Friday, March 02, 2007

Brief Update -- Global models may perform like the recent global markets

The dynamical signal with the MJO is centered at ~10S/140E per several monitoring tools including the multivariate EOF Wheeler index and animations of circulation anomalies. Rough calculations of its phase speed give me anywhere from 5-8m/s depending on what variable you use; for example, velocity potential or OLRA. In any case, this recent MJO event has been a very difficult one to monitor and understand. In fact, I think what has happened during the past week is a consolidation between the MJO dynamical signal and westward propagating convection associated with west Pacific SST boundary forcing. The latter is a slower process that started about mid-January 2007 and may be part of the evolution of a transition from El-Nino to La-Nina.

My point in this short writing is I do not think the global models have “much of a clue” of the responses to the above. Global relative AAM tendency is ~plus 10-15 Hadleys per reanalysis data with much of that signal coming from the equatorial and subtropical atmospheres of the Western Hemisphere. Contributions are from the tropical forcing crossing Indonesia and an increase in the global frictional torque (while global relative AAM remains ~minus 2 sigma). I think the global circulation is trending toward GSDM Stage 2. This argues for ridge amplification across the eastern Pacific (~140W given seasonal cycle) during the next few weeks, most likely week 2. The global models (ensembles) do not depict this very well. They generally want to persist a trough across the Gulf of Alaska downstream of the central Pacific blocking.

I would not want to be a person investing money banking on “good” global model performance during the next few weeks. A decline in, for example, the skill of week-2 ensemble model means going into the middle of this month, appears probable (similar to the recent global markets?). Sources would include not only the excitation of a west Pacific wavetrain forced by Rossby wave energy dispersion tied to the east Indonesia/west Pacific tropical forcing, but also seasonal transition. My thought would be for the Gulf of Alaska trough to dig southeast along the USA west coast (impacting ARB) during week 2, then shift into the Rockies leading to a resumption of a stormy regime for much of the USA. This would be similar to recently observed, but with an Arctic cold air source. If this MJO signal stalls, say ~150E, then shifts back to west, this regime may persist for a period longer than 1 synoptic event.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I will try to post a more complete discussion ~Tuesday next week while at ESRL/PSD. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

No comments: