Saturday, December 06, 2008

It is now “Weeks 2-5”; USA High Impact Weather Likely

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 again being updated daily. There are still some map room issues that will hopefully be resolved soon. WB (2009; likely not to be published until next year), part-1 of a 2 part paper where the GWO is formally introduced, is in press for MWR publication. There is a link in the Appendix to download the manuscript.

Please remember that these discussions are part of an experimental effort involving linking weather and climate. Until this effort is formalized at the national level, many important scientific issues we are easily well aware of cannot be addressed. Stated another way, it is nearly impossible for me to talk about “everything” in these postings, including offering attribution to unpredictable noise. Plans are moving forward to have a one-day workshop on the WB (2009) GWO concepts during February 2009 in Boulder. Stay tuned for details.

Like a broken record, the spatial pattern of global SSTs remains entrenched. There is a strong negative phase of the PDO (extratropics have a role) and the central and eastern equatorial Pacific Ocean has weakly responded to La-Nina. In fact, per latest TAO buoy data, negative subsurface anomalies ~minus 1-3C are present at roughly 150m depth east of 160W. The latter are similar to a year ago. Although recently weakened (more said below), strongly enhanced trades from the equatorial Pacific Ocean to Indonesia have been contributing to the cooling for nearly 2 months.

The warmest SSTs globally extend from north of Australia into the southwest Pacific Ocean, with totals ~29-30C and anomalies 1-2C. These SSTs have cooled during the last few days. Meanwhile, the Indian Ocean has been warming with totals nearing 29C and anomalies roughly plus 0.5C. There are several issues with these SSTs, and I hope to address a few below. (note the initial projection) (link 18).

The wind and convective signals are still out of sync (discussed in last 2 postings), and the dynamics as explained by WB (2009) measure of the GWO is “calling the shots”. The MJO signal is weak, in octant 4 of WH (2004) phase space leaving in the interannual component. Additionally, just as the La-Nina like global atmosphere has been forcing much of the tropical oceans for nearly 2 months, the extratropics have been forcing the tropics for at least the past couple of weeks. I can only wish that I could stop writing right here.

Moist tropical convective forcing has been getting better organized in the region of western Indonesia during the past week, currently centered ~0/100E per full disk satellite imagery. There is some frontal activity defining the SPCZ over those warm South Pacific waters while thunderstorm activity has been on the wane across tropical South America. Finally, activity is increasing across South Africa and starting to spread into the southwest Indian Ocean.

The Eastern Hemisphere tropical forcing has been evolving through a combination of more rapid westward shifting elements forced by the extratropics over the last couple weeks, along with slower processes involving consolidation. The latter have also involved westward shifts from the west Pacific Ocean particularly north of the equator. Regardless of the details, the concentration of moist tropical forcing into the region of ~80-120E particularly since mid-November is consistent with La-Nina. The WB (2009) measure of the GWO considers these types of behaviors.

The Western Hemisphere enhancement during the last ~2 weeks especially across tropical South America is relatively “easy” to link to the wind signal. In other words, monitoring animations of daily mean upper tropospheric vector wind anomalies one can follow Rossby wave energy dispersions (RWDs) through the extratropical and subtropical atmospheres enhancing regions if tropical rainfall. Loosely, there is some signal of these RWDs in the 250mb snr psi composite anomalies for GWO phases 6-7. However, I want to focus on the GWO and not offer attribution to “other things” using a means that is inefficient.

Since September, there have been 3 circuits in phase space of the WB (2009) measure of the GWO (recent two ~3 sigma), each succeeding event seemingly making a larger excursion toward “high AAM” having periods of roughly 30 days. The dynamical processes, loosely involving first poleward AAM transport by the eddies (through RWDs; the early October situation most robust) and then “responding” surface torques, have worked to increase global westerly wind flow anomalies. For example, during mid November RWDs from the anomalous upper tropospheric Dateline twin cyclones placed the wicked ridge from Hell along the USA west coast. Subsequently, there was an enhancement of the surface easterlies increasing the global frictional torque. Much of the global westerly wind flow anomaly has been in the equatorial and subtropical atmospheres having zonal mean magnitudes ~5-10m/s at 200mb.

As discussed in WB (2009), the GWO is an independent mode of subseasonal atmospheric variability. This boreal autumn may be a “clinic” demonstrating that to the “non-believers”. I think the September and October variations may have been “internal” to the extratropics (got to keep this simple!), while the most recent and on-going event is starting to involve the tropics (recall the above about the convection). Having my reasons, I think the seasonal cycle linked to the southward shift of zonal mean Northern Hemisphere westerlies is partly responsible. The latest evolution from phases 4-5 to 8 in GWO phase space has taken more than 2 weeks, much slower than the past two. The extended jet from East Asia ~25 November was a synoptic response. Put another way, we may finally be starting to see evidence that the wind and convective signals are coupling and that presumably may also be the case for the tropics and extratropics.

About 10 days ago, global relative AAM was ~2 standard deviations above the R1 data climatology (GWO1), the highest in about 2 years. Updated through 4 December, with minor variations both the global friction and mountain torques were strongly negative. The recent explosive north central Pacific Ocean cyclonic baroclinic development did a lot to contribute to a negative zonal mean frictional torque of ~2 Hadleys at 30N. This is an example of how the atmosphere eliminates anomalous westerly wind flow. The negative global mountain torque has involved an East Asia component, a response to anomalous surface low pressure that I can link to intensifying Eastern Hemisphere tropical forcing per above. The ongoing cold air outbreak from East Asia will not go into the deep tropics, and thus I do not think there will be a strong positive mountain torque from the Tibetan Plateau, etc.

