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 (link at the bottom) 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.
Western Hemisphere tropical Pacific SSTs remain above normal, with weekly mean anomalies (8/13-19) ~plus 1-1.5C. Somewhat larger anomalies exist just west of South America and from the equatorial date line to about 160E, with magnitudes ~plus 2C. SST anomalies ~plus .5-1.5C were also prevalent across most of the Caribbean and portions of the Tropical North Atlantic (TNA), as well as the Indian Ocean. Actual SSTs in excess of 29C are present across portions of the Indian Ocean, equatorial date line and regions of the Caribbean. Below normal SSTs, with anomalies ~minus .5-1.0C have been observed for a few weeks from ~90-150E particularly just south of the equator around Indonesia to north of Australia. This pattern of cool SSTs near Indonesia with warmth around the date line is typical of mature warm events, not developing ones which may presently be the case.
TAO buoy array and other data continue to tell us of persistent strong westerly wind anomalies (and actual westerlies) on the equator at the date line (anomalies ~5-10m/s), with SSTAs ~plus 2C down to depths of 150m in that region. In fact, there may be 2 downwelling oceanic Kelvin waves moving along the thermocline of the equatorial Pacific cold tongue, with one just east of the date line and the other ~120W. To me, all indications are that the ocean-atmosphere system is tilting toward a warm ENSO event (magnitude unclear), and some indices would offer a weak warm event has already emerged.
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” the global weather during boreal summer. These include suppression (thus far) of the TNA tropical cyclone season (understanding climatology), the increase of TNWP tropical cyclones (ex., southeast China), and the excessive heat much of the USA has dealt with.
Weather-climate linkage signals particularly from the tropics remain very weak. There is evidence, as would be expected per above, that atmosphere-ocean coupling between the SST boundary forced tropical convection west of the date line and circulation response/feedback has been occurring possibly since April (see recent weather-climate discussion for details). However, the MJO signal is virtually non-existent and even the ~50-day and ~30-day tropical forcing signals are anemic at best. The AAM plots, particularly the tendency and mountain and frictional torques still do give some evidence of the latter. There is a fast convective dynamical signal (weakly projects onto a Kelvin wave per Coherent modes Hovmollers) moving through the Western Hemisphere at ~16-17m/s located at about 40W. It is possible this signal will emerge into the Indian Ocean by the end of week 1, especially since thunderstorm clusters are becoming very active across central Africa.
Per animations of 150mb and 250mb daily mean vector wind anomalies (and other fields) I can offer that the tropical convective flare-up ~August 7-10 at roughly 160E first impacted the extratropics via Rossby wave energy dispersions (and other complex processes), and currently the extratropics may be forcing back into the tropics. For instance, about a week after the flare-up blocking appeared across the North Atlantic which subsequently dispersed into northern Asia and led to a recent anticyclonic wave breaking (AWB) event across the Northwest Pacific. During this whole period anomalous equatorial zonal mean westerly flow collapsed with even weak zonal mean easterlies appearing. Additionally, the AWB has also assisted with AAM transport to allow zonal mean westerly flow (anomalies ~plus 5m/s at 200mb) to propagate poleward to ~25N. There have been at least 2 weather impacts (relevant to the USA): 1) a more favorable environment for TNA tropical cyclogenesis, and 2) enhancement of the southwest USA monsoon.
There may be another important repercussion: extratropical feedbacks into the tropics to further develop a possible oncoming warm event. I can also offer that the above discussed AWB, through extratropical baroclinic processes interacting with dynamical processes in the tropics, have allowed for the persistence and even intensification of the anomalous equatorial westerlies near the date line. All of this is very speculative and needs to quantified for any hope of defensibility. Whatever the case may be, the atmosphere is on a path which is presenting tremendous difficulty to diagnose using “conventional thinking” (not to say things like the above have not happened before; they have).
There are also active tropical convective signals rapidly appearing across both the west central Pacific (which has been a region of recent suppression) and equatorial Indian Ocean. Strong convective suppression still continues from ~80-120E. The recent active signals appear to have had assistance from the Southern Hemisphere extratropics. I would speculate that the Indian Ocean convection may propagate northeast toward the Bay of Bengal and Southeast Asia, possibly consolidating the growing convection across the TNWP during the next few weeks. One could also offer 2 separate areas may persist for “a while” given the SSTs and the fast dynamical signal currently moving through the Western Hemisphere.
