Short

Since it unlikely I will not be able to write another posting until (hopefully) next weekend (8-9 September), I thought to do a brief update. There is no change to my reasoning I discussed on 1 September. The spatial pattern of global tropical SSTs supports the possibility of a coupled ocean-atmosphere La-Nina response through at least boreal fall into early winter. Hence the continuation of a low AAM GSDM Stage 1 circulation is most probable.

The dominate global tropical convective forcing has loosely consolidated ~10N/80-90E, per monitoring tools such as full disk satellite imagery and various Hovmoller plots. There is some evidence of another east-northeast shift of the tropical forcing toward western Indonesia and Southeast Asia. I think this movement is a bit farther south than the past 3. However, another burst of strong convection is developing back to the west ~0/60E, illustrating the stationary character of our atmospheric base state.

Global relative AAM tendency has spiked to ~plus 30 Hadleys per R1 data plots through 31 August, and much of that appears to be driven by a decent global mountain torque (also ~plus 30 Hadleys). There also appears to be an equatorial contribution to this positive AAM tendency, perhaps linked with a weak west central Pacific tropical convective flare-up. Bottom line is I think we are having another perturbation to our GSDM Stage 1 regime forced by the global mountain torque and the weak eastward movement of the tropical convection discussed above. When updated, this will be observed as another circuit in GWO quasi-phase space well on the left hand side of the ESRL/PSD plot, similar to the event roughly a couple of weeks ago.

Trough development from the Rockies into the Plains for week 1 is a good bet. A closed low may anticyclonically wave break (AWB; probable to occur during a GSDM Stage 1 base state) across the central Rockies later this week before shifting northeast toward the Great Lakes. I still like the notion of a stronger and colder trough for the Rockies and Plains week 2. However, throughout this upcoming fall the polar jet stream westerlies may remain farther north on average than climatology across North America given our GSDM Stage 1 circulation. Western/central USA troughs may be a function of brief upward GWO orbits. By around mid-late November into December the seasonal cycle contribution may allow a more persistent and at times highly amplified negative phase of the Pacific-North American teleconnection (favoring anomalously strong western USA troughs). WB (2007) has already showed that the latter is probable during GSDM Stage 1 situations for the period of November-March. I hope we can formally illustrate these kinds of concepts much more objectively with signal to noise ratio plots and risk assessment maps for a period such as December-February (DJF).

The outlook internationally is unchanged from 1 September. A modification may be for a somewhat more active Tropical Northwest Pacific Ocean in the region of the South China Sea per above later this week into week 2. Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone information. The inability of Hurricane Dean and thus far Hurricane Felix to exhibit much of a northward movement from the deep tropical Atlantic Ocean is a result of the persistent anomalous zonal mean easterly wind flow throughout the tropical atmosphere. I still see quite a bit of AWB activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. In fact, Gabrielle may be the result of a hybrid frontal wave development off the USA east coast per some models.

Appendix

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

http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg

http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg

http://www.cdc.noaa.gov/map/clim/gsdm.shtml

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

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 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 an update during the weekend of 8-9 September.

Ed Berry