Wednesday, July 05, 2006

Like it or not, Uncertainty Rules

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 been some cooling of the global tropical SSTs during the past week, particularly across the Indian and North Tropical Atlantic Ocean basins. SSTs across the latter regions have lowered as much as 1.0 deg C. However, anomalies remain above normal west of 170W with ~plus 1-2 deg C warmth down to roughly 200m. Actual SSTs in excess of 29C 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 .

The following are links to ENSO discussions.

There is some evidence that a slower mode of tropical convective forcing may be emerging. Per coherent modes Hovmollers of OLR/OLRA and other plots, boundary forcing from the warm SSTs west of the date line has led to roughly a 4-5 week persistence of enhanced convection along and south of the equator. The most active region has been centered ~160E with OLR anomalies ~ minus 30-50 W/m**2. Several convectively coupled Kelvin waves (fast dynamical signals) have propagated eastward through this SST boundary forced convection. For instance, about 2-3 weeks ago (~June 17-18) one of these Kelvin waves returning back into the Eastern Hemisphere lead to an intense convective flare-up across warm equatorial Indian Ocean SSTs. A fast Rossby wave energy dispersion from this flare-up propagated into the northern extratropics, interacting with the slower SST boundary forced Tropical West Pacific convection as it did. The response was a downstream retrogression and amplification of the western North American Ridge, a deep central USA trough and an anomalous anticyclone just east of New England. It was this chain of events that resulted in the tremendous rainfall which affected much of the USA East Coast.

The latest in the “series” of these convectively coupled Kelvin waves (at least 2 of them) has moved into the Western Hemisphere. A response has been for somewhat enhanced rainfall from the east Pacific ITCZ into lower Central America with suppression of the recently active Indian and Southeast Asian monsoon systems. While intense convection still persists across the west central and northwest Pacific (including Typhoon Ewiniar see ), there is also suppression across the Indian Ocean. Linear thinking would suggest the enhanced convection portion of the fast dynamical signal to once again return to the Eastern Hemisphere by roughly the end of week 2. We will see.

All of the above discussed tropical convective variability/forcing have led to another equatorial westerly wind event from 150E to the date line. TAO buoy data through July 4th indicated westerly anomalies of at least ~5-10 m/s, and is the third such event since about March of this year. It is somewhat unusual to see surface westerly flow on the equator this time of year (usually north). However, we observed this kind of behavior during summer 2004 (see weather-climate discussions), prior to the 2004-05 warm event. Attention needs to be paid if another down welling Oceanic Kelvin wave is initiated by these westerlies, which would lead to a warming of the equatorial eastern Pacific. In summary, while slower tropical west Pacific SST boundary forcing may be evolving, convectively coupled Kelvin waves appear to be dominating the dynamical signal.

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).

During the past 4 weeks or so we have seen nice signals of poleward propagating zonal mean zonal wind anomalies. During the first half of June there were zonal mean anomalous easterlies propagating off the equator into the subtropical and lower midlatitude atmospheres, followed by westerlies. In fact, 200mb zonal mean easterly anomalies have propagated to at least 45N, helping to support strong ridges across the northern extratropics. Vertically averaged zonal mean westerly wind anomalies dominate the tropical and subtropical atmospheres, with magnitudes of ~10 m/s around 20N at 200mb (not 10-20 m/s as incorrectly stated on my July 1st posting). A separate maximum of about 5 m/s has appeared during the last few days just south of equator, and is likely linked to the west Pacific SST boundary forced convection.

It must be remembered that these zonal mean zonal wind anomalies do not just propagate poleward, but downward as well (see our paper for references). The global frictional torque, after being positive for about a couple of weeks, has now dropped to ~minus 10 Hadleys based on the reanalysis data through July 2nd. This is telling us that some of the upper tropospheric westerly flow is now reaching the Earth’s surface, which will help bring down the strong positive global mountain torque (roughly a 6 day lag, on average). The AAM tendency is still positive at roughly 10-15 Hadleys, but well off its positive maximum. Based on the 1968-1997 reanalysis climatology, tropospheric relative AAM is ~ ½ of a standard deviation above normal (see links below for AAM plots).

