Saturday, November 10, 2007

El-Viejo Keeps Rocking

There has not been a lot of change to the tropical global SST pattern during the past week. Equatorial Pacific Ocean cold anomalies remain from the west coast of South America to ~160E, with the greatest magnitudes ~minus 3C at about 120W. There is some possibility this cold event may be nearing maturity since the coldest subsurface anomalies have apparently surfaced and magnitudes only around minus 1-2C exist just below the surface east of 150W. Plus 0.5-1C subsurface anomalies (as deep as 150m) remain along the equator from 160W to at least 140E. It is probable for the western tropical Pacific Ocean to remain relatively convectively suppressed thorough at least December (Nemesis is still around), meaning SST anomalies across the warm pool region may increase.

The west central Pacific Ocean still has totals in excess of 30C currently just south of the equator at 140E slowly shifting into the Southern Hemisphere with the annual cycle, linking to a well pronounced positive SST horseshoe into the global midlatitudes. In fact, in the regions of 30-40N and S latitudes, Pacific Ocean SST anomalies are well in excess of plus 3C (totals 22C and colder) as a response to persistent intense subtropical anticyclones. This is a situation of the atmosphere forcing the ocean, unlike the ocean forcing the atmosphere as typically occurs across the deep tropics. Many will argue feedbacks occur from extratropical SSTAs such as these; however, they are only secondary. For instance, should the current intense North Pacific Ocean jet persist, significant SST cooling will occur across that region.

There is once again the warm-cool-warm Indian Ocean to west central Pacific SSTA distribution, with totals in excess of 28C across the equatorial Indian Ocean. I am not going to “go there” in regard to Indian Ocean Dipole (IOD) issues. The Atlantic basin is still warmer than climatology, with a bit of an increase during the past week. Should we start talking about the 2008 North Atlantic tropical cyclone season? Who knows, there may be one or two “Gabrielle-like” systems across the South Atlantic around March 2008.

Repeating from a week ago, the following link is to the WMO news page where a nice discussion about the evolution of this cold event is posted. I recommend reading it, paying attention to the “unusual” characteristics noted.

Full disk satellite imagery and other diagnostic monitoring tools present a nice signal of consolidation of tropical convective forcing ~0/90-100E during the last week. The eastward shift into the west central Pacific Ocean discussed last posting has lost coherence. What is left of it is centered ~0/140-150E, while drifting west. I think a combination of these two regions of tropical convective rainfall centered ~0/120E is probable during the next week (or less). Generally diurnal rainfall is present around central/South Africa with some suppression over tropical South America.

I think we are starting to observe the “composite La-Nina” response of tropical convective forcing. However, I do expect this Eastern Hemisphere region of tropical convection to shift east into portions of the west central and even South Pacific (along the SPCZ) especially early 2008, within envelops of other subseasonal variations. There is little, if any, signal of a MJO at this time. Attention needs to be paid if the above convection starts to move east coherently during the next few weeks.

The global circulation has essentially coupled to La-Nina in terns of SST forcing, tropical convective and circulation responses. Animations of daily mean upper tropospheric vector wind anomalies give a decent signal of twin tropical/subtropical anticylones centered ~110E with twin cyclones near the International Date Line. Anomalous zonal mean easterly wind flow anomalies (~5-10m/s at 200mb) persist across the subtropical atmospheres while weak equatorial westerly wind anomalies have become better defined over the Western Hemisphere. In fact, the latter are coming around into portions of Africa (implications are a monitoring issue). Within this coupled base state (GSDM Stage 1), a fairly well pronounced transient evolution partly linked to global topography has contributed to a strong (daily mean anomalies ~50m/s at 250mb) extended North Pacific jet about the slam the USA Pacific Northwest (more said below).

Global relative AAM remains extremely low, ~2.5-3 standard deviations (precisely AMUs) below the R1 data climatology through 7 November. This situation has been observed since about mid August 2007. While the equatorial Pacific SST pattern suggests the current La-Nina is best described as “moderate”, the global circulation is strongly La-Nina like (which could be viewed as seasonal-time scale extreme weather event). In fact, a defensible argument may be made that the tropical SSTs have been catching up to the atmosphere particularly since August. Other terms of the AAM budget also support the notion of a strong La-Nina circulation, including the inter-hemispheric symmetry of poleward AAM transports across the subtropics, and the mass term. The relatively persistent global positive frictional torque of roughly 10 Hadleys is coming from enhanced Northern Hemisphere trades trying to add angular momentum to the atmosphere from the earth. Typical of a cold event, the atmosphere continues to rotate slower than the earth.

