Friday, May 23, 2008

Ghost Riders Storming the USA Plains (as expected)

Global tropical SSTs are well above average from the East Pacific into the Atlantic Oceans with anomalies ~1-3C and totals near 30C south of Mexico and in the Gulf of Guinea. While La-Nina associated SSTs have weakened considerably, large-scale warm and cold anomaly spatial horseshoes remain well defined across the Pacific Ocean basin (negative phase of the PDO?). SSTs across the Eastern Hemisphere are slowly warming, with totals ~30C across the western Indian Ocean including the Arabian Sea.

Latest 5-day averaged TAO buoy data indicates expanding subsurface warmth (~plus 3C) down to ~200m west of the date line and even 50m west of South America. However, regardless of what tools are believed to be useful, the interannual SST component including ENSO is unclear.

The following are links to global SST and related information. (note the initial projection) (link 18).

Full disk satellite imagery and other aids do not indicate any concentrated region of enhanced tropical convective forcing. Instead, the most robust signal is the strong suppression covering much of the Indian Ocean into Australia. Three-day averaged OLRA have been ~plus 50W/m**2. There has been enhancement across the Western Hemisphere particularly in the regions of Northern South America and Africa. The Northwest Pacific remains sporadic.

Keying on the suppression as well as the 850mb and 200mb winds, the WH (2004) phase space plot indicates ~3 sigma projection (through 22 May) in octant 8. Even with the interannual component left in, the projection is still well over 2 standard deviations. There is other evidence to support what this equatorially confined empirically derived index is telling us, including the WB (2007, 08) GWO. The latter is a dynamical measure of the global circulation that links global-zonal mean-regional scale variations.

The point is that there is strong tropical-extratropical coupling once again occurring, including a very robust MJO signal in the Western Hemisphere and exchanges of momentum between the solid earth and atmosphere. Since about mid-April, dynamical processes captured by the WB (2007, 08) GWO have worked to add global westerly wind flow to the atmosphere. Part of these processes included the extratropics rejuvenating the current MJO. This has included not only a weakening of zonal mean subtropical easterly wind flow anomalies, but also westerly wind flow anomalies being added to the equatorial atmosphere. Hence global relative AAM recently increased too slightly above the R1 data climatology, the highest since ~ 1 December 2007.

During this month, these anomalous equatorial westerlies (~5m/s zonal mean anomalies at 200mb) have propagated poleward and downward particularly into the Southern Hemisphere. Interactions with the midlatitude storm tracks including Rossby wave energy dispersions (RWDs) tied to Northern Hemisphere north-south mountain massifs led to the recent remarkable extension of the East Asian jet and ridge from Hell along the USA west coast. In fact, the global mountain torque was ~plus 20 Hadleys ~ 11 May with much of that contribution from East Asia.

As was expected probabilistically 2 weeks ago, there is currently frictional dissipation of the added westerly wind flow. The global frictional and mountain torques are strongly negative leading to a computed AAM tendency of ~minus 25 Hadleys. Much of that negative global AAM tendency has ~1-2 Hadley zonal mean contributions from the equatorial and subtropical atmospheres. Hence the GWO has orbited in phase space to octant 1 and a regional-scale response has been the retrogression of the “PNA ridge” into the eastern Pacific Ocean. The anomalous North Pacific Ocean jet has collapsed into the unseasonably very strong western USA trough leading to another severe local storms outbreak on Plains as I write.

As stated in previous postings, the WB (2007, 08) GWO, which considers the MJO and other tropical variability, offers a global dynamical weather-climate linkage framework to intelligently evaluate the numerical models. A couple of weeks ago the feeling was that the positive PNA USA ridge pattern was unlikely to persist. What adds confidence to offering a prediction based on the WB (2007, 08) GWO is when the numerical models start to support what scientific thinking suggests. The Day-10 deterministic ECMWF solution as well as the ESRL/PSD ensemble were the first models to capture the “PNA retrogression”. I applaud particularly ECMWF for such as wonderful Day-10 forecast (for the USA). Nonetheless, other models such as the GEFS were slow to catch on, and because of model disagreement so were the official forecasts for particularly week-2. My feeling is that this was a blown forecast of opportunity for several reasons.

As tropical convective forcing returns to the Eastern Hemisphere during the next 1-3 weeks (with the MJO), the WB (2007,08) GWO is probable to circuit into phase 3 as zonal mean easterly wind flow anomalies increase in the tropical/subtropical atmospheres. Supportive are animations of fields such as 150mb daily mean vector wind anomalies. They indicate twin tropical anticyclones ~30-60W, with some “hints” of returning to the Indian Ocean. Thus there may be June rendition of a circumglobal teleconnection of midlatitude ridges as our EL-Viejo base state may get some “new life”. A serious and critical monitoring issue from the interannual perspective will be to see if the tropical MJO remains strong and shifts into the west central Pacific Ocean during the next several weeks (monsoon systems understood).

Shifting northwest with the seasonal cycle, more periodic and anomalous western USA troughs are probable for the next 2-3 weeks. At some point the “Desert Southwest” monsoon ridge will appear; however, that may not be until July (which is climatology). The storm track is probable to remain active from the northern and central Rockies into the Upper Mississippi Valley leading to more rounds of heavy rain and severe local storms. Summertime heat should become established across much of the southern USA. Finally, please see statements from the Tropical Prediction Center for possible upcoming USA tropical cyclone hazards. During weeks 1-2 both the East Pacific Ocean and Caribbean may have the latter.

Intense to severe thunderstorm activity is probable to spread east from equatorial Africa into the Indian Ocean weeks 1-2. In fact, most recent satellite imagery do show clusters of thunderstorms increasing in those regions. The warming SSTs discussed above are often a precursor to the active phase of the MJO. During weeks 2-4, intense rainfall may become established from the Arabian Sea into Southeast Asia, eventually focusing on the Indian and Asian monsoon systems. The tropical cyclone hazard still cannot be ruled out for both the Arabian Sea and Bay of Bengal. The west Pacific Ocean “should” remain relatively suppressed at least week-1. I trust the expertise of the appropriate weather centers internationally to alert the public of additional weather hazards worldwide.


An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (X-axis) can be found at

This phase plot is being re-done, as is the web site. Stay tuned. We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous to the WH (2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.

Links to CPC and PSD ENSO discussions:

The following is a link to information about the stratosphere and other nice monitoring tools:

The following is a link to NCEP model verifications (surf around for lots more). .

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us (soon) to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification.

The WB (2007) paper on the GSDM has been published in the February issue of MWR. In addition, a two-part paper is in ACTIVE preparation by WB that will formally introduce the GWO along with subseasonal composites for variables such as surface temperatures. We want to emphasize notions such as global-zonal mean-regional scale linkages as well as forcing-response-feedback (with subsequent interactions) relationships.

Given shift work and travel, updates are extremely difficult. I should be able to post another discussion the weekend of 31 May – 1 June.

Ed Berry

1 comment:

Skid Mark said...

Ed, i have been reading your weather Blog for sometime now, thanks for keeping us informed!