Saturday, July 29, 2006

The Rise and Fall of AAM

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.

Global tropical SSTs remain generally above normal, especially from the South American west coast at ~20S extending northward to the equatorial cold tongue then on to the date line. SSTAs ~ plus .5-1.5C to at least depths of 100-150m were being observed in those regions per the latest TAO array data (link below). The latter may be partly from the seasonal cycle since SST climatology favors cooling in this region, and perhaps from an earlier very weak Oceanic Kelvin wave currently reaching the South American coast. Latest TAO buoy data continues to observe surface westerly wind anomalies (~ 5m/s) and even actual westerlies along the equator from about 160W to west of the date line, with evidence of another 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. In fact, there has even been some warming during the past week-10 days, especially across the tropical Atlantic Ocean. 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 growing evidence, to me, that the weather-climate system is tilting toward a warm event. In fact, a weak warm event may already be underway. 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.

Coherent signals of tropical convective forcing are still hard to come by. The MJO signal has been essentially non-existent since about March (there is a climatology component to this), and the 20-30 day variability occurring since at least December 2005 (see ESRL/PSD weather-climate discussions) has weakened. Nevertheless, per coherent modes Hovmollers and other combined plots and animations, a case can be made of a weak convectively coupled dynamical signal having returned to the Eastern Hemisphere starting roughly 10-14 days ago. Loosely, this may be a mixed mode consisting of a convectively coupled Kelvin wave and forcing from Southern Hemisphere dynamics. The flare-up observed in the central equatorial Indian Ocean about a week-10 days ago was tied to this process, and has since lead to a rapid eruption of thunderstorm clusters from India through the Bay of Bengal and portions of Southeast Asia (northern tropics). OLRA have been ~minus 50-70 W/m**2 in these areas. This region of thunderstorm activity appears to be drifting east at around 9 m/s (roughly 10 deg long/day) per a “back of the envelop” calculation.

Tropical convection also persists from Southeast Asia toward the equatorial date line, forced by the warm SSTs (see for details of tropical cyclones). I would expect this region of tropical forcing across the Tropical Northwest Pacific (TNWP) to intensify during the next couple of weeks as the region farther west consolidates with this thunderstorm activity (monsoon systems understood, which have been active this year). Should that occur, tropical cyclone activity may increase and threaten locations such as the Philippines, Southeast Asia and possibly Japan.

Any dynamical signal supporting tropical convection across the Western Hemisphere is weak. However, based on near equatorial 200mb velocity potential Hovmollers, there is a suggestion upper tropospheric divergence has stalled around Central America since about July 10th. Satellite imagery (7/29) shows enhanced clusters of thunderstorms from the Amazon Rain Forest into most of Central America and adjacent areas of the eastern Pacific and western Caribbean. In fact, there is moisture transport from this region into the southwest and south central USA as I type. I could even offer there are weak twin subtropical anticyclones tied to this activity leading to subtropical jets (STJs) including one affecting the Pacific Northwest (see links to animations below). A point I will again offer is there may be a dynamical relationship between this Western Hemisphere tropical forcing and the activity west of the date line. The details are unclear, and any speculation I offer cannot be done here (SSTs are one component, roles of the Hadley and Walker Cells are other scientific matters).

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 been some collapse of westerly flow throughout the tropical and subtropical atmospheres, with even zonal mean anomalous easterly flow of ~ 5m/s present at 30N. In fact, on about July 10th there was above average westerly flow present throughout the subtropical North Pacific, replaced by easterlies by around July 26th. I can ascribe this change to the tropical convective flare-up across the central equatorial Indian Ocean discussed above and an earlier wave energy dispersion from a large anticyclone that was over the Tibetan Plateau occurring from ~15-22 July (see animations). The latter was part of a ~50 day mountain-frictional torque index cycle (interacting with the SST boundary forced TNWP convection) discussed in past postings, and this did contribute to recent deadly “heat wave ridge” across western North America.

