Friday, March 21, 2008

Like the Atmosphere -- Delayed

Lack of time precludes a discussion until next weekend. In summary, there is little change to my thoughts as written 15 March. Tropical convective forcing has become better organized across the Eastern Hemisphere and is currently propagating east. The core of this enhanced rainfall is on the equator at roughly 110E. The tropical circulation is responding with anomalous twin upper tropospheric anticyclones around 60E and downstream cyclones near 150E (baroclinic mode). RWDs tied to this forcing are leading to a discontinuous retrogression of circulation anomalies across the extratropical PNA sector. For example, a large anticyclonic gyre is developing ~45-50N/160E having wind speed anomalies ~30-40m/s at 250mb.

A serious monitoring issue is if there may be ocean-atmosphere coupling to the very warm (with deep positive anomalies) west central-South Pacific Ocean SSTs during the next few weeks once the tropical convective forcing comes out. We are also at the time of year when equatorial east Pacific Ocean SSTs are the warmest climatologically especially west of South America. In fact, equatorial SST totals from ~120W to the date line have warmed to at least 25-27C going along with the recent weakening of the cool anomalies (~minus 1-2C; lowest in the region of the date line per TAO buoy data). Bottom line; stay tuned about the future of El-Viejo.

The tropical forcing and other dynamical processes linked to the extratropics are beginning to add some westerly wind flow to the global atmosphere, with a large contribution coming from the Western Hemisphere upper troposphere across the equatorial Pacific Ocean. Hence there is some poleward propagation of the anomalous zonal mean easterly wind flows into the subtropical atmospheres of both hemispheres. However, global relative AAM is still ~3 standard deviations below the R1 data climatology (through 17 March) as part of a strong projection in GWO phase 3. Whether or not we get a robust orbit to phase 5 of the GWO during the next few weeks is unclear.

In any event, I again need to emphasize that whether or not the tropical forcing discussed above is evolving into MJO #4 is irrelevant. That component, working with other dynamical processes considered within the GSDM framework, will have impacts on the global weather including the USA during the next few weeks.

A modification I think that should be added to the USA outlooks discussed last weekend is a growing concern I have about the storm track becoming displaced somewhat to the north. I have been suggesting some possibility of southward shifts at times due to feedbacks from higher latitude blocking (taking into account what follows below).

However, zonal mean easterly wind flow anomalies propagating off the equator into the subtropics suggest (in this current situation) a significant increase in the probability of anomalously strong mid-latitude ridges. Hence, at least for the next few weeks the greatest impacts from western USA troughs moving into the Plains may become more focused across the northern half of the country. The latter is not good news for locations such as southwest Kansas into west Texas where drought may significantly worsen.


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

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, and other information:

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 preparation by WB that will formally introduce the GWO. Given shift work and upcoming travel, updates will be difficult. I will try to issue a discussion the weekend of 29-30 March.

Ed Berry

No comments: