Orbiting Around Stage 1?

Note, the ESRL/PSD R1 AAM plots only have data through 6/22

It is current situations like the present that make writing these postings extremely difficult. For example, the shorter wavelengths of boreal summer are contributing to some of the difficulties(in addtion to my steep learning curve). There are also slowly evolving components that I can “trace back” to December 2006 and even 2002 that I think are also adding to the complexity. Understanding the latter requires disciplined daily monitoring within the GSDM diagnostic framework to have any real hope of improving our understanding of the atmosphere and making better week 1-4 and beyond predictions. At least from speculation, I actually have some comfort where I think the atmosphere is going during the next few weeks and perhaps much longer. If we had a dedicated properly supported web page effort I am confident what I offer below would be more objective (and complete) and not speculation.

Per CPC daily mean analysis, SSTs remain well above average from the west central into the northwest Pacific Ocean with widespread anomalies ~0.5-2C and totals from 29-31C. The warmest waters remain across the South China Sea (SCS). Most of the equatorial Indian Ocean including the Arabian Sea has cooled during the last few weeks given the recent intense tropical rainfall. However, weak warm anomalies are still present along the equator in that region. The trade wind surge during the last week-10 days across the equatorial date line and central Pacific Ocean cooled the SSTs by ~.5-1C. In fact, per TAO data, the oceanic thermocline has been raised east of the date line leading to ~minus 3C anomalies at ~150W/150m. This region of subsurface cool water remains detached from the near normal SSTs above them and the weakly cool anomalies west of South America. Above average SSTs still remain southwest of Mexico into the Caribbean (totals ~28-30C) as well as the eastern Tropical Atlantic (totals ~27-28C).

Again, development of La-Nina remains uncertain. However, we think a tilt toward weakly La-Nina conditions this summer followed by a transitional situation by winter is an option. This notion is partly based on the biennial character of the occurrences of warm and cold events (loosely) since the 2000-01 boreal winter. Also, a strong El-Nino may be “looming”.

The MJO dynamical signal has weakened significantly. For the example, the zonal wave number 1 spatial pattern of tropical/subtropical 200mb velocity potential anomalies has broken up into regional structures during the last few days. In addition, many phase space plots involving the first 2 EOFs of a mutivariate MJO index (equatorially averaged OLRA, 850mb and 200mb vector wind anomalies) no longer show a MJO projection. However, there is still significant tropical convective forcing occurring across the Eastern Hemisphere, and this has and will continue to impact the extratropics until further notice.

Coherent modes Hovmollers suggest that the recent intensification of convection across the west central Pacific may have been linked to a convectively coupled Kelvin wave. Meanwhile, these same plots still show a MJO projection across the northern subtropics centered ~10N/100E per full disk satellite imagery. In fact, a back of the envelope phase speed calculation gives me an eastward propagation of ~4m/s since early this month. Whatever the case (and to save space), we did get our separation of the tropical forcing as discussed a week ago (with weak tropical cyclones across both the Arabian Sea and Bay of Bengal). My feeling is some the tropical forcing centered ~10N/100E will shift generally eastward into the exceptionally warm Tropical Northwest Pacific (TNWP) including the SCS, while convection still remains active possibly as far west as India and the Bay of Bengal (above climatology). I think a component moving into the TNWP is reasonable given there have been ~30-day periods of suppression then enhancement of west Pacific Ocean tropical convection going back to January 2007 linked to extratropical variations.

Restating, from an atmospherics dynamics viewpoint I think the Eastern Hemisphere monsoon systems have become well established, particularly the South Asian system. Animations of daily mean 150mb vector wind anomalies show well developed twin tropical anticyclones from Africa into the Indian Ocean having recent weekly mean anomalies in excess of 20m/s across the Southern Hemisphere (per FNL data). The Webster-Yang index (which also considers the Somalia Jet) shows recent negative vertical wind shear of ~25m/s supporting this observation. The point is the recent intense Indian Ocean tropical forcing (which I can also trace back as a westward shift from central Pacific Ocean forcing starting in January 2007) has fed back to the atmosphere and I think the atmosphere will feedback with additional convection in that region in spite of cooler SSTs. My suspicion is there may be a relatively stationary signal of tropical forcing centered ~10-15N/120-140E starting ~week 2 and continuing through the rest of this summer. Of course, an equally valid option is the 2 regions discussed last week, meaning there is still huge uncertainly. Again, whatever the details it appears probable the Eastern Hemisphere will “call the shots” the rest of this summer, and is one of our reasons for a tilt toward weak La-Nina conditions.

