The spatial pattern of global tropical and even extratropical SST anomalies remains similar to a week ago. Some changes include cooling of the ocean waters generally north of Australia having anomalies ~minus 1-2C as a response to persistent enhanced rainfall, and corresponding warming across the suppressed Indian Ocean. Nevertheless, SST totals in excess of 28C dominate most of the Eastern Hemisphere equatorial and southern tropics extending well into the South Pacific Ocean.
Significant cold anomalies of ~minus 1-3C remain across the equatorial Pacific Ocean from ~155E-west coast of South America. In fact, recent anomalously strong trade winds have expanded and intensified these El-Viejo SST anomalies during the last couple of weeks. These below normal waters extend to at least 100m deep per latest 5-day averaged TAO buoy data (see http://www.pmel.noaa.gov/tao/jsdisplay and other relevant links for SST details). Subsurface warmth is still present ~150m depth in the region of the equatorial date line. The weak oceanic Kelvin wave discussed in the last few postings has lost coherence, likely a response to the enhanced trades.
I again want to make the point the La-Nina SSTs are only a component of the global tropical SSTs forcing (and responding with feedbacks) the atmospheric circulation. In fact, speculation suggests to me that the very warm Eastern Hemisphere tropical SSTs observed for the past several years may have recently tilted the odds toward La-Nina rather than El-Nino. The latter goes back to subseasonal weather events during boreal autumn 2006 which led to a La-Nina global circulation state (as seen from an understanding of the earth-atmosphere AAM budget). An abrupt and unpredicted end to the El-Nino SSTs subsequently occurred.
I can further speculate the warm SSTs discussed above may be a response to the global warming signal “everyone talks about”. Make no mistake; the recent global warming trend is real (started ~1960). So, can we suggest this La-Nina is a response to global warming as a feedback??? Whatever the case, there is lots of uncertainty crossing multiple time and space scales out there. As may be case investing in the financial markets these days, the same may be said about making weather forecasts.
There has not been any substantial change in the character of the tropical convective forcing discussed a week ago. Full disk satellite imagery and other monitoring tools show strong-severe clusters of tropical thunderstorm activity extending from Indonesia-South Pacific Ocean. Consistent with La-Nina, the latter is along a southwestward shifted South Pacific Convergence Zone (SPCZ). However, I think there is a component propagating southeast toward the region of ~20S/180. Recent 3-day averages of OLRA have been less than minus 70W/m**2 in that part of the world (with suppression along the equatorial date line). My own off the cuff phase speed calculation suggests an eastward movement of ~7 m/s within the convective envelope discussed above. I think the latter represents a dynamical signal that will quickly move through the Western Hemisphere during the next 1-3 weeks. Again, who knows about the timing?
The WH(2004) phase space plots show a significant phase 6 MJO signal through 4 January 2008. The MJO partition along with the rest of the Eastern Hemisphere tropical forcing slowed its eastward propagation considerably during the last few weeks due to the atmosphere-ocean coupling discussed in my 30 December 2007 edition. Continuing on my path, I would expect "some" tropical forcing to persist in the region of 120-140E with attendant circulation anomalies while perhaps a MJO dynamical signal moves through the Western Hemisphere south of the equator given the La-Nina SSTs. How the WH(2004) real-time multivariate MJO (RMM) phase space plots maintained by several weather centers show this speculative behavior is unclear.
Animations of upper tropospheric daily mean vector wind anomalies show a strong signal of what would be expected given the spatial distribution of the tropical forcing discussed above. Twin subtropical anticyclones (cyclones) have shifted east to ~120-140E (east of the date line) with 150mb anomaly magnitudes in excess of 40m/s for the equatorial Pacific westerlies. In fact, this Western Hemisphere equatorial anomalous westerly wind flow is now coming back around into the Eastern Hemisphere (more said below). Finally, there is strong cross-equatorial wind flow around the date line from the Southern Hemisphere, tied to the convection passing south of the equator.
Worth repeating from a week ago, there have been several episodes of meridionally oriented Rossby wave energy dispersions (RWDs) into the extratropics tied to circulation features such as these for the last several weeks. For the PNA sector these RWDs have led to robust cyclonic circulation anomalies around Alaska poleward of a central Pacific Ocean anticyclone. Several strong troughs have impacted the USA west coast on into the Plains as a result. The recent (~4 January 2008) severe cyclonic winter weather event that impacted the USA west coast was an extreme manifestation of these RWDs. In fact, daily mean 250mb wind speed anomalies associated with the central Pacific anticyclone was in excess of 70m/s with this event.
