Wednesday, July 23, 2008

Burn the Cookbooks!!!

The views expressed are those of the author and do not necessarily represent those of the National Weather Service.”

Please keep in mind the ESRL/PSD GSDM web link, below, while reading this discussion.

The general thought is “ENSO neutral conditions with some lingering effects of La-Nina circulation” for at least the next few months. While in the views of many there is some truth to a statement such as the latter, care must be taken not to overlook the always important details of the subseasonal dynamics (which do impact seasonal outcomes). For instance, one response has been the warming of the equatorial east Pacific Ocean during the last several months, perhaps reflective of a “~year (-1) of a Rasmussen and Carpenter (1982) warm event scenario”. Please see links below, and stay tuned. (note the initial projection) (link 18)

As expected, Eastern Hemisphere tropical convective forcing has become quite intense, currently centered ~5N/100E. Most numerical and statistical MJO prediction schemes did a poor job with this evolution. Reasons include a failure to capture the role of extratropical dynamical processes explained by the WB (2008) GWO. Continuing, there was also difficulty for many week-2 numerical ensembles to predict the extratropics, with the ACC of the NCEP GFS dipping well below zero for the North American sector. The latter are well known issues by WB, and hopefully an opportunity will arise to quantify it statistically.

Three-day averaged OLRA have exceeded minus 90 W/m**2 with the Indian Ocean severe thunderstorm clusters. The area of enhanced tropical rainfall is oriented northeast-southwest, not only enhancing the monsoon systems, but also frontal bands even across southeast China. Per WH (2004) phase space diagrams, there is a less than 1 sigma MJO projection in octant 3. Leaving the interannual component in, the projection is greater than 1 standard deviation, which is more realistic. I do expect this tropical forcing to continue propagating east and north during the next 1-3 weeks.

During the last couple of weeks there has been some attempt to rejuvenate our low AAM base state. Leaving the westerly QBO phase in, global relative AAM dipped to roughly 1 AMU below the R1 data climatology earlier this month. With the classic transition of Eastern Hemisphere tropical upper tropospheric circulation anomalies to Indian Ocean (west Pacific Ocean) anticyclones (cyclones), equatorial Pacific westerly wind flow anomalies have appeared (roughly 20-25m/s around the date line). Particularly for the Southern Hemisphere, there has been poleward propagation of zonal mean zonal wind flow anomalies including easterlies shifting into the subtropical atmospheres.

These are only a handful of many reasons why global relative AAM tendency spiked to ~plus 20 Hadleys forcing the WB (2008) GWO into approximately octant 4 of phase space on 20 July. Both AAM tendency and the GWO are headed back down (through 21 July), and whether the rejuvenation discussed above is a longer term signal remains unclear (not to contradict other weather-climate signals).

Broadly, there continues to be good tropical-extratropical coupling, including interhemispheric meridional symmetry of zonal mean zonal wind anomalies favoring anomalous midlatitude ridges. Comparing recent daily mean 150mb vector wind anomalies to the MJO composites, phase 3 of 250mb psi is loosely representative (“apples and oranges” issues understood). For instance, there is a well defined Rossby wave energy dispersion (RWD) signal from the west Pacific Ocean cyclones that links up the trough-ridge-trough pattern across North America.

The careful reader will also see these signals in the GWO composites. Avoiding more length and confusion, there is ~50 day period between the recent minima in the time series of the global frictional torque. This leads to the point that decay time scale processes explained by the WB (2008) GWO are currently at least as equally important as any MJO contribution. Hence my feeling (one reason) why the notions suggested by the phase 3-4 composites of the GWO may be representative for roughly weeks 2-3. We need to remember these composites are still centered on 1 July (should be updated soon), and that using these must be defended on sound scientific reasoning, not “shoe horned”. In any case, this thought is consistent with my discussion posted on 12 July.

For the North American sector, perhaps ~days 10-20 (roughly 2-13 August) ridge amplification off the west coast into Alaska leading to an approximate reversal of circulation anomalies across the USA is a possibility. My confidence in this scenario is not high; however, I feel it should be “put on the table”. There is little suggestion of this scenario from the models and certainly the official USA week-2 forecast. In fact, the suggestion of warmth officially across the northern states is also inconsistent with the MJO signal. The "meteorological reasoning" for the official week-2 forecast is based on seemingly subjectively blending various models, including deterministic runs (my dog (angry Rottweiler) does not like this!).

The probable USA temperature patterns can be seen from the 2m air composite signals. However things pan out, a shift toward below normal air temperatures from the Great Lakes extending southward toward the Central Plains should be considered. Once again, locations periodically focusing on the Northern and Central Plains into the Ohio Valley are the most probable storm track for MCS activity. Other locations possibly impacted by periods of heavy rainfall and severe local storms are portions of the eastern states.

Intense to severe tropical thunderstorm activity should shift east and north through portions of India and Southeast Asia during the next 1-2 weeks. Included are frontal bands across southeast China. By week-3 the most intense tropical convection may focus on the northwest Pacific Ocean including the South China Sea and the Philippines. The tropical cyclone hazard may also increase for these areas. Whether or not convection also propagates eastward along the equator to near the date line is unclear (may cause an equatorial westerly wind burst). The tropical cyclone hazard for the Western Hemisphere should be diminished weeks 1-2 (hybrids always understood, including the one currently northeast of the Philippines).

Please see the latest official tropical cyclone forecasts for all basins. I trust the expertise of the appropriate meteorological centers to alert the public of additional weather hazards worldwide.


Links to CPC and PSD ENSO discussions:

The following is a link to information about the stratosphere and other nice monitoring tools:

The following is a link to NCEP model verifications (surf around for lots more)

The following is a link discussing recent global weather and related events

These are probabilistic statements. We hope that an opportunity will arise for us (soon) to allow our dedicated web page effort to mature, expediting objectively and accountability. This web page effort will hopefully include an objective predictive scheme for the GWO with hindcasts.

The WB (2007) paper on the GSDM has been published in the February issue of MWR. In addition, the first of a two-part paper has been submitted to MWR where WB formally introduces the GWO. A pdf version can be downloaded from the following link

Overlapping seasonally varying subseasonal composites for variables such as surface temperature, precipitation, geopotential height and streamfunction anomalies are planned on being posted on the web site mentioned above and presented in part-2 of our paper. We want to emphasize notions such as global-zonal mean-regional scale linkages as well as forcing-response-feedback (with subsequent interactions) relationships. An important purpose is to provide a dynamical weather-climate linkage framework to evaluate the numerical models in a sophisticated manner as part of a subseasonal (and any time scale) forecast process, in addition to a climate service for all users. Relying on the numerical models alone is a cookbook!

Given shift work and travel, updates are extremely difficult. I hope to do another discussion around the weekend of 2-3 August.

Ed Berry

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