Saturday, March 08, 2008

Witch's Brew Update

I want this posting to be shorter. Mature and strong La-Nina SST (and global atmospheric) conditions continue across the Indo-Pacific Ocean sector. The plus 1-3C SST anomaly warming along the coast of South America can be thought of as part of the ENSO cycle working with the seasonal cycle.


The following are links for additional SST information.


http://www.pmel.noaa.gov/tao/jsdisplay/


http://www.cdc.noaa.gov/forecast1/IndoPacific.frcst.html (note the initial projection).


http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/index.primjo.html (link 18)


http://www.wmo.ch/pages/prog/wcp/wcasp/enso_update_latest.html (getting dated, but still useful)


A loud message to deliver is that the tropical convective forcing has become extremely complex during the past few weeks. I have had to significantly reassess my past interpretations of this forcing. There are multiple areas of intense-severe tropical rainfall causing high-impact weather across the global tropics. Broadly, per full disk satellite imagery, etc., two regions of enhancement can be “lumped together”. One area extends from the equatorial Indian Ocean into the very warm (totals in excess of 30C) South Pacific Ocean (along the SPCZ), with the other from tropical South America into Africa. The latter has led to a significant enhancement of the Atlantic Ocean ITCZ this past week.


In addition to the lack of time, it is beyond the scope of what I can present in these discussions the complicated Rossby wave energy dispersions (RWDs) seen during the last few weeks. This is all part of a dynamical system having forcing-response-feedbacks, etc.. An astute meteorologist needs to spend some time monitoring the daily mean evolutions of upper tropospheric vector wind anomalies (just for starters) to gain some understanding for both research and making subseasonal predictions.


For instance, the best defined anomalous twin upper tropospheric tropical anticyclones are over the Western Hemisphere, extending from South America to western Africa. While zonal mean anomalous easterly wind flow anomalies dominate the tropical and subtropical atmospheres, much of that contribution is coming from the Western Hemisphere. Considering a very weak pair of anomalous anticyclones tied to the Eastern Hemisphere tropical forcing, there has been a broad zonal wave number 2 pattern of tropical circulation anomalies during the last week to 10 days.


While the WH (2004) phase space plots of the MJO suggest a roughly 2 standard deviation projection in phase 1 (updated through 7 March), this is currently misleading. This is an example where there needs to be a careful understanding of what to interpret, as well as the science behind techniques such as these. In contrast to past discussions when I was expecting MJO #4, there is little, if any, MJO signal in the real atmosphere. The WH (2004) technique is latching on to the upper tropospheric easterly wind flow anomalies across the Atlantic Ocean too strongly. Hence statistical and numerical predictions of the MJO based on the WH (2004) methodology need to be used with care.


I do think it is probable to see the moist tropical convective forcing get better organized across the Eastern Hemisphere during the next few weeks. This region may cover much of the equatorial Indian Ocean through Indonesia into the Southwest Pacific Ocean. I would also not dismiss the option of 2 regions of tropical convective forcing, roughly the Indian Ocean to Indonesia with a separate region over the Southwest Pacific Ocean. The latter region has the warmest SSTs globally, and I have called this issue our “nemesis as part of the new world atmosphere” in past postings. In any event, whether or not we get MJO #4 is unclear.


Global relative AAM remains very low, ~2.5 standard deviations below the R1 data climatology through 6 March. In addition to the gradual meridional broadening of anomalous zonal mean easterly wind flow anomalies throughout the tropics, contributions have included a persistent negative global frictional torque since early February (~10 Hadleys). There have also been 2 recent episodes of large negative global Coriolis torques of roughly 20 Hadleys spaced 2-3 weeks apart. The negative global frictional torque has come from the midlatitudes of both hemispheres, tied to the storm tracks. During “a typical La-Nina”, the global frictional torque should be, on average, positive due to enhanced trades.


The GWO is strongly in phase 3 (legacy GSDM Stage 1) updated through 6 March. The bottom line is I think it is probable to see roughly 10 day circuits in phase 3 of GWO phase space for at least the next 1-3 weeks. For the USA, that shifts the odds toward more western USA troughs (discussed below).


