This posting will be somewhat less detailed. I will try to post a more complete discussion around Tuesday next week.
Our global circulation state is seemingly appropriate for Friday 13th. R1 AAM data plots show a recent negative global tendency of ~20 Hadleys, suggestive of GSDM Stage 1. Without the QBO global relative AAM is ~ near normal with inter hemispheric symmetry of zonal mean zonal wind anomalies. There are zonal mean anomalous westerlies along the equator (~5-7 m/s at 200mb) flanked subtropical easterlies with westerlies then easterlies poleward. Much of the negative global AAM tendency has come from the mountain and frictional torques as well as the transport signal. The latter has been most pronounced in the Southern Hemisphere.
Tropical convective forcing remains quite active from the central Indian Ocean into the west central Pacific, centered ~10N/90E per full disk satellite imagery. An intense band having three-day averaged OLRA ~minus 50-90W/m**2 extends from the far west Pacific to Typhoon Man-Yi which is starting to impact Japan. Meanwhile, there is a weak dynamical signal of tropical forcing moving through the Western Hemisphere centered on the Americas. A loose phase speed calculation gives me ~17m/s, and it does project onto a convectively coupled Kelvin wave. I think this Western Hemisphere signal is a remnant of the recent MJO. A possibility is for this signal to come back into the Eastern Hemisphere during week 2 and re-invigorate convection across Africa into the very warm western equatorial Indian Ocean (SST anomalies 1-2C). In any event, I think it is probable for our stationary tropical forcing tied to the South Asian Monsoon system to persist for at least the next 2-3 weeks as faster components interact with it. This is consistent with our tilt toward weak La-Nina in terms of ocean forcing and Eastern Hemisphere tropical rainfall distributions.
So, all the above makes a good case for the weather-climate situation to be in GSDM Stage 1, right??? Animations of various wind fields including upper tropospheric and surface daily mean vector wind anomalies support an on-going circulation evolution from GSDM Stage 2 to Stage 3. This observation again demonstrates the need to understand the dynamics of forcing-response-feedbacks/subsequent interactions processes within a weather-climate linkage framework as part of any subseasonal prediction effort.
I think we are finally seeing a more “typical” global circulation response to the past extended period of strongly anomalous Western Hemisphere upper tropospheric equatorial westerly wind flow. These westerly wind anomalies are still present, but with magnitudes of ~5m/s in contrast to ~40m/s a couple of weeks ago. The equatorial westerly wind flow anomalies have been slowly propagating poleward and downward, particularly into the Southern Hemisphere. Nevertheless, the current ~5-10m/s surface west wind anomalies across the North Pacific Ocean ~35N are a response to this dynamic process. Remembering how the Wheeler phase space plot is derived (15N-S domain only), it is these kinds of wind anomaly signals which are leading to its greater than 1 standard deviation MJO projection currently across the Western Hemisphere. However, I argue that any real MJO signal is very weak.
There are weakly anomalous upper tropospheric tropical/subtropical cyclones across the Indian Ocean with downstream anticyclones ~140E. Loosely, this is a reversal of Eastern Hemisphere circulation anomalies from a couple of weeks ago. In fact, map plots of the latest daily mean 150mb vector wind anomalies gives a nice signal of Rossby wave energy dispersions (RWDs) from these west central Pacific anticyclones into the extratropics of both hemispheres. The current western USA ridge is directly linked to the RWD arcing across the North Pacific Ocean from the twin anomalous subtropical anticyclones.
Cutting to the chase, as I type the North Pacific Jet is strengthening (for this time of year). The numerical model ensembles have captured this response. Hence the prediction of a trough-ridge-trough pattern west-east across the USA in about a week looks reasonable. However, even for week 1 uncertainty remains huge in regards to position and amplitude. Other predictions from operational centers may be too over confident in their week 1-2 outlooks. After all, why would that not be the case since we are seeing (in a sense) a Stage 3 circulation with Stage 1 tropical convection! My offering for weeks 2-4 would be for a retrogression of this pattern (again) as tropical forcing remains anomalously strong across the Eastern Hemisphere. It is also probable for the tropics and extratropics to become in-sync as the GWO orbits around GSDM Stage 1 during this time.
Global temperature and precipitation anomalies/high impact weather concerns include the following. Locations from India into Southeast Asia are likely to experience recurrent poundings of severe storms and flooding rainfall. In fact, this situation may worsen weeks 2-4. After Typhoon Man-Yi goes by Japan, tropical cyclogenesis looks less favorable through at least week 2 for the northwest Pacific. The East Pacific should be favorable for tropical cyclogenesis week 1 while the tropical Atlantic stays relatively tranquil ~weeks 1-2. The record cold that has been occuring over much of Argentina and Chile should moderate during week 1.
During week 1 the western USA heat is probable to expand into the Northern Plains/Upper Mississippi Valley while the Pacific Northwest gets welcome cooling. Wet conditions are likely to persist across Texas while thunderstorm activity increases across the Rockies. This situation may last through much of week 2, with temperature anomalies then reversing week 3 due to retrogression. Intense/severe MCS activity may focus on the northern/central Plains later week 2 and week 3 as the process of retrogression occurs.
Appendix
An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (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
These are probabilistic statements, and work is ongoing to quantify in future posts. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor and thoroughness. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I hope to post a decent discussion ~Tuesday next week. In general, due to covering shifts and travel, my postings on this Blog will be irregular through at least August.
Ed Berry
No comments:
Post a Comment