Saturday, September 13, 2008

The Great Wall has Risen

The views expressed are those of the author and do not necessarily represent those of the National Weather Service.”

Please keep in mind the ESRL/PSD GSDM web link, below, while reading this discussion.

The 91-day signal to noise ratio (snr) anomaly composites are now updated daily, centered on the date shown (see product descriptions). On-going map room issues causing missing data and unrepresentative plots continue.

The Tropical Northwest Pacific Ocean (TNWP) still has the warmest SSTs globally, with totals ~30-31C east of Philippines and anomalies at least plus 1-2C. The equatorial Indian Ocean is also warmer than climatology, as are portions of the Caribbean into the Tropical Atlantic Ocean.

Somewhat characteristic of a mature La-Nina, well defined spatial horseshoes of anomalous SST warmth and coolness emanate from the equatorial west central Pacific Ocean and Dateline regions, respectively, into the extratropics (PDO notions understood). Negative SST anomalies across the latter are at least 2C, having totals cooler than 28C. Thus there is an enhanced east-west SST gradient from the Dateline into the Indian Ocean. Additionally, the equatorial central and east Pacific Ocean thermocline continues to be anomalously shallow allowing subsurface anomalies of less than minus 3C at ~150m deep/155W per TAO buoy data. In fact,this observation is similar to about a year ago.

It appears probable that additional episodes of trade wind surges around the equatorial Dateline generating upwelling oceanic Kelvin waves will continue going into the boreal 2008-09 winter. Hence, while the official ENSO discussion acknowledges that a return to a weak La-Nina is possible, I do not think ENSO-neutral conditions are expected to continue through the end of 2008” is a scientifically complete statement. As mentioned previously, it is the global tropical SSTs that must be considered when discussing circulation dynamics, not just “Nino 3.4”. Furthermore, the coupled global weather-climate system is already in “a non-lingering La-Nina mode” (more said below). (note the initial projection) (link 18).

Full disk satellite imagery and other tools show tropical convection remains enhanced across the Eastern Hemisphere. The latter is centered ~10-15N/120E, extending from the Bay of Bengal east-southeast to New Guinea and on into the SPCZ. Projection onto a MJO per the WH (2004) methodology is quite strong, roughly 2 sigma in octant 5 of phase space through 12 September. Phase 5 of the OLR composite anomaly snr done on the WH (2004) MJO index reasonably depicts the spatial pattern of tropical convection. The projection reduces to about 1.5 standard deviations in phase 4 retaining the interannual component. However, consisting of 3 subseasonal events since June, there has been at least a 1 standard deviation shift toward the Indian Ocean/Maritime Continent. This is suggestive of a base state returning to “La-Nina”.

Consistent with the above, the Eastern Hemisphere tropical forcing has significantly slowed its eastward propagation, while shifting much more toward the north. While I do expect a weaker MJO signal to propagate into the Western Hemisphere during the next few weeks, our current subseasonal event is likely to be truncated. Tropical forcing is probable to become somewhat enhanced around the Americas ~weeks 1-2, then particularly so from the Atlantic into the Indian Oceans perhaps weeks 2-3.

Relative to WH (2004) phase space, I think the MJO signal will orbit through the “1 sigma circle” and emerge into phases 8-1-2 by roughly late this month. Again, timing and details are noise, especially given recent Western Hemisphere issues, and uncertainty remains non-trivial. However, summing up: 1) centered on 10 September, phase 5 of the MJO streamfunction (psi) composite anomaly snr plot currently depicts the tropical and PNA circulation characteristics; and 2) roughly phase 1 is probable ~days 10-20 for all fields, essentially bypassing phases 6-7. Interestingly, there is some numerical and statistical model support for (2).

Observationally, animations of upper tropospheric daily mean vector wind anomalies do show twin anticyclones ~150E and rapidly decaying cyclones just east of the Dateline. In fact, around 9 September there were ~20-30m/s westerly wind anomalies at 150mb near the Dateline. As these westerlies attempted to propagate eastward, they literally hit a downstream “wall of anomalous easterly wind flow anomalies”. Outflow from Hurricane “Ike” (pointed out by Weickmann) was involved with these easterlies. This is the second observation of this type of behavior during the last few months. I do think there are some complicated feedback issues between the interannual component of the global circulation base state and on-going subseasonal activity. While the WB (2008) GWO captures some of these dynamical processes, there is much work to be done to completely understand them.

