Orbiting in GSDM Phase Space

Shift work obligations and upcoming travel for at least the next 2 weeks will impact the timeliness and completeness of these discussions, including this writing. However, my thinking from 4/17 remains unchanged. Zonal mean anomalous upper tropospheric easterly wind flow continues to increase throughout the tropical and subtropical atmospheres. Magnitudes are ~5m/s contributing to a negative global relative atmospheric angular momentum (AAM) tendency of about minus 15 Hadleys as of 4/18. Twin upper tropospheric subtropical anticyclones are becoming better established over the Eastern Hemisphere and actual relative AAM is approaching 1 standard deviation below the 1968-1997 NCEP/NCAR reanalysis climatology.

A fairly large region of enhanced tropical convective forcing extends from eastern Indonesia into the South Pacific. This rainfall has been locally forced by warm SSTs and interactions with the extratropics. Upper tropospheric Western Hemisphere westerly wind flow anomalies across the Pacific have been a response, and have contributed to the southern USA subtropical jet. This region of tropical convection is slowly moving east-southeast enhancing the South Pacific Convergence Zone (SPCZ). Meanwhile, monitoring tools suggest a struggling dynamical signal from the equatorial Atlantic into the Indian Ocean.

I think the global circulation remains in GSDM Stage 4, and it now appears probable to me it will stay roughly in that phase plane until the above mentioned dynamical signal becomes robust. In fact, the trajectory shown on the referenced plot in the Appendix may do ~1-2 circles before we finally get a MJO sometime in May. My feeling is those equatorial westerlies discussed above will contribute to another extended North Pacific jet by week 2 (which many ensembles have picked up) and then slam the western USA with more strong troughs by weeks 3-4. Thus after an active week 1 for much of the USA, week 2 should be relatively tranquil followed by exceptionally active May weather weeks 3-4 (shifted slightly north from April). Most readers should know the ramifications of strong mobile western states troughs and a southwest flow storm track on the Plains during May. For instance, several rounds of severe local storms on the Plains, late season snowstorms across the Rockies and summertime heat across the Deep South would be probable.

Appendix

An experimental phase space plot of the GSDM utilizing normalized relative AAM tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

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

These are probabilistic statements, which we will try to quantify in future posts. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I should be able to write an update by late next week.

Ed Berry