The above paragraph is intended to offer a quick attribution to the large negative AAM tendency (GWO2), ~ 30 Hadleys, that has been expected, observed 4 December (averaging periods understood). Relative AAM is approaching the R1 data climatology. Also, particularly across the Northern Hemisphere AAM is once again fluxing poleward, with the maximum divergence ~40N.

Animations of upper tropospheric daily mean vector wind anomalies show twin tropical anticyclones getting better defined ~120E with downstream cyclones around the Dateline. While details are still complicated that will not be addressed here (ex., a well defined Northern Hemisphere positive phased subtropical wave train (Branstator (2002) is present), our zonally asymmetric wave-1 pattern of tropical circulation anomalies is returning. Phases 4-5 of the WH (2004) MJO snr psi composite anomaly plots reasonably depict tropical circulation anomalies.

I think the GWO signal is coming back into the Eastern Hemisphere as I type. Tropical convective forcing is probable to expand and intensify across much of the Indian Ocean and western Indonesia during the next 1-2 weeks. As the wind and convective signals sync up, global relative AAM is likely to decrease (well?) below the R1 data climatology. It is reasonable to offer that a superposition of circulation anomalies depicted by phase 3 (typical of La-Nina) of both the MJO and GWO snr psi composite anomaly plots may be observed by ~week-3. During this evolution the surface trades from east of Indonesia to the equatorial Pacific Ocean are likely to again increase. That may persist and possibly further cool the Nino SSTs.

The current retrogression of extratropical circulation anomalies across the Asia-North American sector, expected for at least the past 2-3 weeks (see past discussions), is directly tied to what appears to be the wind and convective signals syncing up. The relatively repeatable GWO variations have presented opportunities to offer reasonably successful statements of probabilistic predictions.

Typical during an orbit through phases 7-8-1-2 of GWO phase space, all global numerical models and their ensembles have struggled with their circulation forecasts, in this case capturing the on-going PNA retrogression. For example, about 2 weeks ago nearly all models were predicting the ridge from Hell to persist along the USA west coast "until further notice (UFN)”. The latter provided “motivation” for the discussion I wrote on 22 November. I again must emphasize not to rely on the numerical models alone when making subseasonal forecasts! A dynamical weather-climate linkage framework (ex., WB (2007, 2009)) that utilizes real-time data and diagnostics is a must as part of a complete more sophisticated forecast process.

Models have now generally caught on to what some would describe as a large amplitude reverse/negative phase of the PNA teleconnection by week-2. I think it is probable this situation may persist the rest of this month per above. As discussed in past postings, particularly weeks 2-4 there should be episodes of significant precipitation along the USA west coast associated with “cold” digging troughs, leading to an active southwest flow storm track across the Plains. Weather ramifications, including high impact blizzard conditions and intense thundersnow in the cold sectors and severe local storms in the warm sectors of synoptic lows, should be understood. Arctic air may become entrenched from the central and northern Rockies into the Plains while summer-like conditions are experienced over portions of the Deep South.

Whether or not JFM 2009 has similarities to JFM 2008 is unclear. At some point the warm southwest Pacific Ocean will become convectively active. Perhaps there may be a resumption of coherent (truncated?) MJOs and accompanying circulation shifts. I do have my suspicions, and stay tuned.

Internationally, locations focused on western Indonesia and generally from 80-120E along the equator appear to be in for an extended period of intense to severe thunderstorm activity. This enhanced rainfall will impact northern Australia with the seasonal southward shift. The tropical cyclone risk is unclear; however, as convection intensifies across the Indian Ocean, the risk for cyclones impacting Madagascar may increase. The southwest Pacific Ocean islands are likely to have enhanced rainfall at least week-1, and that whole region will be a “wild card” UFN. Generally seasonable tropical rainfall should return to Brasil and South America weeks 1-3, while enhanced thunderstorm activity occurs across South Africa at least week-1.

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. For instance, there is an intense cold air outbreak currently impacting Japan.


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. The first of a two-part paper, where WB formally introduce the GWO (WB (2009)), has been accepted for publication MWR. A pdf of the in press version can be downloaded from the following link:

In addition to the subseasonal snr composite anomaly plots, we hope near real-time discussions with “weather maps” will become a routine part of the ESRL/PSD GSDM web site sometime soon. Part-2 of our GWO paper will discuss the latter. 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 plan on posting a discussion the weekend of 13-14 December.

Ed Berry


Harold Ambler said...

Hi Ed. I will download the paper -- thank you! I seem to recall reading someplace that residual heat from the 98 El Nino had only recently entered the Indian Ocean. Is that way off?

snoman said...

Congratualtions Ed! You have talked about a mid December Arctic blast for weeks and the Northwest is now just hours away from getting pounded. You nailed this one!


Ed Berry said...

Hi Harold,Jim,

Thank you for the comments. Harold, I am not aware of any literature about residual heat from the '98 El-Nino emerging into the Indian Ocean. I would have to check, including learning a lot more about ocean currents/dynamics.

Jim, thank you for the compliment! I think the large GWO variations offered a "forecast of opportunity" to capture our regime shift. However, the dynamics involving the tropical-extratropical interactions are currently extremely complicated and Klaus and I are learning from this. This situation is why one can only make statements of probability.

Best regards,