern Hemisphere tropical convective forcing, extending from the central Indian Ocean into the Tropical Northwest Pacific with the monsoon rainfall on the northern portion of that envelope. A convectively coupled Kelvin wave may be emerging into the western Pacific, and the time filtered coherent modes Hovmollers lend some support to that notion.

Any dynamical signal supporting tropical convection across the Western Hemisphere has become weak and incoherent (based on 200mb velocity potential and other fields). There is some enhancement from the northward displaced eastern Pacific ITCZ (including Hurricane Daniel – links below) into the southwest Caribbean and more recently central Africa.

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 thought would be for the enhanced Tropical Northwest Pacific convection to possibly dominate the tropical forcing during the next couple of weeks, while a somewhat eastward shifted Indian and Southeast Asian monsoon persists. The latter may be enhanced as “Indian Ocean event number 3” propagates northeast. The future evolution and roles of the east Pacific ITCZ and especially African convection is unclear.

Time-latitude sections of 200mb zonal mean zonal wind anomalies still indicate most of the tropical and subtropical atmospheres to be dominated by westerlies. However, magnitudes are only ~5m/s compared to at least 10m/s a couple of weeks ago. Weak easterly wind anomalies have even returned to the midlatitudes of both hemispheres. The latter is a contributing to the expected week 1 retrogression of synoptic features across the Pacific-North American sector (more said below).

Poleward and downward propagation of zonal mean wind anomalies continue. The most recent round of westerly anomalies to propagate out of the equatorial and subtropical atmospheres has reached the Earth’s surface across the midlatitudes (loosely speaking). This has contributed to negative mountain and frictional torques (frictional torque leads by ~6 days), removing some of the anomalous vertically averaged zonal mean westerly flow (with the Southern Hemisphere doing much of the work, as would be expected). In other words, the Earth is taking away AAM from the atmosphere, after giving AAM to it about 3-4 weeks ago (giveith, and now takeith away!).

A much more complete understanding of the poleward propagation of circulation anomalies and forcing-response-feedback relationships requires a vertical average of zonal mean anomalies and a global summation (regional considerations and other dynamical processes understood). This is one reason to make use of the atmospheric angular momentum (AAM) plots from ESRL/PSD (links below). Yes, this is a restatement from a week ago, but I think it is worthwhile, and needed.

Tropospheric global relative AAM is still ~1 sigma above the 1968-1997 reanalysis climatology based on data through July 14th. The more recently updated operational data relative AAM plot shows values to have dipped slightly below normal based on a 1979-1998 climatology which includes 2 strong and 2 moderate warm events (El-Ninos). Through July 14th the mountain and frictional torques were returning to normal (still ~ minus 5 Hadleys) while the tendency was ~minus 30 Hadleys.

A very important point that we hope to demonstrate in our next weather-climate discussion is the roughly clean 40-50 day variation of vertically averaged zonal mean wind anomalies throughout the tropical and subtropical atmospheres observed since at least mid April. This variation has been nicely observed in the global integral of relative AAM, with strong contributions coming from both the mountain and frictional torques. For instance, during much of May we observed reduced trades (lead to a negative frictional torque), followed by enhanced trades during June (positive frictional torque which adds westerly flow to the atmosphere), then a recent reduction. The positive June global frictional torque was followed by a positive global mountain torque by late June, with a collapse afterwards (see links). During these poleward propagating events there is this constant exchange of AAM between these torques, adding and removing westerly flow from the atmosphere (part of the Earth-Atmosphere AAM budget). The most recent addition of westerly flow to the atmosphere (mainly June) contributed to the current heat wave that both much of the USA and Europe are experiencing. Now, both torques are reducing AAM, contributing to the negative AAM tendency. In our GSDM, we refer to this component as a “mountain-frictional torque index cycle”.