The global weather-climate situation is solidly in GSDM Stage 1 as indicated by the Global Wind Oscillation (GWO) updated through 29 October. The GWO continues to orbit around La-Nina. While we can attribute the larger GWO orbits to non-oscillatory slow processes such as friction-mountain torque index cycles, the faster smaller circuits remain a topic of our research. The recent positive AAM tendency of ~20 Hadleys is coming from the tropics and lower midlatitudes. In addition to the current positive global mountain torque of ~15 Hadleys with East Asia leading, I think there has also been a contribution to the positive AAM tendency from the weak eastward shift of tropical forcing discussed a week ago. The strong North Pacific jet is a response from these combined tropical and midlatitude dynamics (bottom line). In any case, when updated, I think there will be another one of those “smaller orbits” of the GWO.

Instead of stating in the Appendix, we are continuing to rework the GSDM and use terminology similar to that of the WH2004 8-section phase space plots of the MJO. In other words, we will be getting way from terms such as “stages” and use phraseology such as “phases” instead. Stages may be used in reference to what will be referred to a the “legacy WB(2007) GSDM”. Hence GSDM Stage 1 will be “GWO phase 3”,GSDM Stage 3 will be referred to as “GWO phase 7”, etc. I know this seems very confusing; however, stay tuned, it will become much easier to understand. We are making these modifications partly in response to users who do not like the word “stages”.

Worth restating, I think the USA seasonal outlooks recently prepared by the Climate Prediction Center (CPC) are the “correct forecast” given La-Nina, keeping in mind to remain diligent for important subseasonal events. The subseasonal evolutions during November-December-January 2006-07 putting an unexpected demise to El-Nino serves as an excellent example. I also want to make clear that blanket statements about the roles of the North Atlantic Oscillation and/or Arctic Oscillation (NAO/AO) for a cold seasonal mean USA outlook are not scientifically defensible. From a subseasonal viewpoint the latter are responses to at times very complex dynamical forcing onto the global circulation and not that predictable especially beyond a few weeks.

Continuing thoughts from the previous posting, the extended North Pacific Ocean jet is likely to collapse during the next 1-2 weeks. Simplistically, it is not unusual to observe jets outrunning Eastern Hemisphere tropical convective forcing, especially boreal autumn. With the exception of episodes of high winds/heavy rainfall across the Pacific Northwest and perhaps portions of Alaska at times, a generally mild and tranquil weather pattern can be expected roughly week-1 for the USA. There will be some trough amplification east of the Mississippi River meaning opportunities for some precipitation and cooler air.

Perhaps most importantly, given the La-Nina coupling and upcoming probable collapse of the North Pacific Ocean jet, I think the possibility of significant amplification across the Pacific North American sector exists roughly days 10-20. Probabilistically, this would lead to blocking around Alaska and a western USA trough. I am making this statement with extremely low confidence, especially since my predictive insights have ____ recently. In fact, I am more confident about the amplification occurring then the synoptic pattern. For example, if the trough ends up across the east Pacific Ocean, while the USA west coast gets possibly hammered with cold systems the rest of the country may stay “boring” (yes, this scenario may also be good for ski resorts west of the Continental Divide). In any event, we need to monitor. If there is actually any truth to this possibility, people planning travel around Thanksgiving may have to deal with possibly intense winter weather should a southwest flow storm track develop across the Plains with an Arctic cold air source. Hopefully, the numerical models will start to support this synoptic evolution (not trying to “monger” winter).

I do think it is highly probable there will be several occurrences of the “classic” legacy GSDM Stage 1 (GWO phase 3) this cold season. Even with that circulation response, cyclonic baroclinic developments across the southwest High Plains may be too progressive for decent precipitation for locations such as the Desert Southwest into western Texas and eastern Colorado/western Kansas unless strong anticyclonic wave breaking of the lows can occur, say, around "4-corners".

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Locations including the Bay of Bengal and the Northwest Pacific Ocean around the Philippines may have concerns for tropical cyclones week-1 and perhaps through week-2. I think flooding rainfall remains a concern for portions of Central America week-1. Finally, episodes of strong/severe synoptic baroclinic cyclonic systems hammering at least portions of Scandinavia, Europe into the Mediterranean countries are probable through week-2.


An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

Links to CPC and PSD ENSO discussions:

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion late next weekend or early the following week after travel.

Ed Berry

1 comment:

Jim H1 said...

Hi Ed,

Could you define the difference between Earth AAM and relative AAM? Also, when do you think the coming blocking pattern will move further off the West Coast, which is where it belongs given the circumstances?

Thank you!