In any event, the anomalous equatorial westerly wind flow from 9 September is propagating rapidly into the midlatitudes via dynamical processes explained by the WB (2008) GWO. More specifically, Rossby wave energy dispersions (RWDs) linked to a recent strong positive clustered global mountain torque event (~30 Hadleys) have interacted with these westerlies. A regional-scale response has been the (expected) return of the “western USA (wicked) ridge from Hell” PNA pattern, shown by the GWO phase 5 psi composite.

Cutting to the chase, forced by the mountains and tropical convection, global relative AAM tendency spiked to ~plus 30 Hadleys a few days ago, causing the actual AAM to peak near the R1 data climatology. The response in the WB (2008) GWO phase space was for an orbit to octants 4-5 having ~1.5 sigma projection. Consistent with the equatorially confined empirical WH (2004) MJO signal (due to tropical-extratropical coupling), the WB (2008) GWO is probable to collapse toward octants 8-1-2 during the next 1-3 weeks as AAM tendency becomes negative. Hence it is probable that phases 8-1-2 of the WB (2008) GWO composite anomaly psi snr plots will be representative of the global circulation by ~weeks 2-3. Synoptically, that means the North Pacific Ocean jet should anomalously expand then collapse into some form of a western and central USA trough(s) during the next few weeks.

All of these notions are consistent with my last 2 postings, and are starting to remind me of what I wrote during the 2007-08 boreal cold season. In general, until I “see something different”, it is looking more likely that a “weaker rendition” of our low AAM “La-Nina” base state is coming back for this upcoming (2008-09) cold season. The WB (2008) dynamical global measure of the atmospheric circulation, GWO, may continue its ~10 to 20-day orbits while drifting toward a state shown by the phase 3 GWO psi composite anomaly, representative of a positive Branstator (2002) circumglobal teleconnection. This suggests western and central USA troughs, but with added subtropical westerly wind flow compared to last winter. Weather ramifications should be understood; however, hopefully the southern High Plains will get some decent precipitation this cold season, instead of drought.

Much of the lower 48 states will see a break next week from the recent extremely intense high-impact weather. Per above, a more active regime focusing on the western and central states may return particularly later week-2 and week-3, especially if tropical moisture transport through the Gulf of Mexico improves. GWO phases 8-1-2-of the 2m air temperature composite anomaly snr plots give a nice evolution of that parameter across the USA during the next 1-3 weeks.

While the equatorial Indian Ocean is likely to remain suppressed through at least week-1, much of Southeast Asia and the TNWP should have enhanced rainfall. Included is a risk for more northwest Pacific Ocean tropical cyclones. At some point attention will also need to be paid to the Bay of Bengal.

Next week, the tropical Atlantic Ocean will continue to “sputter” in terms of tropical convection including possible cyclones. Per above, weeks 2-3 should see a return to intense to severe clusters of thunderstorms perhaps focusing on Africa and the equatorial Indian Ocean, while lesser activity hangs around the TNWP and Americas. Relative to climatology, the risk of Atlantic Ocean tropical cyclones may increase weeks 2-3. Further, hybrid systems may become an increasing concern, especially in the region of the Caribbean, as boreal autumn progresses.

Please see the latest official tropical cyclone forecasts for all basins. I trust the expertise of the appropriate meteorological centers to alert the public of additional weather hazards worldwide (high impact weather events are still continuing).


Links to CPC and PSD ENSO discussions:

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

The following is a link to NCEP model verifications (surf around for lots more). .

The following is a link discussing recent global weather and related events.

These are probabilistic statements. We hope that an opportunity will arise for us (soon) to allow our dedicated web page effort to mature, expediting objectively and accountability. This web page effort will hopefully include an objective predictive scheme for the GWO with hindcasts.

The WB (2007) paper on the GSDM has been published in the February issue of MWR. In addition, the first of a two-part paper, where WB formally introduce the GWO (WB(2008)), has been accepted for publication in MWR. The authors are in the process of resubmitting after minor revisions. A pdf version (before revision) can be downloaded from the following link:

Overlapping seasonally varying subseasonal composites for variables such as surface temperature, precipitation, geopotential height and streamfunction anomalies are planned on being posted on the web site mentioned above and presented in part-2 of our paper. We want to emphasize notions such as global-zonal mean-regional scale linkages as well as forcing-response-feedback (with subsequent interactions) relationships. An important purpose is to provide a dynamical weather-climate linkage framework to evaluate the numerical models in a sophisticated manner as part of a subseasonal (and any time scale) forecast process, in addition to a climate service for all users. Relying on the numerical models alone is a cookbook! I will probably not be able to do another complete discussion the weekend of ~27-28 September.

Ed Berry

No comments: