Saturday, June 03, 2006

Things Got Messy

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

To get the most from these postings, I kindly recommend that at least some perusal of our paper be given. The gist is from taking into consideration the interactions of 4 different subseasonal time scales, a sequence depicting a coherent set of repeatable events has been derived. 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. The following text merely updates what I posted on May 31st .

With the exception of a cool region just off the coast of South America, SSTs across most of the central and east equatorial Pacific not only remain slightly above average, but have increased a bit during the past few days. As will be discussed below, complicated interactions between a Rossby wave energy dispersion (RWD)/anticyclonic wave breaking (AWB) event of an Alaskan block across the North Pacific, and a convectively coupled Kw, have produced anomalous surface westerly flow along the East Pacific cold tongue. These anomalies are ~5-10 m/s, with even some actual westerly wind flow. An oceanic Kw has resulted, and appears to be stronger than the oceanic Kw that occurred earlier during April.

SST anomalies range from ~ plus .5C to even 1.5C around the date line. SSTs vary from ~22C at the South American coast (anomalies roughly minus 3C) to around 31C just south of the equator at 160E (recall that we use SSTs of 29C as a threshold for supporting persistent tropical convection). SSTs of 29C and higher also extend from the southwest Pacific into the IO. At depth, as was true about 3 weeks ago, anomalies around plus 1-2C extend from 50-250m east-west along the equatorial cold tongue, meaning a slightly deeper (and deepening) than normal oceanic thermocline. SSTAs from the IO into the west Pacific are at least plus .5-1C, with the South Pacific horseshoe experiencing ~ plus 1-2C. The Caribbean into much of the north tropical and equatorial Atlantic Ocean basin also has SSTAs ~ plus 1-2C. Finally, plus 1-2C SSTAs are also present along the southwest coast of Africa and from the east coast of Africa into much of the S. IO. Latest prediction from CPC expects ENSO-neutral conditions to prevail during the next 3-6 months (see latest TAO data here, ESRL/PSD data here ).

The following are links to ENSO discussions.

The signal from the MJO remains weak. However, a strong convectively coupled Kw is moving through the WH as I type (phase speed roughly 15 m/s), and appears to be centered ~40-60W. A observed from full disk satellite imagery (links below), tropical convection is quite intense from the East Pacific toward the North Atlantic ITCZs. Tropical Depression #2 is a direct result of this Kw (see for details).

Time-filtered Hovmoller plots of OLR and OLRA (see objectively illustrate the above mentioned convectively coupled Kw as well as other variations of tropical forcing that have been important during the last few weeks. For instance, a convectively coupled Rossby mode lead above average rainfall across most of the Arabian Sea (weekly averaged OLRAs ~minus 70-90 W/m**2 in the latter area) during the past week. This Rossby mode not only may have contributed to the formation Typhoon Chanchu a few weeks ago, but perhaps to the early onset of the active phases for the Indian and Southeast Asian Monsoons. Additionally, the rapid RWD event interacting with the AWB discussed above may have been initiated by the heating from the enhanced convection across the Arabian Sea.

Monitoring tools such as Hovmollers of velocity potential (CHI) and animations of CHI and OLRA now suggest the main tropical convective forcing signal to suddenly be in the WH, due to the Kw. However, I do think this is a transient situation, meaning the most robust tropical convective forcing will quickly return the EH. (see and )

Real-time satellite imagery suggests convection continuing to increase from India southeast toward much of Indonesia. I think this represents an eastward shift from the Arabian Sea. Convection remains strong across Southeast Asia and sporadic from the central Pacific Ocean to just north of the Polynesian Islands (due to warm SSTs). Satellite imagery and other information can be found from the following links: eastern hemisphere, full-disk west Pacific, mtsat, IO, Africa; other imagery here; tropical cyclone statements; the latest 3-day averages of OLR total and anomalies, and other data can be found here.

Statistical and numerical models of the MJO (see ESRL/PSD MJO tools , BMRC MJO tools, CPC MJO tools) are still generally inconclusive given the weak MJO signal. However, both the Wheeler technique and empirical forecasts from the time filtered coherent modes technique do suggest an eastward propagating signal from the North IO through much of Indonesia through week 2. This, in fact, may already be starting to occur.

My feeling is that we are continuing to see the tropical convective forcing reorganizing in the presence of the seasonal cycle (including the onset of the EH monsoon systems). I think it is only a matter of short time before the very warm SSTs from Indonesia into the southwest Pacific force tropical convection more robustly than currently observed. I think the convectively coupled Kw will be well into the EH in about 10 days, and the upper tropospheric divergence with that feature may lead to a rapid intensification of convection in the region of Indonesia by that time. This will have consequences for the global circulation.

Should the equatorial regions from the Maritime Continent (MC) into the western Pacific become the dominant area of tropical convective forcing during week 2, that would be consistent with the eastward (and southward) shift we are starting to see. That is, we may see a shift from the loose area currently centered ~15N/90E to an intense concentrated region centered ~5N/130E, lending some truth to both the empirical and Wheeler methods. Any west Pacific convection may combine with the Indonesian thunderstorm clusters (possibly enhanced by the Kw) while the Southeast Asian Monsoon continues. A MJO signal may evolve out of this possible enhanced region of ~MC tropical forcing.

The interplay between SSTs, tropical forcing and circulation response-feedbacks is ongoing. Zonal mean upper tropospheric anomalous westerly flow has propagated well off the equator (mainly into the Southern Hemisphere (SH) due to the seasonal cycle) and has been replaced by easterlies. Per time-latitude sections of 200mb zonal mean zonal wind anomalies, westerly anomalies ~15m/s have propagated into the SH extratropics with ~5 m/s or so moving into the NH. Roughly ~10 m/s zonal mean easterly anomalies are now present throughout the global tropical and subtropical atmospheres. Plots of recent daily mean 150mb and 250mb vector wind anomalies (see ) present a signal of twin subtropical anticyclones centered ~ 60 deg. east longitude, as well as ~160W thanks to the Kw. Also, weak upper equatorial tropospheric westerly wind anomalies have started to appear across portions of Indonesia, which may signal the development of a MJO.

Rapid wave energy dispersions affecting the PNA sector continue from the EH subtropics. For instance, during the last couple of days yet another blocking structure has appeared around Kamchatka tied with the North IO convection moving east. I believe this may be a “carry over nonlinear eddy feedback process” from the 2005-06 cold season. As shown by plots of zonal mean AAM flux divergence, there is now convergence of AAM around 30-40N (where there was divergence a few days ago), leading to a strengthening of westerly flow across the North Pacific. So, instead of fluxing zonal mean AAM from 30-40N which was due to the AWB of the Alaska block, zonal mean AAM is now being converged in that latitude band. I attribute all that to the eastward shifting of the North IO convection interacting with a high latitude retrogressive transient that started in Europe during late April (latter discussed in past Blogs and ESRL/PSD weather-climate discussions).

AAM anomalies are ~ minus 1 standard deviation below the 1968-1997 climatology, with anomalous zonal mean easterly flow across the subtropics and midlatitudes of both hemispheres (particularly the SH). AAM tendency has returned to near zero based on the reanalysis data, which lags 3 days from the current time. The operational data plot shows that AAM has decreased to 2 standard deviations below the 1979-1998 climatology (see, and I would expect the current AAM tendency to once again be negative (see reanalysis AAM tendency plot ).

In addition to the tropical forcing getting better established across the EH, other components within our GSDM framework contributing to the decline in AAM include the mountain and frictional torques (see plot for mountain torque and plot for the frictional torque; see for all AAM plots). A general global mountain/friction torque index cycle has been going on since early April (loosely with mountain torque going up and frictional torque going down) while global mountain torque has been undergoing ~ 25 day variations. The latter include contributions from north-south mountain ranges such as the Andes and those present across East Asia. I would expect the East Asia mountain torque to soon become negative given the anomalously low mean sea level pressure in that region. The point is there is still westerly flow being removed from the atmosphere by the earth both in the zonal mean and global sense.

I maintain that GSDM Stage 1 still best describes the global weather-climate situation. However, things are messy, and once can easily argue we have a Stage 3 global circulation with generally GSDM Stage 1 tropical convective forcing. Understanding the seasonal cycle, we saw a situation similar to our current one back in December 2005 (see past weather-climate discussions on the ESRL/PSD web site).

The tropical convective forcing returning into the EH and consolidating ~ 0/100E (see around a week ago helped with establishing Stage 1. Now, after some messy rearrangements which were discussed above, I think the circulation working with the warm SSTs is going to re-establish more robust EH tropical convective forcing in the region of Indonesia during week 2. Again, one must always think in terms of forcing, response and feedbacks with some understanding of the dynamics.

I also think there is a possibility for the Indonesian convection to start shifting east toward the western Pacific during late week 2 into week 3, possibly as a MJO. Within the GSDM framework, a transition to Stage 2 would be probable if this occurs. I have discussed this possibility during at least my last 3 Blogs, and I am going to stay with it. Most ensemble prediction systems (EPS) do not show this since models do not predict tropical convection very well after about 4-5 days. However, there are hints of this possibility starting to emerge not only from the Wheeler and coherent modes techniques, but also from the GFS and CDC EPSs.

