Saturday, July 15, 2006

Global Westerly Flow Update

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 SSTs have changed little since my July 12th posting. 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 (recently updated).

What was discussed in the July 12th posting is on track, and I am accordingly going to keep this discussion very short (time constraints also present). Please see that writing for links not given here.

Primary tropical convective forcing continues from the very intense Indian and Southeast Asian monsoon systems, west central and northwest Pacific imposed by the warm SSTs, and weakly from the east Pacific into northern South America. The eastward propagating dynamical signal (~12m/s phase speed) responsible for the Western Hemisphere enhancement is still reasonably coherent, with the divergent component centered ~10N/70W on July 14th.

The persistently enhanced convection from the west-northwest Pacific concerns me the most (for subseasonal and beyond timescales). Animations of upper tropospheric daily mean vector wind anomalies show strong divergent outflows/episodic anticyclonic circulation gyres from this convection continuously interacting with extratropical baroclinic wave packets in both hemispheres. Additionally, near equatorial surface westerly wind anomalies ~2-3m/s is again approaching the date line. This tells me a continuation of the forcing-circulation response and feedback processes from this convection.

Within the framework of the GSDM, I can already see defensible evidence that another summertime rendition of Stage 2 is probable (and statistically useful to offer) within the next few weeks (see previous posting for details). For the Pacific-North American sector, similar to August of 2004 and 2005 as well as the latter part of June 2006, this would suggest an anomalous ridge from ~115/120-140W into Alaska, Hudson’s Bay-central USA trough and southeast states-western Atlantic ridge. Since my July 12th writing, more and more ensemble numerical models (links below) are trending toward a solution like this starting around next weekend (dynamic responses getting better sampling in the initial conditions). Again, synoptic details and timing are white noise, static, wasted effort to predict, etc.

Global relative AAM tendency is ~negative 30 Hadleys per reanalysis data as of July 12th. While there is still ~5m/s 200mb anomalous zonal mean westerly flow particularly just south of the equator (with slightly greater values across the Southern Hemisphere extratropics), a significant reduction has occurred during the past week throughout the subtropical and midlatitude atmospheres of the Northern Hemisphere. A lot of that reduction has come from both the mountain and frictional torques.

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. The generally persistent western Pacific convection (for ~6 weeks) may suggest there is currently a transition from the previous cold event (La-Nina) to perhaps an oncoming warm event (El-Nino). Stay tuned for additional information on the latter per links given, especially from CPC.

Week 1 (16-22 July 2006): Loosely GSDM Stage 3-4 this period. Strong westerly flow across the northern USA with a broad central states ridge looks like a good bet. When the retrogression and amplification of the ridge, etc., occurs, and how long the Desert Southwest local monsoon system remains suppressed, are unclear.

The well advertised heat wave (at least loosely) is spreading across most of the country. The largest positive temperature anomalies should be centered from the Northern/Central Rockies into the Mid/Upper Mississippi-Ohio Valleys. Many locations in these areas will exceed 100F, with high dewpoints well into the 70s and possibly a few 80s for locations such as Iowa. There may be some pockets of intense/severe thunderstorms across the northern part of the country linked to jet streaks and frontal activity. Most/all drought stricken parts of the country will not get much relief. With the trades relaxing across the Tropical North Atlantic and the possibility of reduced shear due to twin subtropical highs (GSDM Stage 4), there may be a concern for tropical cyclones particularly across the Caribbean later this period going into week 2. 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 (23-29 July 2006): A statistically useful option to offer is a transition to GSDM Stage 2. This may result in a ridge from western North America/east Pacific into Alaska, a central USA/Great Lakes states trough (~90-95W?) and a ridge across the southeastern part of the country/western Atlantic Ocean (one summertime version of GSDM Stage 2). Much cooler weather for especially the North Central States with increased rainfall for roughly the central and eastern portions of the country would be expected if this occurred.

Week 3 (30 July- 5 August 2006): Same as week 2, otherwise unclear.

Hot daytime temperatures with highs frequently over 100F and little organized rainfall are likely for Southwest Kansas through much of week 1. I cannot rule out a heavy rainfall surprise somewhere in this part of the world, particularly along a weak cold front ~Tuesday. For weeks 2-3 (timing is white noise), should the global circulation transition to GSDM Stage 2, much cooler temperatures and an improved opportunity for rainfall would exist.

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 a more complete posting the middle of next week. Also, work is starting on another weather-climate discussion.
Ed Berry

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