Tropospheric global relative AAM is about normal based on the 1968-1997 reanalysis climatology through July 26th. However, the global (remember) mountain and frictional torques are ~ minus 5 and minus 15 Hadleys, respectively (a lot coming, again, from the Southern Hemisphere), and the global AAM tendency is ~minus 25-30 Hadleys. The latter is coming from particularly the Andes Mountains as well as the deep tropics. I do think we are going into a negative “phase” of our mountain-frictional torque index cycle, and I am not sure how that will force the circulation in the presence of the already discussed tropical convective forcing including the possible emergence of a warm event (El-Nino). Incidentally, the operational AAM plot based on the 1979-1998 climatology has the global integral at ~ minus 1 sigma, since there are 2 strong and 2 moderate El-Ninos in that shorter climatology.

The tropical convective flare-up that started roughly a week-10 days ago across the equatorial Indian Ocean progressed into the Bay of Bengal about the middle of last week (~July 27th). As shown by animations of 150mb and 250mb daily mean vector wind anomalies, a fast wave energy dispersion passing through the Asian subtropics (subtropical wave train) interacted with this flare-up and led to arguably a rapid Rossby wave energy dispersion (RWD) event across the Pacific Rim. In some sense the result has been a discontinuous retrogression of the “hot western USA ridge” to ~150-160W, with an anomalous anticyclonic gyre near 50N/160W having 250mb wind speed anomalies ~50-60 m/s on July 28th. An anomalous trough is digging just off the Pacific Northwest Coast as I type, and will give folks in that part of the country relief from the heat for at least week 1.

Since AAM tendency is at a negative minimum along with the negative torques discussed above, I think GSDM Stage 4 best describes the current weather-climate situation. The distribution of the Eastern Hemisphere tropical convective forcing also lends some support to that notion. However, there is literally no MJO, and other signals are weak. As discussed below, in this type of situation almost "anything can pop out".

GSDM Stage 4 was not an option I offered in my last writing on this Blog (July 22nd). In fact, I suggested going from GSDM Stage 2-3 may occur at this time, meaning, globally, we would add westerly flow instead of removing it, like we are right now. This just shows how good I am with offering predictive insights at times, and anyone who makes predictions needs to recognize the limitations (whatever the time/space scales). As I have preached before, any predictive information must be done with statistically useful scientifically defensible statements of probability (and the proper resources, support and commitment/discipline).

For weeks 1-3, I think we are dealing with a weather-climate system where a warm event may be emerging, faster modes of tropical convective variability may continue, our mountain/frictional torque index cycle seems to be continuing, all interacting with fast extratropical wave energy dispersion processes. My suspicion is the tropical convective forcing across the west central and northwest Pacific may become dominate during weeks 2-3. However, uncertainty is very high after week 1. Good model agreement offers some confidence for week 1.

Week 1 (30 July – 5 August 2006): GSDM Stage 4-1 is most probable. Synoptically we should see a trough across the western USA with a Southeast states ridge. Cooler weather is likely for much of the Pacific Northwest-California through the Northern Rockies while the central and eastern USA have potentially deadly heat/humidity. An active severe local storm/MCS track is likely from the Northern/Central Rockies into the Upper Mississippi Valley/Great Lakes through about the middle of this period. This track should shift southeast as the cooler air does the same. The Southeast into the Mid Atlantic states are likely to stay very warm this whole period. Other types of weather should be apparent in this kind of active pattern. There may be a window of opportunity for some kind of North Atlantic tropical cyclone development during this period going into week 2. Please see for the latest tropical cyclone information.

Week 2 (6-12 August 2006): A transition to GSDM Stage 2 may occur, suggesting the trough would shift toward the central USA while ridge amplification occurs from around/just off the west coast into Alaska. Synoptically there may be a deep trough from Hudson’s Bay into the Northern Plains (~90-100W) with a Southeast states ridge. This would suggest cool/wet for the Central and North Central states, warm/wet for much of the East and warm/dry for the West and Pacific Northwest. An eastward shift of this pattern would return the heat into the western part of the country.

Week 3 (13-19 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???

A welcome cool down seems like a good bet for Southwest Kansas by the middle of next week (~ Wednesday) after a few days of maxes ~100-108F. Highs may lower into the upper 70s-80s for especially Wednesday, with a warm up into the 90s by next weekend. I would also like to think there should be a few opportunities for storms and rainfall beginning Tuesday night through Friday. For week 2, if a situation similar to August 2004 returns, that would be good news for additional rainfall and relatively cooler temperatures. We will have to see. In any event, whatever the details I would think rainfall should be near normal weeks 2-3.

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 on about the middle of next week. 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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