The mountain-frictional torque index cycle has been going on in the presence of the 20-30 day tropical convective variability and SST boundary forced convection across the Tropical Northwest Pacific (monsoon systems understood). Finally, fast Rossby wave energy dispersions have occurred with these other dynamical processes. So, even though the MJO signal has been essentially “not there” since at least March, at least 3 other components as described by the GSDM have been present, but still with a WEAK overall signal. In a sense, the stationary boundary forced convection can be thought of as a “stationary MJO” within the context of the GSDM (thus all 4 components).

In my postings during the last couple of weeks, I have offered that the anomalous global and zonal mean westerly flow throughout the atmosphere would collapse. I was basing that on the SST boundary forced western Pacific convection and attempting to time the 20-30 day mode. Loosely, my feeling was there would be a transition from what has recently been GSDM Stage 3 to Stage 2. This process is occurring as I type. However, and very important, I was not totally in tune with the mountain-frictional torque index cycle already discussed, which is very hard to do since it is a stochastic process. Also, there is no way myself or anyone will know the synoptic details of these kinds of interactions beyond days 2-3 (generally). The point is we can only offer defensible predictive information making use of probability statements.

Animations of daily mean anomalies of 150mb and 250mb vector winds continue to show twin somewhat distorted subtropical anticylones ~ 150E, with another pair ~105-110E. The former is linked with the SST boundary forcing while the latter may be linked to the Indian Ocean convection, all discussed above. However, for about the past week, the anticyclonic circulation gyre ~35N/105E (the northern member of the twin pair) has had 250mb wind speed anomalies ~30-40m/s. While attribution to this feature cannot be done here, it is possible the East Asian topography has played a role, going back to the mountain torque component. This anticyclone is interacting with baroclinic wave packets which will contribute the expected western North American ridge by this upcoming weekend.

Uncertainty continues extremely high. It is plausible to expect GSDM Stage 2 week 1, meaning western North American ridge and central/eastern USA trough by this coming weekend into early next week. Most numerical ensemble prediction systems agree with this. I also like the notion of continued retrogression of the western USA trough to ~140W by week 2, as many numerical models are showing. This would be a maturing of the Stage 2 response. However, should the convection across the Tropical Northwest Pacific become very intense during week 2 (per above), the synoptic pattern across North America “could be almost anything”. I am also concerned that more westerly flow may once again be added to the atmosphere later week 2 into week 3, meaning a possible return to GSDM Stage 3. Recall there is still slightly above average zonal mean and global westerly flow throughout the atmosphere. How much additional reduction there will be before going back up is unclear.

Week 1 (20-26 July 2006): GSDM Stage 2 is most probable. While intense heat returns to the western USA by this weekend, cooler and wetter weather can be expected for the central and eastern states. Severe local storms and heavy rainfall should spread from the Mississippi Valley into the eastern states with the developing trough. Toward the end of this period the ridge may shift back into the eastern Pacific while generally cyclonic flow dominates much of Canada and the northern USA. Please see for the latest tropical cyclone information.

Week 2 (27 July - 2 August 2006): A mature GSDM Stage 2 would be probable at the start of this period, possibly followed by Stage 3 at the end. Synoptically there may be a deep trough from Hudson’s Bay into the Northern Rockies and Northern Plains with a southeast states ridge. Above average westerly flow along the USA/Canada border may return afterwards. Temperatures would again warm to above average values for much of the southcentral into the eastern part of the country while at least more seasonal readings occur across the northern states. An active MCS track may become quite robust from the Northern Rockies/Plains to Great Lakes this period.

Week 3 (3-9 August 2006): Unclear. However, an option to monitor will be a return to a situation similar to the current one should a summertime rendition of GSDM Stage 3 become quite robust.

A welcome cool down (likely brief) seems like a good bet for Southwest Kansas by this weekend, with maximums perhaps in the 80s on roughly Saturday. At least one or two rounds of scattered thunderstorms are probable ~Thursday night into this weekend. Afterwards temperatures will warm, possibly back to ~100F by the middle of next week. Above average temperatures with a few opportunities of scattered storms appear probable going into August.

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 early August.
Ed Berry

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