The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).
http://www.cdc.noaa.gov/MJO/Predictions/wb2006.pdf
From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.
Our latest weather-climate discussion dated August 18th, 2006, has been posted on the ESRL/PSD MJO web site at
http://www.cdc.noaa.gov/MJO/Forecasts/climate_discussions.html
Please see past postings for web site links. I am going to discontinue inserting most of them in an effort for brevity. I also need to do the same with these postings.
The distribution of global tropical SSTs have remained fairly persistent during the last couple of weeks. Western Hemisphere tropical Pacific SSTs remain above normal, with weekly mean anomalies (8/20-26) ~plus 1-2C. The largest magnitudes were centered near the equatorial date line and just west of South America, where daily mean anomalies close to plus 3C were observed on September 4th for the latter (seasonal cycle of SSTs contributing). Anomalies of roughly plus .5-1.5C were still present across the Atlantic and much of the Indian Ocean. Below normal SSTs ~minus 1-2C continue around Indonesia, particularly south of the equator. Hence we see a pattern of a “warm Indian Ocean-cool Indonesian region-warm date line region” in regard to SSTAs, typical of a mature warm ENSO event.
Persistent westerly wind anomalies (~3-5 m/s from about 140E-180) and even actual westerlies have continued to deepen the oceanic thermocline along the equatorial cold tongue. In fact, latest TAO buoy data show anomalies ~plus 4C at 100m depth ~140W, and I may be able to make the case for 2 oceanic Kelvin waves currently propagating east along the cold tongue thermocline.
Actual SSTs in excess of 29C (threshold we use to maintain significant tropical convective forcing) remain present across portions of the Indian Ocean, equatorial date line and regions of the Caribbean. These SSTs are most extensive from ~150E-180 within 10 degrees of the equator (see links for details). An important point is SST boundary forcing has been contributing to multiple regions of enhanced tropical convection, with episodic flare-ups particularly across the Indian Ocean and date line areas during the past several weeks. This has masked any real coherent signals of tropical forcing that may still exist, and for all intents and purposes there is no MJO signal, nor any more modes of 30 and 50-day processes. Much of the tropical forcing is random with little organization.
This is interesting since the spatial distribution of global tropical SSTs may not be random, but slowly evolving in a coherent manner. I have a thought that as the seasonal (southward) migration of tropical convection occurs west of the date line; perhaps by the December 2006-January 2007 time frame SST boundary forced convection in the region of the date line may start to dominate. A secondary region may also continue across the Indian Ocean. This evolution would be consistent with the onset of a mature phase of a warm event. Again, I reiterate that I think the atmosphere is tilting toward a warm ENSO event (magnitude unclear), and some indices would offer a weak warm event has already emerged. What I stated above is pure speculation which I cannot defend at this time. The following are links to ENSO discussions.
http://www.cdc.noaa.gov/people/klaus.wolter/MEI/
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.html
Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and remember that official USA information on anything related to ENSO comes from CPC.
http://www.cpc.ncep.noaa.gov/products/predictions/90day/
From monitoring, I do think this developing situation (possible warm ENSO) has had at least “episodic impacts” to the global weather during boreal summer. These include general suppression (thus far) of the Tropical North Atlantic (TNA) tropical cyclone season (understanding climatology), the increase of Tropical Northwest Pacific (TNWP) tropical cyclones (ex., southeast China), and the excessive heat much of the USA has dealt with.
Obviously weather-climate linkage signals are very weak. There are interactions which are certainly not linear and random that has been occurring for a while, and I only want to summarize the (hopefully) important behaviors for brevity. Right now I think the most robust tropical convective signal is emerging across the equatorial Indian Ocean (OLRA ~ minus 50-70 W/m**2), and I think Rossby wave energy dispersion (RWD) processes moving through the North Atlantic/Scandinavia are contributing. Other regions remain north of the date line, around the Americas and finally the TNA into Africa. Various plots of velocity potential suggest the latter is a convectively coupled dynamical signal which may emerge into the Indian Ocean during the next week or so.