To summarize, I think we have a weather-climate situation consisting of 1) a likely emerging warm event, 2) fast Western Hemisphere convectively coupled dynamical signal about to propagate into the Eastern Hemisphere, and 3) a lot of weak unclear speculative signals as discussed above, which is seemingly a situation becoming “the norm”. We also need to think about the role of the seasonal cycle as transition to boreal autumn approaches.
The AAM plots present little global signal. There are strong AAM signals from the Southern Hemisphere particularly from the frictional and Coriolis torques, and earth components contributing to a global relative AAM signal of ~minus ½ sigma. Since about June 1st, zonal mean westerly flow at 200mb has been well above average from 25-40S with anomalies as great as 15-20m/s, allowing for a very active storm track there. The latter may also be a warm ENSO signal.
GSDM Stage 3-4 best describes the current weather-climate situation (considering the AAM budget). Zonal mean anomalous westerly flow is propagating well into northern subtropical atmosphere, while weak zonal mean easterly flow replaces it along the equator. Also, tropical convective forcing across the Eastern Hemisphere appears to be on the increase. I did not expect this situation a week ago. My thoughts were for GSDM Stage 2 at this time, and any I prediction I based on that was a poor assessment. For instance, I had thoughts of a fairly cool airmass for the middle of the USA starting this weekend into next week, and that does not look likely (there will still be a polar Pacific airmass moving into the Plains).
Where we go from here is unclear. Given there are 2 areas of tropical forcing emerging across the Eastern Hemisphere, past experience would tell me GSDM Stage 4-1 for week 1. The ECMWF deterministic run through day 7 supports this notion. Models such as the NCEP GFS 500mb ensemble mean suggest a GSDM Stage 3 like circulation for week 2. My humble thought would be to speculate a convective signal to propagate into the TNWP by the end of week 2 or week 3, perhaps leading to GSDM Stage 2 during that time. Whatever the case, careful daily monitoring within the GSDM framework will be needed to capture the possibility of a significant signal emerging from what is essentially mostly noise.
The numerical models have also been struggling with this situation. While these numerical prediction schemes have little information about the impact of tropical convective forcing after about day 5, they do take into account non-linear feedbacks. My advice to anyone having to make predictions is to consult as many numerical ensemble prediction schemes as possible. That, along with monitoring using the GSDM, should give some hope for success for roughly week 1. Confidence in the following outlooks is as low as it gets. I would feel this way even if there was excellent ensemble numerical model agreement through week 3 since I can point to sources of uncertainty with the GSDM framework that numerical models will struggle with.
Week 1 (24-30 August 2006): GSDM Stage 4-1 may be most probable, meaning a subtropical ridge across the Deep Southeast USA and relatively low amplitude NE-SW trough across the western states. The SW USA monsoon should remain quite active. There may be an active MCS storm track across much of the middle of the country. Flooding from heavy rain producing thunderstorms is a concern for much of the Desert Southwest into the Central Plains. A window of opportunity for tropical cyclogenesis should continue across the TNA. Please see http://www.nhc.noaa.gov/ for the latest tropical cyclone information.
Week 2 (31 August – 6 September 2006): As seasonal transition to stronger westerly flow continues across the country, loosely GSDM Stage 1-2 may occur. That would suggest more mobile troughs coming into the western USA with a subtropical ridge across the Southeast. The Desert Southwest monsoon may start to weaken. Everything else is unclear.
Week 3 (7-13 September 2006): Unclear to offer anything statistically useful to folks who need it.
Another decent episode of rainfall (including a flooding hazard) is probable for Southwest Kansas at least through this upcoming weekend along with near-below normal temperatures. I can see (with my crystal ball) another scenario for a frontal episode of rainfall late next week; however, upper tropospheric moisture transport from the deep tropics may be less by that time. Temperatures may warm to above normal before this. Climatology will start working against us for rainfall as we get into this fall. We will see. I can always dream up scenarios to work against it.
I will try to do another posting this upcoming weekend. I will be on travel from 8/27-9/2, and the next posting may not be until 9/6.
Ed Berry
No comments:
Post a Comment