I continue to state forecast uncertainty is higher than has been true recently. Sources include tropical convective flare-ups from other ocean basins such as the North Atlantic and the roughly 25-day variations of the global mountain torque, especially from the Andes Mountains this time of year. The sudden and not predicted (by me) emergence of a strong WH convective signal thanks to the Kw did not help matters any. I have also got to start wondering what the ramifications from the warming of the East Pacific cold tongue due to the oceanic Kw will have on the future of the weather-climate situation.

In the outlooks that follow, I am still going to key on a possible intensification of the tropical convection ~ MC during week 2, possibly shifting toward the west Pacific warm SSTs as a MJO during week 3. Current satellite trends and monitoring tools maintain my confidence for that notion. I am also considering the mountain/frictional torque index cycle and ~ 25 day variations of at least the East Asian mountain torque. All of these components may be increasing by week 3 (seasonal cycle and other considerations understood).

Week 1 (4-10 June 2006): GSDM Stage 1 is most probable, but with complications such as a summertime rendition of an extended East Asian/North Pacific jet. The models and I have struggled with this during the past week. All models generally show, with low ensemble spreads, the pattern of an east Pacific trough, Rockies to Great Plains ridge and East Coast trough through much of next week. There will be at least a couple of short-wave troughs moving along the relatively strong westerly flow just north of the ridge. This pattern translates to above normal temperatures and below normal precipitation for about the western 2/3 rds of the country with the opposite for the East. Excessive heat may become a concern for portions of the Intermountain west and Plains (understanding dewpoints will be generally low) with heavy rainfall for portions of the East and Northeast. Finally, some severe local storm and MCS activity is likely for especially the northern Great Plains into the Great Lakes/Ohio Valley states.

As I discussed in my last Blog, this “ridge pattern” for the lower 48-states would likely not persist, in opposition to the predictions of most numerical models. Models such as the ECMWF, GFS and Canadian indeed now do predict this ridge to move east while a trough develops across the western USA, all starting by roughly next weekend.

Week 2 (11-17 June 2006): GSDM Stage 1 would be most probable, possibly transitioning to Stage 2 late. I am slowing down my evolution from the May 31st Blog thanks to the nonlinear feedbacks (discussed above) that I did not expect. Also, understanding there will be a northwestward shift, the general pattern of a western states trough and south central to southeast USA ridge similar to last spring would be probable (including the well known temperature and precipitation anomalies). The added westerly flow across the North Pacific looks to delay a true summertime rendition (with the shorter wavelengths) of GSDM Stage 1 for the USA.

If we transition to GSDM Stage 2, like was observed during especially the summer of 2004, that situation tends to favor cool/wet across the central part of the country with warmth/dryness along the west coast and southeastern USA (please see for example).

This type of pattern can be a prolific producer of heavy rainfall and severe local storms from roughly the eastern Rocky Mountain States of Colorado-Montana through the Central/Northern Plains, Mid/Upper Mississippi Valley into the Great Lakes and Ohio Valley regions.

Week 3 (18-24 June 2006): GSDM Stage 2 continuing and/or emerging would be most probable.

Specifically for southwest Kansas, I think we can say there is some truth to my outlooks for weeks 2-3 since most models and their ensemble are now lending support (all part of a complete forecast process) . Well above average temperatures and below normal precipitation is a good bet through most of next week. However, 1 or 2 surprises may occur, and will have to be monitored. By next weekend, even with the storm track to our north, jet streaks rotating northeast along the STJ may produce some decent storms along a dryline/lee side trough. Also, a front could easily stall across this part of the world even with southwest flow. If we transition to GSDM Stage 2, our rain chances will increase and temperatures will cool down. That transition may start during week 2. Whatever the case may be, at least for the next few weeks, the notion of a “sufficating hot/dry anticyclone” building across this region appears less probable.

Latest CDC Ensemble Forecast

Latest NCEP Ensemble Forecast

Additional NCEP Ensemble Output

Latest Canadian Ensemble Output

Latest Deterministic ECMWF Forecasts

Please see the CPC Drought Monitor for areas of dryness and the latest official outlooks and statements from 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.

All weather-climate discussions can be obtained from the following link:

These weather-climate discussions are done as time permits.

I will attempt another Blog posting during the middle of next week.
Ed Berry

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