Saturday, August 12, 2006

The New World Atmosphere

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.

There has been little change in the global tropical SSTs since my August 9th posting. Latest TAO data suggests the third (weak) downwelling oceanic Kelvin wave since mid-March was located around 165W based on the latest five-day averaged TAO data (link below). This feature was accompanied with SSTAs ~plus 1-2C down to about 100m.

The trades have increased across a good portion of the north tropical oceans during the last week, particularly across the Western Hemisphere. This includes the trade wind surge across the central and eastern equatorial Pacific. Surface easterly wind anomalies have been ~3 m/s, and have led to some reduction of the anomalous warmth across the central Pacific. There are still pockets of surface westerly wind anomalies from near the date line into Indonesia. In direct response to a recent consolidation of tropical convective forcing ~10-15N/120E, the Eastern Hemisphere monsoon systems have intensified (more said below). In fact, loosely surface cross-equatorial anomalous southerly flow generally exists from Africa to the date line, with magnitudes of ~5-10m/s. The recent (and expected) upsurge in tropical cyclone activity across the TNWP has contributed this cross-equatorial flow.

Additional global SST information can be obtained from latest TAO data here, ESRL/PSD data here, CPC data

http://www.cpc.ncep.noaa.gov/products/predictions/threats/index_gloss.html,

and BMRC at

http://www.bom.gov.au/bmrc/ocean/results/climocan.htm .

From continuous monitoring and various diagnostic and dynamical tools, there continues to be evidence that the weather-climate system is still tilting toward a warm event. The current trade wind surge discussed above has dampened some of the positive SSTAs associated with it. The following are links to recently updated and informative ENSO discussions.

http://www.cdc.noaa.gov/people/klaus.wolter/MEI/

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

To me, it is unclear what the magnitude of any possible warm event may be, including global impacts. I should also point out that the various modes of subseasonal variability (addressed in this Blog and part of the GSDM) which can initiate slower processes such as a warm event constantly impact the atmosphere in terms of forcing-circulation response and feedbacks. The latter includes extreme weather events.

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/

Details of consolidations and other behaviors of tropical forcing have been presented in previous writings and will not be repeated. Per 3-day OLRA plots, tropical forcing remains very intense from ~120E-170E at about 10-15N with magnitudes ~minus 50-90 W/m**2. Enhanced tropical convection is also spreading into the Western Hemisphere to about 150W, while the East Pacific ITCZ becomes more active. There is also another flare-up occurring along the equator in the western Indian Ocean (~40E), possibly related to dynamic forcing from the Southern Hemisphere. The latter is not the “first time” this has occurred during summer 2006. In fact, the latest 30-day OLRA plots show negative anomalies of ~15-25W/m**2 in this region (with suppression of ~plus 50-70 W/m**2 from the central equatorial Indian Ocean into Indonesia – see plots). These features have been propagating fairly rapidly north only to enhance the monsoon systems. What will this latest episode do???

I have not addressed a mode of tropical convective variability in these writings which may have been obvious to many others. Since roughly mid-late April 2006, there has been ~50-60 day enhancement of tropical convective forcing across the west central and TNWP, centered ~160E. After late April, there was enhancement during late June and also right now, with suppression during May and July. The weather-climate discussion we are working on will go into additional detail.

The AAM plots have shown a fairly clean atmospheric response signal to this ~50-60 day tropical convective mode. I should emphasize there are a whole spectrum of processes always going on in the atmosphere. The reasons why this ~50-60 day variability appeared are unclear to me. These may be a natural response to the many other faster dynamical processes I have been talking about in the presence little or no MJO and an emerging warm event. I do not know if these ~50-60 days modes are oscillatory, stationary, something purely stochastic, etc. Whatever the case, it is there and is a concern for attribution and prediction.

I can really make things even more “messy” by offering the tropical convective forcing we are now seeing coming into the Western Hemisphere may be our 30-day mode propagating east while the Indian Ocean flare-up may evolve into another convectively coupled Kelvin wave (and I am just getting started!). This is why disciplined monitoring every day is critical with the GSDM framework to have any hope of mitigating “weather surprises”. In the week 1-3 outlooks I will offer, I am going to focus of the ~50-60 day western Pacific tropical convective forcing signal. Even though my uncertainty will remain tremendously high, I do have slightly better confidence for weeks 1-3 than previously.