Animations of upper tropospheric daily mean wind fields also present an emerging signal of anomalous twin tropical/subtropical anticyclones ~120E. This is a fundamental change from a week ago where cyclones were present. I think this change is the result of the eastward propagation of the convection north of the equator discussed above. Rossby wave energy dispersions from the extratropics have maintained anomalous upper tropospheric Western Hemisphere equatorial westerly wind flow with weekly means ~25-30m/s west of South America. Tied to this westerly flow are subtropical cyclones just east of the date line and even across the western Atlantic. The AAM tendency zonal mean anomaly plot as well as time-latitude sections of 200mb zonal mean zonal wind anomalies show this anomalous westerly flow is propagating into the subtropical atmospheres, especially the Southern Hemisphere. As I type a summertime version of the East Asia Jet is expanding into the North Pacific as a response.

So, where is the atmosphere going? For the past couple of weeks I have maintained there would be an evolution to GSDM Stage 2 leading to a cool/wet regime focused on the central USA. As of ~18 June, per Global Wind Oscillation (GWO) phase space and R1 global AAM tendency plots, GSDM Stage 2 represented the weather-climate situation. Tied to the very strong subtropical westerly flow discussed above and extratropical Rossby wave energy dispersions linking both the recent Indian Ocean forcing to the anticyclonically wave breaking subtropical central Pacific lows, the PNA trough-ridge-trough pattern I expected set up is going to be about 20-30 degrees of longitude farther east. These same Rossby wave energy dispersions led to the current negative phase of the NAO and recent anomalous ridges across the North Atlantic Ocean. The model forecasts of ridge conditions across the Rockies a couple of weeks ago were correct; however, there has been a catch-up game to the magnitude of the amplitude tied to the dynamics discussed above. A cooler and wetter situation is becoming established across the central states but for a somewhat different synoptic evolution than I advertised a week ago. This serves as another example where this type of predictive information must be expressed probabilistically since synoptic details and timing are white noise after about 5 days (as is the prediction of tropical convection by the models).

As of the 20 June the GWO phase space plot (5-day average) has started an orbit back toward GSDM Stage 1. The global AAM tendency on 22 June was barely positive since (in addition to other terms) both the global mountain and frictional torques started to decrease. The former has a contribution from lowering mean sea-level pressures (MSLP) across East Asia while the latter is from the trades gradually weakening. Global AAM is still ~1 standard deviation below the R1 climatology, and the frictional torque itself remains well above average (~15 Hadleys). Linked to the Eastern Hemisphere tropical forcing I suspect MSLP anomalies across East Asia may become strongly negative during the next week helping to drive down the global mountain torque to well below normal. This may be part of the ~40 day variations of the global mountain torque discussed in past postings. This will drive AAM tendency downward and may force the GWO close to Stage 4 (if this has not already happened). Overall, orbits (magnitudes unclear) around GSDM Stage 1 may be story the rest of this summer.

What this means to me is there should be a relaxation of the anomalous Western Hemisphere equatorial and subtropical upper tropospheric westerly wind flow. It is not unreasonable to expect anomalous zonal mean easterly flow developing throughout the tropics during the next few weeks. Finally, the East Asian Jet (for this time of year) should retract allowing the USA trough-ridge-trough pattern to retrograde after week 1. Interestingly, this notion is consistent with the statement posted in my 22 June discussion, “However, a more probable offering would suggest circulation anomalies to then consolidate leading to a ridge west of North America and a trough across the central part of the USA by week 3.” More ensemble members are starting to suggest this possibility. It would not surprise me to see enough retrogression to allow even an anomalous trough (well against climatology) across the western USA ~weeks 3-4 for a period of time should the Eastern Hemisphere tropical forcing remain stationary and very intense.

International weather ramifications suggest little change for those portions of India into Southeast Asia and China recently ravaged by severe thunderstorms and flooding rainfall for at least the next 1-2 weeks. Tropical cyclones may become a concern weeks 2-4 for the TNWP, and I cannot rule out another Bay of Bengal “surprise”. Convectively coupled Kelvin waves may enhance convection along the East Pacific ITCZ by week 2 while the tropical Atlantic remains “quiet” weeks 1-2.

The generally warm/dry west-southeast and cool/wet south central-northeast USA situation looks intact week 1. I think we will see the ridge amplify closer to the west coast week 2 and possibly back into the East Pacific ~140W weeks 3-4. Whatever the details, I feel confident this regime is retrogressive. During weeks 2-3 intense/severe summertime MCS activity with flooding rainfall may become an issue throughout much of the central USA along with the anomalously cool conditions. Most of the Far West is likely to stay “very hot” and dry through much of week 2, with maybe some relief weeks 3-4 should a trough set up for a period of time. Portions of the Deep South are probable to have above normal summertime heat with relatively suppressed diurnal thunderstorm activity.

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

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. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion this Friday.

Ed Berry