The tropical convection across the Indian Ocean is linked with upper tropospheric westerlies (see animations of 150mb and 250mb daily mean vector wind anomalies), perhaps suggestive of extratropical dynamical forcing. The coherent modes Hovmoller plots suggest a convectively coupled Kevin wave to develop from this enhanced rainfall and move east. My own thought would support that notion, possibly leading to increased enhancement of tropical convection from the date line region toward the Philippines sometime during week 2. Convection may also remain enhanced across the Indian Ocean as the weak dynamical signal comes into the Eastern Hemisphere. Models such as the CDC and NCEP ensemble maintain precipitation across these 2 areas during week 2 largely responding to the SSTs.
Per animations of 150mb and 250mb daily mean vector wind anomalies, around September 2nd nearly symmetric RWDs linked to twin subtropical anticyclones ~120E occurred in both hemispheres. Circulation anomalies were roughly 20-30m/s at 150mb. The Northern Hemisphere version went across the Pacific Rim and lead to the anomalous cyclone that was across the Northern Plains while its counterpart led to a significant cold outbreak across much of South America. This pattern has dispersed, with currently a lot of northeast-southwest tilt (AAM transport signals were still weak 3 days ago per reanalysis data) to the anomalies from Asia to North America. There is a “hint” that the twin anomalous subtropical anticyclones are currently shifting west toward the Indian Ocean convection. Finally I should mention that twin anticyclones (with generally weak anomalies) keep “coming and going” around the equatorial date line region. The latter is likely a warm ENSO signal.
Global AAM signals are much too weak to discuss. There are regions particularly across the Southern Hemisphere where zonal mean contributions are large particularly from the frictional and possibly the Coriolis torques. Based on the reanalysis data plots through September 2nd, global tropospheric relative AAM is ~minus 1 sigma below the 1968-1997 climatology.
A time-latitude section of 200mb zonal mean zonal wind anomalies can give some sense (this is only 1 level verses a zonally and vertically averaged quantity) as to how global relative AAM is being distributed. Loosely, during the past week zonal mean westerly wind anomalies (magnitudes ~10m/s) have dominated the extratropics poleward of 50 degrees of both hemispheres. Weak easterlies have been present across the equatorial and subtropical atmosphere of the Southern Hemisphere and midlatitudes of the Northern Hemisphere. Finally, weak zonal mean westerly wind anomalies are present across the subtropical atmosphere of the Northern Hemisphere. The latter are contributing to a split-flow pattern from the Pacific into North America, and a recent positive AAM tendency along the equator of ~2 Hadleys may have assisted. As anyone can now see, this situation is a mess, and if the reader looks at the plot reference above, my description does not do the complexity justice.
To summarize, I think we have a weather-climate situation consisting of 1) a likely emerging warm event, 2) a convective signal emerging in the Indian Ocean, and 3) mostly noise. We also need to think about the role of the seasonal cycle as transition to boreal autumn approaches, which also adds a huge source of uncertainty.
Signals are simply too weak to “fit in” to the GSDM framework. From spending lots of hours studying animations of various fields, AAM plots, Hovmollers of “everything”, many ensemble prediction schemes, diagnostics from other worldwide meteorological centers, etc., my suspicion would be to see an emergence to GSDM Stage 2 sometime during the next 1-3 weeks. A key observation will be to watch for a rapid increase in the tropical convection from around the date line to near Southeast Asia during the next few weeks (along with a strong positive maximum in AAM tendency).
Initially troughs may amplify across the western USA with a ridge over the Deep South. Should we see a mature GSDM Stage 2 there would be a large amplitude ridge from ~130W into Alaska with an anomalously deep trough across the middle of North America. This would suggest dry/warm for the far western states and a tilt toward cool/wet for roughly the eastern half of the country. I am not going to make any attempt to break down this possibility week by week. Like it or not, this is the reality we are currently dealing with in regard to making any kind of weather forecast in our situation. Confidence is as low as it gets and any useful probabilistic statements must indicate that.
Week 1 (6-12 September 2006): See above. Please see http://www.nhc.noaa.gov/ for the latest tropical cyclone information.
Week 2 (13-19 September 2006): Same as week 1.
Week 3 (20-26 September 2006): Same as week 1.
In general I think we are looking at cooler than normal temperatures with some opportunities
for rainfall through at least week 1 for Southwest Kansas. Afterwards, unclear.
It is unlikely I will be able to do another posting until about the middle of next week (roughly Wednesday, 9/13). Maybe signals will become clearer by then!
Ed Berry
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