Before getting into any outlooks, I need to also point out recent PNA responses to complex non-linear feedbacks from an intense Kamchatka anticyclone (wind speed anomalies ~40m/s at 250mb at times) during roughly the past couple weeks. The latter is one of those RWD feedbacks (and essentially random processes having little predictability) alluded to above, and this did its part of maintaining a positive East Asian mountain torque. Hence there was a local source of westerly wind flow, and an unexpected extension of the East Asian combined jet across the Pacific Ocean happened this past week. Rossby wave energy dispersing downstream from this jet led to a strong west coast ridge and the baroclinic storm development currently impacting the eastern USA. This evolution has delayed the shift toward western USA troughs by 1-2 weeks per my 23 February discussion. Someone could then comeback and say, “it is always delayed!” The North Pacific Ocean jet stream is collapsing.


Nothing has changed about considerations involving the stratosphere since last week. Please see the following link for details.


http://ds.data.jma.go.jp/tcc/tcc/products/clisys/index.html


Summarizing, there is no clear MJO signal in spite of what the WH (2004) phase space plots suggest. I do think it is probable to see the moist tropical convective forcing get better organized across the Eastern Hemisphere during the next few weeks, possibly enhancing La-Nina. Of course, we need to keep in mind ALL issues tied to seasonal transition to boreal spring. The GWO is probable to orbit in phase 3 for at least the next 1-3 weeks. Hence, even though I missed the strong North Pacific Ocean jet this past week, I can almost linearly move up the outlooks from last week up one week.


More troughs impacting the western and central USA, probable for GWO phase 3, appear likely weeks 2-3. Most numerical ensemble prediction systems have been capturing this possibility during the last few days. Blocking structures that are also likely to occur at the higher latitudes may shift a southwest flow storm track across the Plains southward at times. This will increase the risk of high impact weather to above climatology centered on the middle of the country. Locations such as the Upper Mississippi Valley hit hard by severe winter weather may not only experience more of it, but also have “spring” delayed. Violent outbreaks of severe local storms including tornadoes may occur from the lower Mississippi Valley into Ohio Valley and portions of the Deep South. The latter is probable to shift northwest into the northern and central Plains/Upper Mississippi Valley going into May and June.


Finally, I continue to have concerns about the dryness intensifying from far southwestern Kansas into western Texas. There are reasons tied to this base state where storm systems may remain too progressive at least for the next few weeks such that these locations get “dry intrusions”. Hence there is an increased risk of high fire dangers and dust storms for these areas. However, a southward shifted storm track and eventually the seasonal cycle may mitigate that.


Still unchanged, per WMO and other information, quite a bit of severe weather internationally continues to occur, tied to our on-going complicated weather-climate situation. I continue to leave it to the expertise of the appropriate weather centers internationally to alert the public of these risks.


Tropical South America into central and South Africa have an increased risk of severe frontal thunderstorms/flooding rainfall week-1 and possibly week-2. Much of the equatorial Indian Ocean into Indonesia as well as Northern Australia and the Philippines are likely not done with enhanced rainfall. In fact, I think this situation may again worsen weeks 2-3. Locations east of Australia into the South Pacific Ocean islands are likely to remain convectively active at least weeks 1-2. Finally, concerns for additional tropical cyclones remain from the South Indian Ocean into the Southwest Pacific Ocean. That risk may shift north of the equator during the next few weeks.


Appendix



An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (X-axis) can be found at


http://www.cdc.noaa.gov/map/clim/gsdm.shtml



We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous to the WH(2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.


Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.



Links to CPC and PSD ENSO discussions:


http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml



http://www.cdc.noaa.gov/people/klaus.wolter/MEI/ (recently updated)


These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us (soon) to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. In addition, a paper is in preparation by WB that will formally introduce the GWO. Given shift work and upcoming travel, updates will be difficult. I will try to issue at least a short posting next weekend.


Ed Berry

3 comments:

snoman said...

Hi Ed,

Will there be any potential effects from the strongest GLAAM anomalies being pushed into the lower levels of the atmosphere? I noticed we are almost as low as last summer / fall now in spite of the upper 7 levels being around zero. Also, what do you think it will take for the Pacific NW to finally have a cold winter? This one ended up being another huge disappointment for cold weather fans here.

Thank you! Jim

Ed Berry said...

Hi Snoman,


Please see the discussion I just published. That may answer some of your questions. A key feature for a PAC NW cold winter is AK blocking. Various feedbacks tied to a warm west Pacific Ocean has been one of the culprits to the recent lack of PAC NW cold winters, including this past winter.

Ed Berry

Ed Berry said...

Jim,

So there is no confusion, I am referring to the March 15th discussion in my previous response.


Ed Berry