Saturday, June 17, 2006

Tropical Convection is Back East

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

The gist is that 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. 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 extreme 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 of at least 29-30C. The oceanic thermocline along the equatorial cold tongue remains depressed more than normal, with anomalies ~plus 1-2C at depths from 100-250m. 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 extensive area of near-equatorial tropical convection from the west central Indian Ocean to the west Pacific, with strong positive anomalies around 80E and a lesser region north of Australia. The convection across the Indian Ocean has been drifting north as well as east, which should strengthen the Indian Monsoon during the next couple of weeks. I would expect a solid consolidation of this forcing to take place north of the equator ~120E during the next couple of weeks. Afterwards, going with the obvious says to expect an eastward movement, possibility as a MJO, into the west Pacific, with a northward shift as it does (seasonal cycle). Compared to a week ago, tropical convection has become relatively suppressed over the western hemisphere ITCZs, particularly around Central America.

There is inconclusive to weak agreement among the statistical and numerical models that the current tropical convective forcing across the eastern hemisphere will behave as discussed above by the end of week 2. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and for the details.

Zonal mean 200mb wind anomalies of ~ minus 5-15 m/s continue to propagate off the equator into the subtropics of both hemispheres, with zonal mean westerly anomalies appearing along and 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.

AAM anomalies have decreased to more than ~ minus 2.5 standard deviations below the 1968-1997 climatology, but with its tendency becoming positive (discussed below), based on the reanalysis data which lags 3 days from the current time. The operational data plot shows that AAM has decreased to more than 3 standard deviations below the 1979-1998 climatology (see The largest contributions to the low global AAM continue to be from the Southern Hemisphere subtropical atmosphere, with a general zonal mean AAM distribution featuring anomalous easterlies throughout the subtropics and high latitudes, and anomalous westerly flow in the midlatitude atmospheres. The 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 90-150E (one can argue others), with increasing divergence due to the tropical convection in that region. During the last week or so westerly wind anomalies have been increasing along and north of the equator from the Indian Ocean into the East Pacific. I would like to argue these westerly anomalies are appearing as the result of wave breaking of extratropical baroclinic wave trains/packets interacting with the eastern hemisphere tropical convective forcing (leading to “expected AAM fluxes”). However, that is an outstanding research issue, and needs attention for both theoretical and prediction concerns. In any case, the appearance of these upper tropospheric equatorial westerly wind anomalies is one of the components leading to the observed positive tendency in global relative AAM. I would expect the equatorial westerly wind anomalies (and AAM) to continue increasing 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 in a summertime rendition of GSDM Stage 1, particularly since the frictional torque is positive while the global mountain torque remains (barely) 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 another week, followed by a transition to Stage 2 during week 2 and perhaps remaining so during week 3. Forecast uncertainty remains very high, particularly given the time of year and especially with timing.

I still 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. Should that be the case, this may be good news for a large part of the central USA where severe drought is being experienced. 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. Given the recent SST behaviors, perhaps the GSDM Stage 2 response may persist for more than a week or two.

Week 1 (18-24 June 2006): Tropical moisture transport from the deep tropics through the Gulf of Mexico into the eastern half of the CONUS is 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, parts of South Texas may receive locally excessive rainfall due to nearly stationary low latitude “upper level lows” south of the ridge axis. Please see for the latest tropical cyclone information.

Week 2 (25-01 July 2006): A transition to GSDM 2 looks more probable to me than it did when I did my last posting. Adding to my confidence is more and more ensemble members of various numerical models are starting to capture this possible circulation change (as already dictated from weather-climate monitoring and the GSDM framework). What may be happening is that the initial conditions of the models are getting a better representation of the current weather-climate situation (consistent with both our work and experience). This suggests 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 (02-08 July 2006): GSDM Stage 2, possibly transitioning to Stage 3 (need to leave this possibility open), would be my best offering as we head into the Fourth of July weekend and after. 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.

It appears like Southwest Kansas is going to heat up again by early next week, with maximums possibly ~100-110F around next Tuesday (with a “blow torch” strong south-southwest wind). A trend toward gradually cooler (highs still in the 90s) with increasing opportunities (best to our north) of nocturnal MCSs is probable starting roughly the middle of the week. However, I think a more decided trend toward cooler and wetter appears more likely starting next weekend into the following week. Indeed, there may be a 3-5 day period of above normal rainfall and below normal temperatures (highs only from the mid 70s-80s). Week 3 may not be excessively hot while precipitation chances decrease. Whatever the case may be for the details, I again state I do not think our weather-climate situation supports an extended period of tremendously excessive scorching apocalyptic 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 on about the middle of next week.
Ed Berry

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