Sunday, November 18, 2007

El-Viejo Update

This posting will be short. One motivation for attempting to “get something out” is my concern for a change to a significant to perhaps severe winter regime focusing on the USA Rockies and Plains by week-3 (roughly the first week in December). Of course, there are always timing issues, and the above statement is made from my conservative perspective when it comes to making any kind of weather predictions.

The same weather-climate issues discussed on my 10 November, 2007, posting remain. The dominate tropical convective forcing has shifted east to ~0/140E during the past week. This was more of an eastward push than I expected a week ago; however, not surprising. Several tropical cyclones have been and still are occurring across the Indo-Pacific region. I am optimistic that the Bay of Bengal tropical cyclone season is “over with”. The low confidence amplification across the Asian-North American sector I also offered a week ago is in progress as I type. Numerical ensemble prediction schemes have been “playing catch-up” to this initial first regional scale response to La-Nina and tropical convective forcing.

Global relative AAM has risen slightly to ~minus 2 standard deviations below the R1 data climatology (14 November last update) with the recent development of equatorial zonal mean upper tropospheric westerly wind flow anomalies. The latter are most robust west of South America, having magnitudes ~20-30m/s. Anomalous twin upper tropospheric subtropical anticyclones have redeveloped ~120-140E, forced by the tropical convection. Rossby wave energy dispersions from these anticyclones will deepen a trough into the Rockies and Plains by the middle of this upcoming week, leading to what may be this cold season’s first winter storm for the Upper Mississippi Valley and Great Lakes regions.

The punch line is that the above mentioned tropical forcing may be the final step for mature coupling to La-Nina. Typically for the ENSO “cycle”, often a MJO or other tropical convective forcing leads to mature coupling during January of the annual cycle. We are concerned this process is starting sooner. Zonal mean subtropical easterly wind flow anomalies remain very intense poleward of the equatorial westerlies already discussed across both hemispheres. At this time, an evolution toward phase 5 of the Global Wind Oscillation (GWO; legacy GSDM Stage 2) appears to be in progress. Updated through 14 November global relative AAM tendency was ~plus 20 Hadleys with the calculated tendency ~plus 30 Hadleys.

Even though statistical tools such as the WH2004 RMMs already indicate and predict a robust MJO during the next couple of weeks, I disagree with that notion. One important point to remember is that these types of techniques do remove the ENSO signal, and that is a problem given the current situation. I do think it is probable for a fast dynamical signal to propagate through the Western Hemisphere over the next couple of weeks. However, as already indicated by satellite imagery, the tropical convective forcing ~140E is starting to “break apart”. One portion should retreat back to the west-northwest while the other enhances a westward shifted South Pacific Converge Zone (SPCZ). Additionally, tropical convection is re-firing across the warm Indian Ocean just south of the equator. Bottom line is tropical forcing should re-intensify back to west ~80-120E during the next few weeks, forcing the GWO back to ~phase 3 (legacy GSDM Stage 1).

For the PNA sector, I think there is a real possibility for a corresponding westward shift of circulation anomalies by early December, perhaps leading to a blocking ridge into Alaska ~140-150W and extratropical cold lows even impacting the USA west coast before coming inland. Cold Arctic air has been recently building up across portions of Siberia into northern Canada. If there is any correctness to these notions, this Arctic air may dump into the USA including locations west of the Continental Divide starting week-2. Weeks 3-4 may see the classic moist southwest flow storm track across the Plains, with intense cold focusing on the Rockies and Northern Plains while the Deep South has above normal temperatures. Areas such as the Desert Southwest and the southwest High Plains may also have decent precipitation in spite of the La-Nina signal. Other weather impacts may include snowfall for Pacific Northwest cities such as Seattle and severe local storms and heavy rainfall from the lower Mississippi Valley into the Ohio Valley.

Finally, any seasonal mean DJF 2007-08 anomalous cold air surface temperature signal which could in reality occur for portions of the USA may be dominated by December. It will be interesting to observe subseasonal activity starting January as tropical convective forcing shifts toward the very warm west central and southwest Pacific Ocean. Will we start to see our "Nemesis"? Is El-Nino in the cards for 2008-09, perhaps even a strong one???

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center and other international centers for tropical cyclone statements. Areas including the Northwest Pacific Ocean around the Philippines, as well as from the South Indian into the Southwest Pacific Oceans including the "paradise islands" may have concerns for tropical cyclones through week-2. Finally, episodes of strong/severe synoptic baroclinic cyclonic systems hammering at least portions of Scandinavia, Europe into the Mediterranean countries/northwest Africa remain probable through week-2.


An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (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 the WH2004 “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Links to CPC and PSD ENSO discussions:

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment composite maps, composite 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. I will try to post another discussion around late next weekend or early week-2.

Ed Berry

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