Animations of daily mean 150mb and 250mb vector wind anomalies (need to look at the totals also) present a very complex circulation (understatement!). There is some evidence that the Atlantic equatorial westerly wind anomalies have worked back into the Indian Ocean, especially at 250mb (consistent with suppression). Meanwhile subtropical westerlies continue across the North Pacific with 250mb anomalies still ~20 m/s. Rossby wave energy dispersion across north Asia is interacting with the divergent outflow from the west Pacific tropical convection, maintaining an anomalous cyclone across the North Pacific. The latter is combining with the above average subtropical Pacific westerlies, leading to strong westerly flow approaching the USA. Across North America a moderate amplitude ridge exists across the Rockies with a trough to the east.

Within the GSDM framework, largely from the zonal mean wind signal from the subtropical atmosphere and the AAM budget (including a possible mountain-frictional torque index cycle), I think there is some argument for a boreal summertime rendition of a Stage 3 circulation. The relatively strong westerly flow across the North Pacific (loosely speaking) and cyclonic flow particularly south of Alaska to the Pacific Northwest (and downstream ridge-trough pattern) also support that notion. This possibility has been suggested in the last couple of postings.

Uncertainty reigns supreme where the atmosphere goes from here, especially after week 1. There are forecasts of opportunity, including prediction of extreme weather events, which do occasionally arise for weeks 2-4 (especially week 2). However, uncertainty dominates, and in our current situation, I think even more than “usual” (boreal summer and other issues understood). It is possible that a transition from previous cold event (La-Nina) to perhaps an oncoming warm event (El-Nino) may be part of the issue. Stay tuned for additional information on the latter per links given, especially from CPC.

Week 1 (6-12 July 2006): More westerly flow is coming into the western USA (per above), leading to a deamplification and downstream progression of the current western CONUS ridge (split flow displaced northwest). Most models are showing this solution. Moisture transport from the deep tropics of the east Pacific is occurring into the Desert Southwest while there is improvement from the Gulf of Mexico. An active storm track with the strong westerlies (for this time of year) may become established from the Pacific Northwest to New England, while a weaker southern branch crosses the southern Rockies into the central Plains. In this type of synoptic pattern, moisture transport from the Gulf of Mexico should be robust due to lee-side Rocky Mountain troughs.

Active and possibly severe MCSs/Derechoes (along with areas of heavy rainfall) from the Northern and Central Plains into the Great Lakes and Ohio Valley will continue to be a concern. The moisture transport into the southwest states may become suppressed while possibly excessive heat/humidity builds across the Plains and heads east. The Tropical North Atlantic looks to remain unfavorable for tropical cyclogenesis due to upper tropospheric anomalous westerly flow. Please see for the latest tropical cyclone information. Other types of summertime weather/hazards around the country in this situation should be apparent.

Week 2 (13–19 July 2006): While tropical convection may remain enhanced across the western and northwestern Pacific (while maybe shifting slightly to the west), at least a weak dynamical signal could emerge across the Indian Ocean, all by the end of this period. A synoptic pattern similar to week 1 may start this period. Afterwards, no statistically useful information can be offered.

Week 3 (20-26 July 2006): Same as week 2. One option to monitor will be for the North American ridge position to shift back toward the west coast into Alaska, leading to a downstream trough across the central USA and a southeast states ridge.

A decent opportunity of rainfall exists for Southwest Kansas this upcoming weekend, followed by a return to summertime heat next week. With good Gulf of Mexico moisture transport, at least diurnal thunderstorm activity focusing along the lee-side trough then moving east would be expected. Forecast is unclear, afterwards. However, I think rainfall opportunities will continue the rest of this summer (as opposed to totally dry, per earlier fears).

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 am going to be on travel this upcoming weekend. Hopefully I will be able to post an update on about the middle of next week.
Ed Berry

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