Friday, April 25, 2008

Postponed; A break from El-Viejo???

Shift work and other issues preclude a complete discussion until hopefully next weekend. In summary, the friction-mountain torque index cycle variation addressed on the 19 April posting is evolving. A response has been the expected “downward turn” in the WB (2007, 08) GWO phase space, in phase 4 updated through 22 April.

Magnitudes of daily mean vector wind anomalies at 250mb on 24 April were in excess of 30m/s over northeast Asia which is directly attributable to the ~25 Hadley East Asian mountain torque updated through 22 April. This jet streak, as part of fast Rossby wave energy dispersion (RWD) processes interacting with the intensifying Eastern Hemisphere tropical convective forcing (more said below), is likely to dig a trough into the western USA next week. More and more numerical models have been trending toward the above with varying solutions. From a weather prediction point of view, I would favor the slower and farther south solutions such as the deterministic ECMWF model valid mid-late next week.

The tropical convective forcing has been steadily getting better organized across the Eastern Hemisphere during the last week. Full disk satellite imagery and other tools suggest the centroid of this intense-severe tropical rainfall at ~120E just north of the equator while extending from the eastern Indian Ocean east-southeast into the South Pacific Ocean Convergence Zone (SPCZ). Having ENSO removed, WH (2004) phase space plots show little projection onto a MJO. Leaving the interannual variability in, there is a greater than 1 standard deviation MJO projection in WH (2004) phase space, phase 4. Coherent modes Hovmollers suggest a weak MJO projection, along with some eastward propagation during the last couple of weeks. Regardless of whether or not we have a “true” MJO, as often observed this past boreal cold season, tropical-extratropical coupling is again occurring leading to approximately the same phases of both the GWO and MJO (latter leaving ENSO in). The dynamics explained by the GWO (including AAM transports in addition to the surface torques) give some reasoning to these types of interactions. Again, the GWO is a dynamical measure of the global and zonal mean circulation.

I think it is probable for the Eastern Hemisphere tropical convective forcing to shift into the west central and southwest Pacific Ocean during the next few weeks. While the details of GWO evolution are always unclear (red noise processes are part of it), I also think it is probable to see a circuit into phase 5, before once again collapsing. Animations of upper and lower tropospheric daily mean vector wind anomalies not only present a strong signal of the expected tropical baroclinic response to the convective forcing, but also interactions with RWDs outside the tropics, all supportive of the above. Additionally, various diagnostics show strong zonal mean easterly wind flow anomalies propagating poleward well into the subtropical and midlatitude atmospheres, ~25-30N and ~40S. Tied to the above, zonal mean westerly wind flow anomalies have been increasing in the equatorial upper troposphere during particularly the last week-10 days. Much of that is from the Western Hemisphere Pacific Ocean linked to the twin cyclones near the date line. An important monitoring issue is if these anomalous westerlies also propagate poleward and downward during the next few weeks.

Once the tropical convective forcing comes out into the west Pacific, meridionally oriented RWDs are likely, perhaps shifting a trough farther east into the Plains. Given higher latitude blocking forced by zonal mean AAM transport considerations, a cold and wet pattern for much of the lower 48 states focusing on the central states is a forecast option by ~weeks 2-3.

In the longer term, La-Nina SSTs continue to moderate. However, the global and zonal mean circulation base state still has a good memory of this past “season of the witch”. Stay tuned.

Links below are to additional SST information. (note the initial projection) (link 18).

There is no change to the USA and global outlooks discussed on 19 April. As mentioned above, the greatest negative anomalies for below normal temperatures may focus on the central states ~weeks 2-3. Beyond that, retrogression of existing circulation anomalies not only due to subseasonal activity but also climatology may occur shifting an active storm track northwest. I continue to leave it to 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

WB are in the process of redoing the GWO phase space plot to make it appear more realistic, physically. 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:

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. Given shift work and upcoming travel, updates will always remain extremely difficult. I will try to post another discussion next weekend, 3-4 May.

Ed Berry

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