Saturday, July 22, 2006

Balance of Momentum

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).

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.

SST anomalies across the extreme eastern equatorial Pacific remain ~plus 1C, with somewhat larger anomalies extending downward to nearly 100m. The latter may be partly from the seasonal cycle since SST climatology favors cooling in this region, and perhaps from an earlier very weak Oceanic Kelvin wave currently reaching the South American coast. Above average SSTs dominate the eastern Pacific Ocean equatorial cold tongue all the way to the date line, extending at depth to ~200m (deeper than normal thermocline). Latest TAO buoy data still shows surface westerly wind anomalies (~ 5m/s) and even actual westerlies along the equator west of the date line. Deepening of the 20C isotherm suggests evidence of another downwelling oceanic Kelvin wave crossing into the Western Hemisphere. Actual SSTs in excess of 29C still dominate from the South Pacific into the Indian Ocean, and nearly all of the Tropical Northwest Pacific. Additional global SST information can be obtained from latest TAO data here, ESRL/PSD data here, CPC data,

and BMRC at .

From continuous monitoring and various diagnostic and dynamical tools, there is some evidence the weather-climate system is tilting toward a warm event. However, the magnitude of any possible warm event and global impacts are unclear (some additional discussion below). The following are links to ENSO discussions (recently updated).

Please also see the following CPC link (and others therein) for further ENSO, etc., insights.

Coherent signals of tropical convective forcing remain very weak. There has been a loose consolidation deep convection ~10-15N/120-140E during the last few days, while the monsoon system remains active from India into Southeast Asia. Possibly linked to jet streak dynamics with the Southern Hemisphere, yet another small convective flare-up has occurred across the central equatorial Indian Ocean. Three-day averaged OLRA with this activity has been ~minus 50 W/m**2. If I am monitoring correctly, this is the third such kind of event this month, which is much more frequent than observed since the start of 2006. Tropical convection has also increased nicely across the African Sahel during the past week. Attribution to the latter is still unclear (one can always speculate). Finally, enhancement of tropical convective forcing remains from the East Pacific into Northern South America, with 3-day averaged OLRA ~minus 30-50 W/m**2. Category 4 (at least) Hurricane Daniel and recently developed Tropical Storm Emilia continue across the East Pacific (please see for details).

Since I went into some length about the possible modes of variability with the tropical forcing during my last writing, I will not repeat all that here. The Wheeler Phase Space Diagram shows a weak projection onto a MJO from the Western Hemisphere into Africa (links below). However, with tropical forcing signals so weak right now, almost anything can suddenly “pop out”. I do not think there is any true MJO signal at this time, and feel there has not been even a weak one since March 2006.

I am suspicious the slow feedback processes between the SSTs, convection and atmospheric response may be growing and expanding. There may now also be coupling between the moist convection and warm SSTs starting to occur with the East Pacific SSTs, as well. If this is true, that may have been initiated by processes still going on across the Tropical Northwest Pacific (coupling) including the weak oceanic Kelvin waves warming the East Pacific equatorial cold tongue in proper phase with the seasonal cycle. Of course, the enhanced East Pacific tropical convective forcing is well north of the cold tongue (need SSTs ~ >=29C to sustain the storm clusters). I suspect other processes involving the northern extratropics working with the Tropical West Pacific convection started a warming of the north tropical (~5-15N) East Pacific earlier this year (~March; we need to remember the western Pacific convection has been shifting north with the seasonal cycle). The point is these are very complicated dynamical processes which only someone who monitors these things daily may be able to detect. To me, I think it is plausible to propose that the tropical western and eastern Pacific Ocean basins are feeding back off of each other through a complex forcing-response-feedback scenario which cannot be described here. I can physically see how this could further enhance the already slightly above average SSTs from west of the date line to the coast of South America during the coming months (a warm event?). However, this is pure (but I felt useful) speculation on my part, and let us see what happens. Please see for details of the western Pacific tropical cyclones.

Empirical, statistical and numerical prediction tools continue to be inconclusive for useful information about the future evolution of the tropical convection. Please see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools, and for the details (and draw your own conclusions). These tools generally rely on a moderate to strong MJO signal, which is nearly non-existent at this time, per above.

