Saturday, May 31, 2008

Ghost Riders -- Part Deux

Global tropical SSTs remain well above average from the East Pacific into the Atlantic Oceans with anomalies ~1-3C and totals near 30C south of Mexico and about 29C along the equator east of South America. La-Nina associated SSTs are relatively weak with negative anomalies only ~0.5-1C along and east of the equatorial date line. Large-scale warm and cold anomaly spatial horseshoes remain well defined across the Pacific Ocean basin (negative phase of the PDO?). SST anomalies across the Eastern Hemisphere are also fairly weak (generally less than 1C) having a loose warm-cool-warm spatial pattern from the Indian into the west central Pacific Oceans. The latter connects to the warm Pacific Ocean horseshoe, with totals ~28.5-30C.

Latest 5-day averaged TAO buoy data continues to indicate expanding subsurface warmth (~plus 3C) down to ~200m west of the date line to west of South America (~plus 4C at 50m). Restating from a week ago, regardless of what tools are believed to be useful, the interannual SST component including ENSO is unclear. However, careful monitoring of upcoming subseasonal variations is extremely critical (only a handful of these issues can be addressed below given this presentation medium). These variations WILL impact the global weather including the USA, irrelevant of any “Nino or whatever” indices, classifications, and so forth. Anything to the contrary is not scientifically defensible.

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

Full disk satellite imagery and other diagnostic monitoring tools show there has been a significant increase of intense-severe clusters of thunderstorms from the equatorial Arabian Sea and Indian Ocean east-northeast into Southeast Asia (3-day OLRA ~minus 50-70 W/m**2). Enhanced tropical rainfall persists across northern South America and particularly equatorial Africa. The nemesis of the west Pacific Ocean is still hanging around, with Typhoon Nakri spawned since my last writing.

Updated through 30 May, the latest WH (2004) phase space plot shows a much weaker MJO projection, barely above 1 sigma, than a week ago, in octant 1 (with and without the interannual component). Should this be interpreted as a decaying MJO? There are literally an endless number of responses I could offer to that question. However, the point is we have an important evolution of tropical convective forcing and associated circulation anomalies shifting from the Western into the Eastern Hemispheres.

Continuing on the above, strong tropical-extratropical coupling continues (recall last week’s discussion of events since mid April). As was observed earlier this year, once again there is a well defined inter-hemispheric meridional symmetry of zonally symmetric zonal mean circulation anomalies. However, there are some subtle differences (a few discussed below) involving the exchange of momentum between the solid earth and free atmosphere. The WB (2007, 08) GWO, which captures the global dynamical AAM processes including any MJO contribution, has orbited in phase space to nearly octant 3. Without further confusing the reader, another point is that the global circulation, at least for now, is returning to a La-Nina base state.

Updated through 29 May, global relative AAM has dipped to ~minus 1.5 AMUs below the R1 data climatology, with its tendency ~minus 5-10 Hadleys. While the global integral of the surface torques (and tendency) may not appear that large (~0 for the torques), that is anything but true for specific latitude bands of zonal mean frictional and mountain torques as well as the AAM transports.

This above is another example where I feel rather comfortable knowing “how we got here”, but have an extremely difficult time sharing that. It is only a simple matter for me to go back through numerous animations of many fields, daily averaged and longer periods, linking those observations to WB (2007, 07) GWO variations and be able to offer scientifically defensible null hypotheses.

Most broadly, the symmetry I am writing about involves anomalous zonal mean westerlies ~60-70N and S, as well as 30-35 N and S. Anomalous easterlies are present in the upper midlatitudes and the tropical and subtropical atmospheres. Hence the zonal mean frictional torque anomaly is ~plus 2 Hadleys near 45S and ~plus 1 Hadley around 50N. The latter is significant since this is opposite to what was seen during roughly February, and represents one of those subtle base state changes I am discussing. Obviously the overall gradual increase in global relative AAM since roughly early March in addition to the dynamical processes leading to MJO #5 are another example of slow base state changes.