Please see http://www.npmoc.navy.mil/jtwc.html and http://www.nhc.noaa.gov/ for tropical cyclone concerns. Empirical, statistical and numerical prediction tools continue to be inconclusive for useful information about the future evolution of the tropical convection, and that is not going to change for the foreseeable future (bravely spoken!). Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/index.primjo.html for the details. These tools generally rely on a moderate to strong MJO signal, which is nearly non-existent at this time.

However, I think it is worthwhile to again note that the Wheeler index did have a ~1 sigma signal for about a couple of days last week, centered on Southeast Asia and the TNWP (see link), not inconsistent with the above and tropical wind fields (discussed below).

http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/RMM/phase.Last90days.gif.

There has not been much change to the distribution of 200mb zonal mean zonal wind anomalies since August 9th. There has been a slight increase in the magnitude of anomalous westerly flow ~ 15N with magnitudes ~5m/s (plots can be generated at http://www.cdc.noaa.gov/map/time_plot ; see for yourself some of the details).

If the reader generates a time-latitude section of 150mb zonal mean meridional wind anomalies since April 1st, 2006, a loose relationship between increased zonal mean divergence of ~3 m/s and enhanced convection across centered ~140E can been seen, following the ~50-60 time scale. Of course, this enhanced divergence will shift northward with the seasonal cycle, and may suggest a local increase in the Hadley Cell. The latter may be true at this time.

Global AAM signals are relatively weak, and links are given to the plots. However, I want to make the point that the global frictional torque is already at ~plus 10 Hadleys above the 1968-1997 reanalysis climatology as of August 9th, with a large contribution from the above normal trades. I think we are going to see behaviors similar to roughly April into early May and mid-late June into early July meaning another strong positive phase of a global (remember) mountain-frictional torque index cycle linked to the ~50-60 day tropical convective mode. Recall that the global frictional torque leads the mountain torque by ~6 days.

Animations of 150mb and 250mb daily mean vector wind anomalies still depict loose twin subtropical anticyclones centered around 120-140E in a region of anomalous divergence. Westerly wind anomalies ~15-25m/s continue to be present throughout much of the tropical Pacific into Northern South America, while anomalous easterly flow slowly collapses from the Indian Ocean into Africa. I might also mention that I can link the recent appearance of the large anticyclonic circulation gyre just southeast of Greenland to a recent fast Rossby wave energy dispersion (RWD) initiated by our 30-day convective mode (is that what it is?) crossing the date line into the Western Hemisphere.

To summarize, I think we have a weather-climate situation consisting of 1) the ~50-60 day tropical convective mode, 2) the ~30-day convective mode, 3) other faster modes of tropical convective variability like those caused by the Southern Hemisphere which are nothing but noise, 4) the ~50-day mountain-frictional torque index cycle going perhaps into a positive phase, 5) the rapid extratropical Rossby wave energy dispersions/baroclinic wave packets, 6) weak or non-existent MJO, which may be part of a “new world atmosphere” emerging (I need to add some “strange humor”), 7) possible ENSO and/or global warming signal, and 8) seasonal transition into boreal autumn. The latter includes the strengthening Northern Hemisphere polar vortex, which should displace toward the TNWP convective forcing initially. Restating, I can list a lot of components within our GSDM framework; however, it still all adds up to a weak predictive signal at best.

GSDM Stage 1-2 best describes the current global circulation/weather-climate situation. My thoughts are about another week or so of this situation, with an evolution to GSDM Stage 2 weeks 2-3. Understanding seasonal transition, if there is any truth to our ~50-60 day mode remaining coherent, weeks 3-4 may see a transition to GSDM Stage 3-4, possible evolving to GSDM 4-1 weeks 4-6. Of course, this timing is not statistically useful (noise), but is offered out of concern for the North Atlantic hurricane season. From the subseasonal viewpoint, a scenario of conditions becoming favorable for North Atlantic tropical cyclogenesis as the climatological peak approaches must be monitored.

