Saturday, November 03, 2007

The Devil (El-Viejo?) Reigns Supreme

Basin wide (within 5 deg of the equator) moderate La-Nina SST anomalies have intensified slightly during the past week with magnitudes ~minus 1-3C with the coldest along the equator at ~125W per recent 5-day averaged TAO buoy data. These kind of negative SST anomalies along the equatorial Pacific Ocean cold tongue are quite impressive in the presence of the seasonal cooling cycle. The cold anomalies extend down to roughly 200m deep west of South America and as far west at the surface to at least 160E. Strengthening of the near equatorial trades has assisted with the cooling SSTs around and west of the date line.

The west central Pacific Ocean still has totals in excess of 30C along the equator at 140E slowly shifting south with the annual cycle, linking to a well pronounced positive SST horseshoe into the global midlatitudes. Interestingly, weak cool anomalies have recently appeared across the North Indian Ocean, while the Atlantic basin is still warmer than climatology.

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.

Most dynamical and statistical forecast tools from global weather centers suggest this La-Nina to persist into at least early 2008. That notion is probable based on current subseasonal events (discussed below). However, the weak subsurface warming ~150-160W with anomalies ~plus 1C/150m, discussed last week, remains. Sparing lengthy details, I am unclear how this whole ENSO situation will behave during 2008. I would expect anomalous warmth to “build up” particularly west of the date line as boreal winter and austral summer take their course.

Monitoring satellite imagery and other diagnostic tools during the last few weeks tells me there has been a somewhat unexpected eastward shift of the Eastern Hemisphere tropical convective forcing from the Indian Ocean into Indonesia. Some projection onto a MJO was present, and this may have been, in fact, the truncated event I was expecting per past postings. However, I think the eastward movement has ceased, with consolidation and strengthening of the tropical forcing centered ~0/120-140E currently in progress. Other regions of enhanced tropical rainfall are also present across portions of west central-southeast South America/Brasil and Africa. There is also a weak flare-up across the west central Pacific responding to anticyclonically wave breaking cyclones from both hemispheres.

The gist is the global circulation is again coupling to La-Nina. I can see this even from plots such as Hovmoller representations of 250mb meridional wind anomalies in bands such as 10-40N. Global relative AAM is extremely low ~3 standard deviations below the R1 data climatology through 1 November. Since at least late September there have 2 “pulses” of poleward propagation of anomalous zonal mean easterly winds into the global subtropical atmospheres. Most recently zonal mean AAM anomalies ~minus 2 AMUs have appeared just south of 30N. This translates to zonal mean easterly wind flow anomalies as large as 10m/s at 200mb.

Staying on the topic of the AAM budget, a decent poleward transport signal has oscillated ~35N (weaker Southern Hemisphere signal) during the last week or so. This is why any astute synoptician would have been observing a lot of northeast-southwest (northwest-southeast) tilted eddies across the Northern Hemisphere (Southern Hemisphere) recently. The global frictional torque has increased to ~plus 10 Hadleys with a large source coming from the northern subtropics due to the intensifying trades. The global mountain torque has become slightly negative with a decent signal coming from the Andes Mountains.

In any case, I can very easily see how recent the AAM signals both globally and in the zonal mean are lining up nicely with the La-Nina coupling (SSTAs -> tropical convection-> circulation response, then subsequent interactions). The mass term in the AAM budget shows strong positive zonal mean anomalies (~.5 AMU) across the northern midlatitudes with the opposite over the tropics. This all translates to anomalously intense midlatitude ridges across the Northern Hemisphere. Another way to pitch this is that the atmosphere is currently rotating slower than the earth (increasing the length of the day by ~.005 seconds!).

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.

Animations of upper tropospheric daily mean vector wind anomalies give a nice signal of twin tropical/subtropical anticylones centered ~120E with twin cyclones just east of the date line. Anomaly magnitudes at 150mb are roughly 20-30m/s, including the equatorial westerlies across the Pacific west of South America. This is the typical baroclinic response to tropical convective forcing as part of the re-coupling to La-Nina.

The recent eastward shift of the Eastern Hemisphere tropical forcing is leading to an extension of the East Asian Jet as I type. This is an extratropical feedback response to zonally oriented Rossby wave energy dispersions interacting with the convection. Whether or not there will be a positive East Asian and global mountain torque with this event is unclear to me. However, not typical of GSDM Stage 1 and La-Nina is the upcoming development of an anomalous cyclonic circulation gyre across the central North Pacific Ocean per models (which I agree). This reminds a little of what we observed during November-December 2005 having a “Stage 3 circulation with Stage 1 forcing”. The Tropical Northwest Pacific Ocean (TNWP) was quite convectively active then. I speculate that since this La-Nina is stronger (and basin wide) than the 2005-06 event, and the TNWP SSTs do not appear as warm, we will not see our current North Pacific jet extension be as persistent and intense.

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. Of course, once established, the decay time scales of the NAO/AO can provide some predictive information. Enough said!

As I stated a week ago, for especially the PNA sector, predictability has been VERY LOW for both myself (frustratingly) and the numerical models during the last couple weeks. In fact, week-two anomaly correlation coefficients (ACC) for both North America and the Northern Hemisphere for the NCEP ensemble mean have at times been near or below zero, per their web site. I think both the recent positive global mountain torque event discussed last week and the eastward shift of the tropical forcing discussed above have contributed to poor model performance. Yes, boreal autumn is also a tough time of the year for the numerical models, in general.

So what useful predictive insights can I offer today? I do think my “classic” GSDM Stage 1 response will occur “sometime”, favoring a western USA trough and southwest flow storm track across the Plains. However, that is not likely through at least well into week-two given the feedback issues discussed above, along with the seasonal cycle. The North Pacific Jet is very likely to blow across the northern part of the country by that time, in the presence of a low amplitude western states ridge and eastern USA trough. As the coupling strengthens, I can see a scenario of a brief GSDM Stage 3-Stage 4 evolution which may lead to a slower moving trough/closed low(s) across the southwest states perhaps later week 2-week 3. Until then, expect generally “boring” weather for the lower 48 states, except for those who enjoy dry weather and high fire dangers in places. As I have stated before, the latter is typical of autumn. Important exceptions are portions of Alaska and the Pacific Northwest where periods of very heavy rainfall and high winds are probable, and possibly the Great Lakes states for lake-effect snowfall and general "clipper systems".

Again, I emphasize the need for daily monitoring. To me, it is highly probable there will be several occurrences of the “classic” GSDM Stage 1 this cold season. Since timing is unclear, only monitoring may catch the evolution, and that may be before the models get it. Even with that circulation evolution, cyclonic baroclinic developments across the southwest High Plains may be too progressive for decent precipitation for that region unless strong anticyclonic wave breaking of the lows can occur over the Desert Southwest. During this upcoming cold season, given an Arctic cold air source, episodes of severe winter weather including substantial snowfall and blizzards may occur from the Northern Plains into the Upper Mississippi Valley while the Ohio Valley region has heavy rainfall and possibly severe thunderstorms.

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 through week 2. Heavy tropical rainfall may be probable for portions of South America and Africa also through week 2. I will let “Gabrielle” rest her soul.


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 around next weekend after travel, and before another trip the following week. I did fail to keep this discussion short!

Ed Berry

1 comment:

James H said...

Hi Ed,

Some of us like your long posts!

Do you think the Pacific NW will score an Arctic blast or two this winter, given the circumstances?