Summing up, there has been a tendency for the WB (2007, 08) GWO to slowly shift away from La-Nina since mid March. Please keep in mind that while I type “La-Nina”, “El-Veijo”, or whatever, this is only a component of a complex global (ocean-land-atmosphere) interannual signal I am really talking about (to not have a “failure to communicate”). Remembering that, subseasonal events constructively and destructively interfering with this basic global circulation state (which are “unusual for La-Nina” – see past postings) have led to the barrage of anomalous western USA troughs since at least December 2007. Numerous high impact/extreme weather events of several types (tornadoes, blizzards, flooding rainfall, etc.) affecting most of the country have accompanied these troughs.

Okay, so where do we go from here? Zonal mean flux convergence of AAM transport (greater than 12 Hadleys) has maximized ~40N with sinks to the north and south. A similar process is occurring in the Southern Hemisphere. The dynamics tied to the tilts of midlatitude eddies interacting with the surface torques and the tropical forcing are at least partly responsible. Regional-scale responses have included a robust extension of the Australian jet (austral winter) and soon the jet across North Pacific Ocean. The latter is what the models are keying on for the next western USA trough to slam the northern and central Plains (etc.) with possibly another vicious and destructive round of severe local storms next week.

Animations of upper tropospheric daily mean vector wind anomalies continue to present the expected circulation responses consistent with the WB (2007, 08) GWO. Twin upper tropospheric tropical anticyclones are becoming established (coming back) in Africa and the western Indian Ocean, while cyclones develop in the region of the date line. Rossby wave energy dispersions (RWDs) are strongly zonally oriented across the midlatitudes.

My feelings are for the tropical convective forcing to become better established from roughly the Indian Ocean into the Southeast Asia (impacting the local monsoon systems) by week-2, if not sooner. I also think this enhanced rainfall may take on more of a west-northwest to east-southeast orientation from around India and particularly Southeast Asia into the west central and northwest Pacific Ocean weeks ~2-4 (also impacting Indonesia). Remember timing is white noise and I am offering a statement of probability. Should significant tropical forcing shift east along the equator with an accompanying westerly wind event, interannual ramifications may become non-trivial. Regardless, the latter is a real possibility (and worth the speculation), and may start the process of transitioning to an equatorial Pacific Ocean warm event.

The WB (2007, 08) GWO may not only orbit to phase 5 during the next 1-3 weeks, but even drift toward the “El-Nino side”. The WH (2004) MJO phase plot may also do a similar behavior. Focusing now on the USA, above average subtropical/midlatitude zonal mean westerly wind flow suggests more troughs to penetrate inland into the western states going well into June. Of course, these will be shifted northwest, and impact mainly the Northern Plains and Upper Mississippi Valley as they progress. Whatever the case, I do think the general pattern of “cool northwest-warm southeast” with wetness in the middle is probable through at least weeks 2-3. Included are possibilities of additional rounds of severe local storms (and flooding rainfall) including tornadoes perhaps focusing on Iowa and surrounding states. Taking into account the WB (2007, 08) GWO, should tropical forcing start to shift farther east during boreal summer, cool and wet may also shift into the Plains and even portions of the eastern states (ridge from Hell returns to the west coast?).

As discussed above, portions of India/Indonesia into Southeast Asia/China may be impacted by intense to severe thunderstorms during the next few weeks. During weeks 3-4 that area of enhanced rainfall may become focused from Southeast Asia into the Philippines to the west central Pacific Ocean. Please see the latest statements from the JTWC and TPC for tropical cyclone concerns. While the risk for the latter may decrease across the East Pacific Ocean and Caribbean weeks 1-2, there may an increased tropical cyclone risk for the northwest Pacific Ocean weeks 2-4. I trust the expertise of the appropriate meteorological 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). .

The following is a link discussing recent global weather and related events.

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 7-8 June.

Ed Berry


Justin Gehrts said...


Thanks for making these weekly posts. While most of the "why" is way over my head, the ramifications of the "whys" are very helpful. I work at a TV station in Rockford, IL and often share your thoughts with my chief meteorologist so we can look out for and key in on the trends you discuss. It's a great additional resource - keep up the good work!

Ed Berry said...

Hi Justin,

Thank you for the kind comments! Hopefully Klaus and I will have a dedicated web page at some future time where we can illustrate some of the subseasonal behaviors with figures. That would be a start to help with communicating the "whys".