Again, relationships I discussed in my last Blog posting between the mountain-frictional torque index cycle and the 20-30 day tropical convective variability still apply (for now), and will not be repeated. Time-latitude sections of 200mb zonal mean anomalies show anomalous westerly flow is once again slowly increasing throughout the tropical and subtropical atmospheres. Zonal mean westerly anomalies are ~5-10 m/s from about 25N-30S, with the largest magnitudes south of the equator. This has contributed to a positive AAM tendency ~20 Hadleys as of July 19th per reanalysis data, and actual relative tropospheric AAM is increasing (as would be expected).

Tropospheric global relative AAM is near average per the 1968-1997 reanalysis climatology based on data through July 19th. The more recently updated operational data relative AAM plot shows values to have dipped to ~1 sigma below normal based on a 1979-1998 climatology which includes 2 strong and 2 moderate warm events (El-Ninos). I would think tropospheric relative AAM would return to above average (at least for the reanalysis climatology) during the next week or so. In addition to the character of the tropical convective forcing discussed above, the global mountain torque is also increasing (much of the contribution from the tropics including the Andes) and that component will also increase the westerly flow.

The global frictional torque is actually returning to ~ minus 10 Hadleys, with much of that contribution coming from the Southern Hemisphere extratropics (austral winter). Now comes trying to understand the Earth-Atmosphere AAM budget given the tropical convective forcing issues already discussed above and the other matters given attention in the July 19th posting. I can physically see why there are a lot of low pressure/strong baroclinic extratropical storms across the Southern Hemisphere extratropics, leading to the negative frictional torque, as a response to all the feedbacks. As long as the tropical convection and north-south mountain ranges keep adding westerly flow to the atmosphere, eddy feedback processes such as “strong low pressure areas” in the Southern Hemisphere will be needed to maintain AAM balance.

Animations of daily mean anomalies of 150mb and 250mb vector winds present fairly clean fast wave energy dispersions interacting with generally persistent twin subtropical anticyclones ~150E. The anticyclonic gyre ~35N/105E referred to in the previous posting has contributed to a well-defined baroclinic wave packet helping to amplify the current ridge across western North America. It is also apparent there is a large meridional character to the extratropical flow across both hemispheres with some symmetry. GSDM Stage 2 best describes the global weather-climate situation.

Uncertainty remains extremely high. Since I want to keep this posting from not getting much longer, the “bottom line” is I do think a transition to GSDM Stage 3 is a useful offering during weeks 1-3. However, I cannot offer anything about timing and synoptic details. More and more ensemble numerical models show the western North American ridge deamplifying and moving east while a trough develops along the West Coast. Additionally, the polar jet stream westerlies are predicted to increase along the US/Canadian border, all during week 2.

Week 1 (23-29 July 2006): GSDM Stage 2 continuing is most probable. The intense heat from the western USA should start to shift into the Plains during this period, while at least seasonal weather continues for the eastern states. Severe local storms/MCS may be a concern from the Central/Northern Plains into the Ohio Valley. Please see for the latest tropical cyclone information.

Week 2 (30 July - 5 August 2006): GSDM Stage 3 may return, meaning much of the USA may have weather similar to what was observed from ~13-20 July. That is, intense heat may affect a large part of the country once again.

Week 3 (6-12 August 2006): GSDM Stage 3 may persist.

I am concerned the hot weather with maxes above 100F may return to Southwest Kansas as early as Tuesday (7/25), but more likely starting next weekend into at least week 2. At least a few rounds of scattered storms are probable week 1 before the “real heat” starts.

The time -filtered coherent modes Hovmoller plots of OLR and OLRA are at, velocity potential Hovmollers at , and an animation of velocity potential overlayed on OLRAs are at

Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa, ; other imagery here. Latest tropical cyclone statements can be found from, while the latest 3-day averages of OLR totals and anomalies and other data can be found here (animations of various fields from the operational data) (Global Tropical Hazards Assessment available from this site, along with other useful information) (reanalysis AAM plots) (operational AAM plots)

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts (link to our Weather-
Climate discussions) (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 about the middle of next week. Work is also on-going to post a weather-climate discussion on the ESRL/PSD MJO web site hopefully by early August.

Ed Berry

No comments: