Friday, October 31, 2008

The Dog is Back – No 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 91-day signal to noise ratio (snr) anomaly composites are now updated daily, centered on the date shown. Please see product descriptions. Part-1 of WB (2008), where the GWO is formally introduced, is in press for publication in MWR. There is a link in the Appendix to download the manuscript.

The general spatial pattern of global SSTs, tropical and extratropical, has not significantly changed for the past several weeks. Warmest ocean waters have shifted south of the equator in the warm pool region, where totals in excess of 30C exist just northeast of New Guinea. Widespread SSTs ~29C and greater extend from the TNWP into southwest Pacific Ocean. However, anomaly magnitudes are not that large. Finally, the strongest equatorial trade wind surge in at least 6 months from the Dateline to Indonesia is weakening. Negative anomalies across the central Pacific are ~1C. An upwelling oceanic Kelvin wave impacting the subsurface of the central and eastern equatorial Pacific Ocean during the next few months would be expected. See links below. (note the initial projection) (link 18).

Similar to about a month ago, the WB (2008) measure of the GWO has decoupled from the tropical convective forcing (stated loosely for our purposes), including the MJO component. Keeping a complicated story simple, dynamical processes tied to meridional momentum transports (both hemispheres), positive surface torques (including from the above mentioned trade surge) and the baroclinic eddies greatly reduced anomalous zonal mean easterly wind flow across the subtropical atmospheres.

This is why the extratropical eddies have been northeast-southwest tilted (northwest-southeast Southern Hemisphere) in a base state favorable for anticyclonic wave breaking and anomalous midlatitude ridges for at least the past 2-3 weeks. Furthermore, anomalous zonal mean westerly wind flow has been shifted well poleward across the Northern Hemisphere, ~10-15m/s @ 200mb/60-70N. A rapid evolution through phases 8-1-2 of the WB (2008) GWO snr 250mb psi composite anomalies best describes the extratropical circulation through at least week-1.

Eastern Hemisphere tropical convective forcing remains intense, extending from Southeast Asia across the equator ~130E into the warm southwest Pacific Ocean. There has been a strong MJO component, recently projecting ~2 sigma in WH (2004) phase space. Through 30 October the latter was located in phase 5, phase 4 with the important interannual signal left in. Phase 5 of the WH (2004) 250mb snr composite psi anomaly plots is fairly representative of current tropical circulation anomalies.

While there continues to be a preference for zonally oriented Rossby wave energy dispersions (RWDs) given the GWO, the MJO component of the tropical forcing has been unquestionably impacting the global extratropics including the USA. For instance, per animations of upper tropospheric daily mean vector wind anomalies, ~22 October RWDs from the twin subtropical west Pacific Ocean cyclones contributed to west coast ridge amplification and baroclinic storm development on the Plains. There is actually a weak signal supporting this notion from phase 4 of the WH (2004) MJO snr 250mb composite psi anomaly plots.

Yes, on average midlatitude signals from tropical forcing are weak especially this time of year; however, individual cases can be/are exceptions. For instance, linked to GWO dynamics briefly addressed above, wind speed anomaly magnitudes associated with the MJO forced RWD heading toward the USA west coast 7-10 days ago were ~40-50m/s at 250mb. This is an example why daily monitoring is critical, and why looking for classic cookbook responses like “pineapple expresses” do not work most of the time in the real atmosphere (the Rottweiler is very angry!). Finally, there is also the issue that the WB (2008) measure of the GWO is a dynamical global index while tools such as the WH (2004) measure of the MJO are empirical and equatorially confined. The former is needed to more completely understand tropical-extratropical interactions, and our recent paper gives some attention to that matter.

Cutting to the chase, the MJO component has been propagating east ~5 deg long/day since roughly mid October. This movement is slowing, and the tropical convection with it has lost some intensity. Additionally, tied to the GWO, tropical convection has already been increasing across the Western Hemisphere. Animations of upper tropospheric daily mean vector wind anomalies present a strong signal of a RWD/baroclinic wave packet from a central Pacific Ocean anticyclone all the way into Europe. GWO processes like these should contribute to subtropical transition bringing the anticyclones (cyclones) into the Western Hemisphere (Eastern Hemisphere). However, I think it more probable that the subtropical anticyclones will be stronger than the cyclones, and the latter short-lived.

Updated through 28 October, the global frictional torque is ~minus 10 Hadleys, due to midlatitude frictional dissipation and weakening trades. Although this negative response is similar to around mid September, roughly 50 days ago, the physical processes responsible then and now are not. That said, per reasons discussed in WB (2008) and the current situation with the tropical convective forcing, both the GWO and the MJO should couple in ~octants 7-8-1 of phase space approximately mid-late November. In some sense, the global circulation at that time may be a superimposition of the circulation anomalies depicted by the corresponding GWO and MJO snr 250mb composite psi anomaly plots.

Again, timing is simply noise and my confidence is lowered because of seasonal cycle issues. Further, this is only “scientific speculation” until we develop better tools. All one can say physically is the following. As the tropical forcing shifts into the western Pacific Ocean during the next 1-2 weeks, a sequence of events involving first a positive global mountain torque followed by equatorward momentum transport would be probable. Subsequently, an extended North Pacific jet, then finally (in the GWO composite sense) a zonally oriented RWD/baroclinic wave packet collapsing the jet leading to a western USA trough.

However, I think this reasoning is scientifically defensible and provides additional predictive information to evaluate numerical models should they “catch on”. As discussed a week ago, the latter suggests more strong troughs to impact the western USA leading to an active storm track on the Plains weeks 3-4. A cold regime may follow during December focusing on the central states (while extending from perhaps the Pacific Northwest into the Great Lakes and northeast states).

In general, the character of the global circulation remains La-Nina. At some point during the upcoming boreal winter a quasi-stationary state residing in octants 3-5 of GWO phase space may evolve. Weather ramifications for all the above should be understood from the snr composites.

Internationally, intense thunderstorm activity should continue weeks 1-2 from particularly the TNWP/portions of Indonesia into the SPCZ. In fact, severe tropical thunderstorm flare-ups including cyclones are a concern on both sides of the equator. Locations impacted may include the Philippines/South China Sea and the Polynesian Islands. There are already Western Hemisphere flare-ups including increasing diurnal thunderstorm activity across tropical South America. Locations around the Americas including Brasil should see an increase in rainfall ~weeks 2-3 (maybe longer). Africa into the equatorial Indian Ocean are probable to be suppressed until ~ later week-2 and week-3. Regardless of the details, I would be surprised not to see much of the Indian Ocean into Indonesia strongly convectively active going into December.

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. For instance, much of Western Europe has recently been active, and Arctic air is rapidly building from North America into Asia.


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. The first of a two-part paper, where WB formally introduce the GWO (WB (2008)), has been accepted for publication MWR. A pdf of the in press version can be downloaded from the
following link:

We hope additional overlapping seasonally varying global and zonal mean subseasonal composites for variables such as surface air temperature, precipitation, geopotential height, winds and streamfunction anomalies can be posted on the ESRL/PSD GSDM web site. Part-2 of our paper will discuss the latter. 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! I plan on posting a complete discussion the weekend of 8-9 November.

Ed Berry

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