Week 1 (13-19 August 2006): GSDM Stage 1-2 seems probable. Per the NCEP GFS and other models, there look to be at least 1 or 2 more mobile troughs to dig along the USA west coast, then move inland through the northern states. This would suggest additional episodes of heat to expand from the Central Rockies into much of the Central/Southern Plains and Deep South. An active severe local storm/MCS track would be possible from the Northern/Central Rockies into the Mid/Upper Mississippi Valley/Great Lakes-Ohio Valley. While tropical cyclogenesis may become active across the East Pacific while the TNWP remains quite active, the Atlantic should remain suppressed.

Week 2 (20-26 August 2006): Transition to GSDM 2 would be expected, which means synoptically I may finally see the ridge shift back into the western USA. A downstream central and eastern states trough would be expected. Many models responding to initial conditions are supportive. The anticyclone developing just south of Greenland could shift toward the west, and may contribute an extended period of a negative NAO (I can talk indices too!). The NAO is already negative linked to the flare-up discussed above. While the western states warms/dries, the central and east would be expected to become cooler and wetter. Tropical cyclone development across the North Atlantic may remain suppressed while the East Pacific may be active.

Week 3 (27 August – 2 September 2006): GSDM Stage 2 may mature, perhaps transitioning to GSDM Stage 3. Synoptically this could be a period similar to late June (West Coast ridge maybe into AK, Mississippi Valley/Plains trough, etc.) understanding the seasonal cycle, with the expected weather.

I hope Southwest Kansas can get some much needed rainfall week 1. Timing of fronts with the diurnal cycle of thunderstorm activity has been just one of many factors not favorable for rainfall this summer. I can see this same scenario for portions of this area on Sunday, August 13th. Hopefully the dynamics with the mobile troughs will mitigate some of that. Weeks 2-3 are unclear given our climate sensitivity, especially if the ridge retrogrades. Broadly I would expect a dry regime as the ridge shifts back toward the West Coast while temperatures remain near-above average. At some point a decent surge of cooler than normal air may be possible as Labor Day approaches. To improve our rain chances we need the westerlies farther south with a trough position across the central Rockies (there are other favorable patterns).

The time -filtered coherent modes Hovmoller plots of OLR and OLRA are at http://www.cdc.noaa.gov/map/clim/olr_modes/), velocity potential Hovmollers at http://www.cpc.ncep.noaa.gov/products/intraseasonal/vpot_tlon.html , and an animation of velocity potential overlayed on OLRAs are at http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ir_anim_monthly.shtml.

Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa, http://www.jma.go.jp/en/gms/ ; other imagery here. Latest tropical cyclone statements can be found from http://www.nhc.noaa.gov/, while the latest 3-day averages of OLR totals and anomalies and other data can be found here
http://www.cdc.noaa.gov/map/clim/glbcir.anim.shtml (animations of various fields from the operational data)

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml (Global Tropical Hazards Assessment available from this site, along with other useful information)

http://www.cdc.noaa.gov/map/clim/aam.rean.shtml (reanalysis AAM plots)

http://www.cdc.noaa.gov/map/clim/aam.shtml (operational AAM plots)

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts

http://www.cdc.noaa.gov/people/jeffrey.s.whitaker/refcst/week2/

http://www.cdc.noaa.gov/MJO/Forecasts/climate_discussions.html (link to our Weather-
Climate discussions)

http://www.cpc.ncep.noaa.gov/schemm/z500ac_wk2_na.html (model performance; please navigate to others)

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from the Storm Prediction Center not only for severe storms, but also fire weather concerns. Finally, the CPC USA Hazards Assessment for offers additional insights not only for possible week 1 high impact weather, but week 2 as well.

I will try to do another posting around Tuesday, next week. Work is also on-going to post a weather-climate discussion on the ESRL/PSD MJO web site hopefully by about the middle of this month. Due to shift work obligations (graveyards), my next couple of postings may be quite brief, with the possibility of missing next weekend.

Ed Berry

1 comment:

Eric said...

This pattern looks a lot like August 1989 according to some WestPac experts.