Saturday, October 04, 2008

Global Circulation Following the Markets – Going Down

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.

Writing like the record that never stops playing, overall spatial patterns of global SSTs, tropical and extratropical, have changed little during the past several weeks. However, some warming has recently occurred across the equatorial west central and southwest Pacific Ocean, with totals in excess of 30C and anomalies ~plus 1C. In fact, latest 5-day averaged TAO buoy data suggest these "moderate" warm anomalies extend to a depth of around 150m from the Dateline to 140E. The warming was a response to, at best; a modest equatorial westerly wind event (~5m/s anomalies) forced by both GWO and MJO-related positive convection anomalies. Stated previously, an important subseasonal monitoring issue going into this upcoming boreal cold season is the occurrence of additional equatorial westerly wind events tied to convection shifting into the west Pacific Ocean.

The anomalous warmth across the equatorial Indian Ocean has expanded during the last week, with totals in excess of 29C and anomalies at least plus 0.5-1C (large for the Indian Ocean). As discussed below, it is only a matter of short time before tropical convection becomes widespread in that part of the world. I will give my Nino 3.4 and ENSO neutral comments “a rest”, for now. (note the initial projection) (link 18).

Following through from my last discussion, 27 September, the tropics and extratropics appear to be getting back in sync. However, I think this dynamical process is occurring in the Western Hemisphere, perhaps from the Americas into Africa. This is farther west than I would have thought a couple of weeks ago; nevertheless, my notion of expecting it appears to be correct. Full disk satellite imagery shows enhanced tropical rainfall from the East Pacific Ocean extending through the Atlantic into Africa. In fact, an intense rainfall event may be setting up for portions of northwest Africa.

There have been several complicated dynamical processes that I have been observing leading to the current tropical-extratropical circulation coupling. It is nearly impossible for me to explain that level of detail in these discussions. In summary, these included 1) fast ~10-20 day global mountain-frictional torque index “cycles” since about 10 September, with roughly 10-20 Hadley variations for the global mountain torque; 2) meridional momentum transports, including a recent rapid quadrapole of zonal mean sinks and sources, forced by the eddies; 3) a slower ~50-day variation of the global frictional torque linked to the tropical convective forcing and dissipation of extratropical westerly wind flow anomalies in the storm track regions; and 4) Rossby wave energy dispersions (RWDs) excited by the tropical convective forcing. Inter-hemispheric meridional symmetry involving these processes has become quite robust.

The bottom line is that the WB (2008) measure of the GWO, a dynamical representation of the global circulation which considers the MJO, best explains the physical processes forcing the atmosphere. Interactions of the zonally oriented RWDs have literally dragged any MJO component of tropical forcing into the Western Hemisphere. For instance, the upper tropospheric anomalous twin tropical anticyclones that were ~120E a week ago have shifted to about 90W. Stating that convectively coupled Kelvin waves led to most of the Western Hemisphere tropical forcing (and anywhere else) is not a scientifically complete statement, and overly simplified thinking. The empirical WH (2004) measure of the MJO, keying on the equatorial wind signal, shows a less than 1 sigma projection in octant 8 of phase space through 3 October. Leaving in the interannual signal gives a larger than 1 standard deviation projection in octant 1 of phase space, which is more representative.

All of these complicated processes have worked to bring global relative AAM down to about 1 sigma below the R1 data climatology through 2 October. With a computed tendency of ~minus 15 Hadleys, the WB (2008) GWO has done a rapid orbit to roughly octant 1 of phase space, following a slower evolving similar trajectory. Anomalous zonal mean easterly wind flow; for example, greater than 5m/s at 200mb ~15S, is returning to the tropical and subtropical atmospheres. The anomalous zonal mean westerly wind flow that propagated into the midlatitudes a few weeks ago is undergoing frictional dissipation (per above). The latter is why there was the large orbit in GWO phase space, from octants 4-5 to 8-1, during September. Furthermore, the decided ~1 sigma shift toward the low AAM portion of GWO phase space is a response to the tropical-extratropical coupling including a persistent negative global frictional torque since about mid-September.

Summing up, the global circulation has returned to a weakly low AAM base state. I have concerns that as tropical convective forcing becomes anomalously intense across the Indian Ocean into Indonesia the next few weeks, the low AAM situation may intensify. Additionally, trade wind surges in the region of the Dateline have been stronger than any equatorial westerly events thus far. Hence, from a global tropical ocean and circulation perspective (not Nino 3.4), the odds may be tipping toward a La-Nina situation this upcoming boreal cold season. Regardless of what happens, subseasonal activity may continue to be quite robust (ex., perhaps simultaneous west Pacific and Indian Ocean tropical forcing) suggesting an increased risk of global high impact weather during the next several months.

Regionally for the PNA sector, GWO phases 8-1 of the 250mb psi snr composite anomaly plots best depicts the current circulation, while phases 2-3 for the MJO 250mb psi composites depicts tropical circulation anomalies. Recall that the global circulation is still “getting back together”. By later week-2 into week-3, while the “usual” 8-1 to 4-5 variations continue, I like the phases 2-3 of the snr 250mb psi plots for both the MJO and GWO. In the longer term, the MJO may again tilt toward octants 3-5 with the GWO displaced toward octants 2-4 of their phase spaces.

Stronger than I would have thought a couple weeks ago (seasonal cycle issues), at least a “preview” of the barrage of western and central USA troughs may be starting. Week-1 is a response to the North Pacific Ocean jet collapsing, typical of a GWO 8-1 transition leading to zonal wave number transition. During weeks 2-3, while more western states troughs are probable (per many models), these may reach their maximum amplitudes in the middle of the country, as shown by the snr 250mb composite psi anomaly plots. As was observed during 2007-08, progressing into DJF, JFM and so forth, GWO and MJO snr phases 2-3 suggest troughs to deepen farther west, across western North America including the USA. The corresponding 2m air temperature snr plots for GWO phases 2-3 suggest a greater than climatology probability shift of anomalous cold in the middle of the country. That may mean the first freeze going into this cold season.

As already offered in past postings and implied above, a rendition of the weather-climate situation similar to the 2007-08 cold season may occur for 2008-09. That suggests more western and central USA troughs but with added (reduced) subtropical westerly (easterly) wind flow anomalies. This may mean a more variable storm track due to occasional positive PNA patterns. In any case, I would be concerned of a tilt toward a greater likelihood of high impact weather for December-March possibly affecting most of the lower 48 states.

Intense to severe tropical thunderstorm activity is likely to increase across Africa and the equatorial Indian Ocean ~weeks 1-2, and then expand east and northeast into Bay of Bengal, Indonesia and Southeast Asia during ~weeks 2-4. The TNWP Pacific Ocean should be relatively suppressed for approximately the next 1-2 weeks. However, the west central Pacific Ocean is a “wild card” until further notice. Keeping in mind the latter, the risk of Bay of Bengal tropical cyclogenesis may shift to slightly above climatology ~weeks 2-4.

The tropical cyclone risk for East Pacific Ocean should continue week-1 then diminish afterward. Focusing on the region of the Caribbean, at least a climatological risk of tropical cyclones may continue through at least week-2 given the possibility of hybrids.

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. There has been an increase during the last week.


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, where WB formally introduce the GWO (WB (2008)), has been accepted for publication and resubmitted to MWR. A pdf version (before revision) can be downloaded from the following link:

Hopefully we can get the revised version posted soon.

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! I should be able to do a discussion next weekend, ~11-12 October.

Ed Berry

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