Saturday, May 10, 2008

Hell Ride Update

Numerous issues dictate that this writing must be shorter. In fact, given matters briefly mentioned in the Appendix particularly scientific concerns, brevity may become the norm (perhaps with more updates). Stay tuned. I will never be close to satisfied with these discussions until we have a properly supported dedicated web page effort (with figures, composites, verification, etc…..). The latter is the right and only way to provide useful information about the dynamics of subseasonal atmospheric variability, having ~week1-4 predictions expressed probabilistically for users to make important decisions.

There is little change to the global SSTs as discussed in my 3 May posting. Perhaps most notable is the westward shift of equatorial subsurface warmth, ~4-5C at 150-200m depth from 140-160E per 5-day averaged TAO buoy data. That is “not good” for those cheering for El-Nino. However, May is a critical month for ENSO variability given Indian Ocean-Asian monsoon issues, annual cycle of solar input to the tropical oceans, etc. Also, as more folks are starting to recognize, all wide tropical ocean basins (and outside) have global-regional scale circulation impacts. Please see links below. (note the initial projection) (link 18).

The story with the tropical convective forcing is complicated, and it should be. Arguably a weak “rejuvenation” of MJO #5 (for the past boreal cold season) occurred during the last few weeks. The WH (2004) MJO phase space plot suggests a projection in octant 6 (barely so leaving ENSO in) updated through 9 May. Dynamical processes explained by the WB (2007, 08) GWO contributed to the latter, and led to severe Tropical Cyclone Nargis and currently Super Typhoon Rammasun. A westerly wind burst on the equator accompanied the tropical forcing as it propagated into the west central Pacific Ocean. Intense-severe thunderstorm activity has now weakened across the west central Pacific Ocean as has the SPCZ. The lower tropospheric westerly wind anomalies are also shifting off the equator.

Full-disk satellite imagery has shown a nice consolidation of enhanced rainfall centered ~120E from 5-15N during the last few days. The region of intense-severe tropical convection extends from the Bay of Bengal into the northwest Pacific Ocean. This is farther north than climatology, and may signal an early onset to the monsoon systems in those regions, particularly Southeast Asia.

As demonstrated by the WB (2007, 08) GWO, since mid April global westerly wind flow has been increasing. That has produced anomalous zonal mean westerlies in the equatorial and subtropical atmospheres while removing anomalous midlatitude easterlies. Hence the global relative AAM has increased to near the R1 data climatology putting a “dent” in our El-Viejo base state. One circulation response has been the extension of the East Asian Jet (EAJ; for this time of year having 250mb wind speed anomalies ~30-40m/s at ~40N) and a negative phase of the NAO (latter not by some cookbook process “thinking”).

Tropical forcing returning to the Eastern Hemisphere and physical processes tied to the earth-atmosphere AAM budget (mainly the frictional and mountain torques) are now removing global westerly wind flow. That does include the subtropical atmospheres, where zonal mean AAM tendencies are ~minus 4 Hadleys around 10-20N through 8 May. Hence, the WB (2007, 08) GWO is orbiting in phase space toward octant 1, meaning global relative AAM is probable to decrease during the next few weeks.

After weeks of struggling, the numerical models are performing better (link below). Their prediction of the “wicked ridge of the USA west coast (ridge from Hell per others)”, a large pattern change from the past several weeks, for the next 7-10 days is reasonable (along with the downstream USA storm discussed 3 May). In fact, Whitaker and others (ex., MWR 1998) have suggested, from an ensemble numerical modeling perspective, the positive phase of the PNA to be a relatively predictable pattern for roughly week 1-2 time scales. My thoughts are to understand why this appears to be the case, and believe the large-scale character to extended EAJs is a contributor (keeping in mind the dynamical processes that cause that response). .

While having underestimated the above mentioned “wicked ridge (arguably a systematic bias of mine!)”, there is no change to my outlooks from 3 May. By ~ weeks 2-3, I do think tropical convective forcing will become more robust across the Indian Ocean while possibly remaining intense from northern Indonesia/Southeast Asia-west central Pacific Ocean. The GWO may circuit through phases 1-2 into phase 3 by that time. A probable regional scale response will be for the +PNA pattern to slowly retrograde weeks 2-3. Thus anomalous cold/wet likely to dominate much of the eastern two-thirds of the USA should shift northwest to what has been a familiar regime since at least last January. Weather ramifications should be understood. Whether or not there are additional episodes of adding global westerly wind flow and perhaps a base state change linked to interannual variations (including ENSO) is unclear.

Locations such as western Kansas had significant welcome rainfall last week. The anomalous zonal mean westerly wind flow (~5m/s at 200mb) leading to a regional-scale response of a strong STJ into the USA Desert Southwest was a direct contributor. However, this upcoming week looks generally dry (white noise "surprises" can "always" happen), and what may become a northwest shifted storm track by week-3 means the dryness concern is still there.

Locations from portions of the Bay of Bengal into Indonesia/Southeast Asia and the Philippines are likely to get hammered with intense to severe thunderstorm activity week-1, and may signal an early onset to the wet monsoon phase. This region of severe tropical rainfall may expand into the west central and South Pacific Ocean weeks 2-3, while persisting across especially Southeast Asia and the Philippines. Locations from east Pacific Ocean ITCZ into South America are likely to be sporadic weeks 1-2. Per above, what may be a poor excuse of a dynamical MJO signal linked to the GWO appears probable to increase convection across the Indian Ocean (and perhaps portions of equatorial Africa) weeks 2-3. As mentioned last week, our nemesis of 2 dominate regions of tropical forcing may return going into boreal summer.

Please see the latest official JTWC statements about Super Typhoon Rammasun. While an on-going oncern for the northwest Pacific Ocean including the South China Sea the next few weeks, the risk of tropical cyclone development may again increase by week-2 across the Bay of Bengal (perhaps sooner). That would include coastal sections of Myanmar. I trust the expertise of the appropriate weather centers internationally to alert the public of additional weather hazards worldwide.


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

This phase plot is being re-done, stay tuned. We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous to the WH (2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.

Links to CPC and PSD ENSO discussions:

The following is a link to information about the stratosphere and other nice monitoring tools:

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 (soon) 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. In addition, a two-part paper is in ACTIVE preparation by WB that will formally introduce the GWO along with subseasonal composites for variables such as surface temperatures. We want to emphasize notions such as global-zonal mean-regional scale linkages as well as forcing-response-feedback (with subsequent interactions) relationships.

Given shift work and travel, updates are extremely difficult. I may not be able to post another discussion the weekend of 24-25 May. Stay tuned.

Ed Berry

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