Wednesday, June 14, 2006

United Tropical Convection?

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 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.

Also, I am making attempts to shorten these postings for a variety of reasons. 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.

With the exception of the East Pacific, SSTs are above average throughout most of the global tropics. The largest anomalies extend from the South Pacific into the Indian Ocean, with actual SSTs at least 29-30C. Global SST information can be obtained from latest TAO data here, ESRL/PSD data here, and CPC data

The following are links to ENSO discussions.

Latest satellite imagery shows an intense cluster of thunderstorms along the equatorial Indian Ocean, with a much larger region extending from the Bay of Bengal east-southeast into the west central equatorial Pacific. A consolidation of eastern hemisphere tropical convection appears to be in progress (as discussed in previous postings), with the centroid at ~10N/110E. In addition, a separate region of enhancement appears to be breaking off and moving fairly rapidly into the western hemisphere, perhaps as a convectively coupled Kelvin wave (see links to coherent modes Hovmollers below). Finally, thunderstorm activity remains very intense from northern South America into western Africa, along the Atlantic ITCZ.

Since December 2005 there have 6 episodes of tropical convection evolving across the eastern hemisphere, with subsequent eastward movement. The periods have been ~30 days, including MJO components. My feeling is we are likely seeing this behavior again, understanding seasonal variations (such as northward propagation during the Boreal summer). It is probable that during the next 2-3 weeks the consolidated region of tropical convection will approach the date line, with one or more convectively Kelvin waves moving rapidly through the western hemisphere. I would also expect other basins such as the tropical Atlantic to be fairly active (due to warm SSTs), especially if there are convectively coupled Kelvin waves.

There is some agreement among the statistical and numerical models that the current tropical convective forcing across the eastern hemisphere will move east during week 2, as discussed above. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools for the details.

A time-latitude section of 200mb zonal mean zonal wind anomalies indicates anomalies of ~ minus 5-15 m/s propagating off the equator into the subtropics of both hemispheres, with zonal mean westerly anomalies just north of the equator. Strong zonal mean westerly wind anomalies (~15-25 m/s; largest in the Southern Hemisphere) prevail across the northern and southern hemisphere extratropics, indicative of inter-hemispheric symmetry due to tropical forcing.

The trades remain relaxed with even actual surface westerly flow along the Atlantic ITCZ (with anomalous convergence); with some enhancement west of the date line (anomalies ~ minus 2-5 m/s). Interestingly, the Somalia Jet (western Arabian Sea) remains much weaker than normal, indicative of enhanced convection well to the east and a suppression of the Indian Monsoon. I would expect to see this situation change during the next couple of weeks as the current enhancement along the equatorial Indian Ocean moves north.

AAM anomalies have decreased to more than ~ minus 2 standard deviations below the 1968-1997 climatology, with its tendency still negative, based on the reanalysis data which lags 3 days from the current time. The operational data plot shows that AAM has decreased to nearly 3 standard deviations below the 1979-1998 climatology (see The recent increase in the zonal mean anomalous westerly flow across ~25-40N and S may be due to eddy feedback and transport processes.

Animations of daily mean 150mb and 250mb vector wind anomalies loosely show twin subtropical anticyclones around 100-140E as well as across the Atlantic and Africa, linked to the tropical convection. During the last few days wave breaking from extratropical baroclinic wave packets has been fluxing AAM out of the northern subtropical atmosphere at around 15-20N, and this process has lead to the zonal mean westerly anomalies just north of the equator. Regionally these westerly anomalies have appeared across both the central Pacific and Indian Oceans. I would expect the equatorial westerly wind anomalies to increase across the central and east Pacific during the next couple of weeks particularly if the Maritime Continent convection shifts toward the West Pacific.

Within the framework of the GSDM, I think we are transitioning from Stage 4-1 (summertime rendition), particularly since the frictional torque is becoming positive while the global mountain torque remains negative (see plot for mountain torque and plot for the frictional torque; see for all AAM plots, including tendency). My thoughts are to remain in Stage 1 for about the next 10 days followed by a transition to Stage 2 by week 3. Forecast uncertainty remains very high, particularly given the time of year and especially with timing.

I do think it is probable a decent area of enhanced tropical convection will come into the west Pacific during the next 1-3 weeks, along with a GSDM Stage 2. During August of 2004 and 2005 we observed this behavior, and it was a “prolific” heavy rain and severe local storms producer for the middle of the country (along with cooler than normal temperatures). Please see our past weather-climate discussions for details.

Week 1 (15-21 June 2006): Tropical moisture transport from the deep tropics through the Gulf of Mexico into the CONUS should be well established. GSDM Stage 1 is most probable, meaning a belt of strong westerlies along and south of the USA/Canada border within a Gulf of Alaska to Pacific Northwest trough-south central states ridge and trough just off the East Coast. Active and possibly severe MCSs/Derechoes (along with areas of heavy rainfall) from the Northern Rockies to the Great Lakes states with above average heat from the Plains into the parts of the East would be expected. Additionally, the Central/Southern Plains and especially parts of Texas may receive locally excessive rainfall due to nearly stationary low latitude “upper level lows” south of the ridge axis (residuals of wave breaking events). Please see for the latest tropical cyclone information.

Week 2 (22-28 June 2006): A transition to GSDM 2 would be probable if the tropical convective forcing moves into the west Pacific. That would suggest ridge amplification off the USA west coast into Alaska with a trough developing across the Rockies and Plains and a ridge across the Southeast. Synoptically this may appear as retrogression with amplification of the wave train discussed for week 1.

Week 3 (29 June-05 July 2006): GSDM Stage 2, perhaps 3, would be my best offering as we head into the Fourth of July weekend. Should we see Stage 3, it is probable the Upper Mississippi Valley and Great Lakes would have below normal temperatures with warmth across the Pacific Northwest and Inter-Mountain West. Roughly the eastern half of the country would be wet.

After a couple of hot days with scattered high based storms, cooler and wetter weather is likely for Southwest Kansas from ~6/16-18. In fact, locally excessive rainfall may be a concern. Next week into the weekend we should warm to above normal readings, with at least a couple of opportunities for scattered diurnal thunderstorm activity. Precipitation opportunities will increase if we evolve to GSDM Stage 2 during week 2, along with much cooler temperatures. Week 3 may not be excessively hot while precipitation chances decrease. Whatever the case may be for the details, I do not think our weather-climate situation supports an extended period of tremendously excessive scorching heat with no rainfall for at least the next month.

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)

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)

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 attempt another posting this upcoming weekend.
Ed Berry

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