Having good initial condition information largely from the extratropics (ex., blocking around Scandinavia), most models did a superb job advertising this extreme weather event starting around Christmas. Of course, the magnitude of the anomalies was underestimated particularly at longer leads given the role of the tropical forcing. Additionally, as shown in the WB ((200?); publication is planned) composites for both the GWO and MJO, extreme USA west coast precipitation events are most probable to occur in GWO phases 7-8 and to a lesser degree MJO phases 7-8. However, the recent storm occurred with GWO phase 3 and MJO phase 5. The latter again emphasizes the roles played by the dynamics of non-linear feedback processes that I can only wish the resources existed for us to demonstrate in real-time. I should also add that discussion was presented in the 18 November 2007 posting of suspicion that a higher probability existed for these kinds of non-linear feedbacks within our 2007-08 La-Nina base state.
Global relative AAM remains low ~minus 1.5 standard deviations below the R1 data climatology updated through 1 January 2008. Since mid-December 2007, in the wake of the earlier MJO, there has been substantial poleward propagation of anomalous zonal mean easterly wind flow anomalies into the subtropical atmospheres. At 200mb zonal mean anomaly magnitudes are ~10m/s poleward of similar anomalies for the equatorial westerlies. These subtropical easterly wind flow anomalies have been contributing to intense midlatitude ridges, particularly across the North Pacific Ocean.
Although slightly positive ~20 December 2007, a strong East Asian Mountain torque did not occur as speculated a week ago. It was, instead, a perturbation in a low AAM base state. However, largely from the strong subtropical Northern Hemisphere trades, the global friction torque has peaked to plus 20 Hadleys. The latter is ~50-60 days after the last big frictional torque event, consistent with the GWO.
Since late December 2007 an intense sink-source transport signal has appeared ~10-40N with flux convergence of this transport ~40N. The dynamics responsible for this poleward transport signal were another contributor to the recent severe USA west coast storm. AAM tendency has been slightly positive since early December largely from processes tied to the friction and Coriolis torques. The point is we have a mature La-Nina base state where secondary processes in the atmosphere are trying to generate westerly flow to offset it. The GWO updated through 31 December shows the atmosphere “drifting” in phase 3 (legacy GSDM Stage 1). I again emphasize that measured in terms of the AAM budget, the global circulation has been strongly La-Nina, nothing moderate about it.
Uncertainty remains HUGE (above whatever climatology anyone chooses) for any subseasonal, etc., prediction. I do think there will be a weak GWO circuit possibly through phases 5-8 during the next 2-3 weeks. Per above, a weak MJO dynamical signal appears probable to propagate through the Western Hemisphere, only to come back around before the end of this month. Most models are suggesting some form of east Pacific Ocean ridge amplification by around week-2. Given the cross equatorial wind flow per above that notion is reasonable. However, systematic bias issues with the NCEP GFS ensemble tell me to favor the slightly farther west solutions of other models such as the ESRL/PSD week-2 ensemble mean. I also think that during ~weeks 2-3 more strong cold storms/troughs are once again probable to slam the USA west coast.
Hence I remain a broken record for any week 1-3 USA outlooks. After relatively warm and weak systems week-1, a colder and wetter regime appears probable from the Desert Southwest (bucking the composite La-Nina signal) into the Plains and east week-2. I would expect a westward shift of this situation weeks 3-4. I also think it is only a matter of time before blocking develops in the region of the Gulf of Alaska allowing Arctic airmasses to interact with a southwest flow storm track across the Plains. With very low confidence, I offer this synoptic evolution is possible around week 3 (~19-26 January). In addition to the winter weather hazards, concerns also must be expressed for an above average occurrence of severe local storms for locations such as the lower Mississippi into perhaps the Ohio Valleys not only weeks 1-3, but perhaps the rest of this cold season.
Internationally, heavy-severe rainfall/thunderstorms (including tropical cyclones) will be a concern for Eastern Hemisphere-South Pacific Ocean areas already discussed above for weeks 1-2. Heavy rainfall and severe thunderstorms may impact much of Brasil and South Africa by week-3, if not much sooner. Bitterly cold Arctic air will continue to dominate much of Siberia into Alaska. Alaska will see significant warming once the blocking develops.
An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (X-axis) can be found at
We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous the WH(2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.
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 (soon) 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. Given travel, I am unclear when my next posting will be. Please stay tuned.