Saturday, September 06, 2008

Still Tilting Toward an El-Viejo Reprisal (The Dog Rules)

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 anomaly composites are now updated daily, centered on the date shown (see product descriptions). There have been on-going map room issues causing missing data and unrepresentative plots.

The warmest SSTs globally persist across the Tropical Northwest Pacific Ocean (TNWP) with totals exceeding 30C east of the Philippines and anomalies at least plus 1-2C. The on-going trade wind surge at the equatorial Dateline has led to ~minus 2C anomalies having totals less than 28C. As discussed below, these relatively cold SSTs must be a consideration to the characteristics of any eastward propagating MJO signal.

Subsurface negative anomalies continue to shift east along the equatorial East Pacific Ocean thermocline (~4C at 140W/125m per TAO data), responding to an upwelling Kelvin wave generated during July. A similar response may occur with the current trade surge.

Finally, it is my feeling that the JJA ONI of -0.1C based on Nino 3.4 does not scientifically completely represent the interannual component of the global weather-climate situation. The roles of other tropical ocean basins and their impacts on global circulation dynamics must be considered. (note the initial projection) (link 18).

Full disk satellite imagery and other tools indicate there has been a slow coherent eastward shift of Eastern Hemisphere tropical convection during ~the last 25 days. A convectively coupled Kelvin wave preceded this rainfall, and appears to be propagating into the Western Hemisphere. The region of enhanced convection is centered on about 5N/120E, extending from the eastern Arabian Sea east-southeast into the west central Pacific Ocean. A "back of the envelope calculation" gives a phase speed of ~3.5-4m/s (~3 deg long/day).

The WH (2004) methodology gives roughly a 1 sigma projection in octant 4 of phase space, and nearly a 1.5 standard deviation shift in octant 3 leaving the base state in. Phase 4 of the composite streamfunction (psi) anomaly plots centered on 3 September nicely represent current tropical and even PNA circulation anomalies. I think the latter is true given a growing MJO signal. However, I also think the interannual component will remain significant.

Global relative AAM is still well below the R1 data climatology, more than 1 sigma (~1 Atmospheric Momentum Unit (AMU)), removing the stratospheric westerly phase of the QBO, through 3 September. Using this same measure (no QBO signal), relative AAM is still up about 2 AMUs compared to a year ago, meaning more subtropical westerly wind flow. However, Rossby wave trains interacting with the tropical convective forcing and midlatitude mountains (explained by the WB (2008) GWO) have led to a strong negative AAM tendency of roughly 15 Hadleys. Much of this negative AAM tendency is coming from the Southern Hemisphere. Hence the WB (2008) GWO continues to “hover” around octants 1-3 of phase space.

Constructively interfering with the MJO signal, phase 3 of the GWO composite psi anomaly plot is representative of the on-going “Branstator (2002)-like” positive phase of a circumglobal teleconnection. The subtropical sink-midlatitude source AAM transport signal, strong in the Southern Hemisphere and weak for the Northern Hemisphere, support this notion. Synoptically this means our global circulation base state favors stronger than normal subtropical and midlatitude ridges.

So, where does the global circulation go from here? First, I think there is some coupling between the GWO and the MJO. Secondly, and arguably not emphasized enough and stating redundantly, I think there is an eastward propagating Eastern Hemisphere MJO signal.

Most models suggest propagation into roughly octants 5-6 of WH (2004) phase space by late week 2 or 3. Given the cold Dateline SSTs and other reasons I have, I agree with the eastward shift, but possibly truncated. The enhanced tropical forcing is probable to propagate east-northeast, focusing on the warm TNWP by ~week 3. A weaker MJO signal should then continue into the Western Hemisphere afterward. Emphasizing from past discussions, should this and other subsequent subseasonal events lead to eastward propagation of tropical convective forcing along the equatorial West Pacific Ocean, a slow (interannual time scale) evolution to a warm event is possible.

The WB (2008) GWO is likely to continue its 10-20 day periodicity orbiting from octants 8-1 to 4-5 in phase space, leaning toward low AAM, while “riding along” with the MJO signal. Phases 4-5 of the MJO psi (and temperature) composite anomaly plots appear reasonable for weeks 1-2. Phases 6-8 may be representative for ~weeks 3-4. Regardless of the white noise details and timing, 1) I am concerned another prolonged period of Western Hemisphere-Indian Ocean active convection is possible after week-two (MJO phases 8-1-2), and 2) at some point there is likely to be an anomalous extended North Pacific Ocean jet collapsing into a western USA trough, like that shown by phases 8-1 of the WB (2008) GWO. Keeping in mind issues of boreal autumn, the latter may be most probable the last 10 days of this month into early October. My confidence is still low in the outlooks that follow.

Per above, currently much of the USA is being dominated by an anomalous trough centered ~100W. Most numerical models significantly underestimated this feature a couple of weeks ago (discussed in my last 2 postings). I agree the trough will persist roughly week-1, in the presence of synoptic events. Ridging is probable to return to the western states while the trough moves into the eastern USA week-2 (phases 4-5 of the GWO composites). Weeks 3-4 were discussed above. The probable temperature outcomes can be seen from the composite anomaly plots.

Again, possible impacts onto the USA from additional tropical cyclones including “Ike” remain a concern, and please see the latest official statements from the Tropical Prediction Center. I think the North Atlantic Ocean tropical cyclone hazard may decrease to below climatology by ~week 2, and then perhaps increase afterward.

In general, heavy precipitation is likely for much of the country east of the Rockies next week. Climatology going into October suggests generally dry weather for much of the country, and that should be the case for at least approximately week-2. A more active situation may return for the western and eventually central states weeks 3-4. My thoughts for this coming cold season remain unchanged (see 30 August posting).

As already discussed, intense to severe tropical thunderstorm activity is likely to propagate east and northeast initially impacting Southeast Asia and Indonesia, then focusing on the Philippines by week-3. Portions of the equatorial Indian Ocean may see a break from the recent rainfall by week-2. While a "wild card" until further notice, the tropical cyclone hazard is probable to increase above climatology for the TNWP weeks 2-3. For various reasons, the East Pacific Ocean ITCZ and Americas will be “sporadic” in terms of enhanced convection, possibly becoming active by weeks 3-4. Equatorial Africa may be somewhat suppressed weeks 1-2.

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 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 has been submitted to MWR where WB formally introduce the GWO (WB (2008)). A pdf version 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 hope to do another discussion next weekend, ~13-14 September.

Ed Berry

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