Saturday, February 23, 2008

Warning: Beaten Down El-Viejo Circulation Getting another Injection

Mature and strong La-Nina SST conditions continue across the tropical Indo-Pacific Ocean sectors with no significant changes since my discussion a week ago. There has been cooling north of Australia and warming across the equatorial Indian Ocean, with anomaly magnitudes ~1-2C. The latter is consistent with the locations of enhanced and suppressed tropical convective rainfall, respectively. The warmest SSTs globally are across the tropical South Pacific Ocean having totals ~29-31C, while those across the Indian Ocean are ~28C.

Per latest 5-day averaged TAO buoy data, the equatorial Pacific Oceanic thermocline remains very steep, with negative SST anomalies ~minus 3-5C down to about 150m deep east of 150W. West of the date line positive anomalies ~plus 3C exist down to ~200m depth. Strong trades (anomalies ~5-10m/s) along the equator from ~140W to the warm pool appear to be dampening the oceanic Kelvin wave discussed in my previous posting. The following are links for additional SST information. (note the initial projection)

Many scientific issues have been raised in regard to our on-going strong cold event during the past several weeks. These have included the strong MJO tropical convective variability and zonally symmetric zonal mean zonal wind anomalies meridionally symmetric about the equator. I am not going to repeat that here. However, an important point to remember is that the characteristics all El-Ninos and La-Ninas are different (trend signals understood). Composite signals, by definition, are going to average out what many others may consider as “climatic noise”. This is why the dynamics of the subseasonal aspects of cold, warm, and whatever other shoe-horned termed events are extremely important to monitor and understand in real-time. The latter contribute to high-impact weather that often does not fit the composites (and impact the seasonal mean). Weather and climate are linked, and one of the purposes of the GSDM is to provide a framework of that linkage for monitoring and predictions.

Full disk satellite imagery and other tools suggest that the MJO signal has become a bit less coherent, and that there are currently multiple regions of moist tropical convective forcing. The most intense region globally of strong-severe tropical convection is still back in the Eastern Hemisphere centered ~120E just south of the equator. The latter has been slowly expanding west into the South Indian Ocean while still affecting the Philippines to northern Australia with flooding rainfall. Weaker regions of moist convection are across the South Pacific Ocean, tropical South America and even South Africa.

WH (2004) phase space plots now give a less than 1 standard deviation projection in phase 8 (removing the ENSO signal). Whatever the case, animations of upper tropospheric winds and velocity potential indicate that the dynamical signal of the MJO is in the Western Hemisphere. My own loose back of the envelope calculation from a Hovmoller plot of equatorial velocity potential has the signal propagating east ~15m/s. It is typical at this phase of the MJO to see multiple regions of enhanced tropical convection. I continue to lean toward the MJO WH (2004) RMM phase space predictions provided by centers such as CMC, UKMET and ECMWF. These suggest that the convective signal will strongly return to the Indian Ocean by late week-2 and week-3 along with suppression across the Western Hemisphere. I do not expect intense convection to persist across the somewhat cooler waters (compared to 4-6 weeks ago) of the South Pacific Ocean like that observed during mid-January.

Identifying important circulation features from animations of upper tropospheric daily mean vector wind anomalies is not as clear as a week ago. Briefly, the twin subtropical anticyclones with the current MJO have weakened considerably (if not completely dispersed) across the Western Hemisphere Pacific Ocean. Furthermore, consistent more with the GWO (discussed below), Rossby wave energy dispersions (RWDs) have become more zonally oriented during the last few days which are propagating along waveguides too complicated to discuss here.

There is still a signal of merdionally oriented RWDs arcing from the central Pacific Ocean to the anomalous (but weakening) anticyclone across Canada continuing across the North Atlantic Ocean to a large anomalous cyclonic gyre across western Asia. The latter RWD path may play a role in both re-firing Indian Ocean tropical convection (Southern Hemisphere symmetry understood) and finally force the East Asian mountain torque to become negative. The positive East Asian mountain torque has been providing a local source of atmospheric westerly wind flow for the past several weeks, including the recent extension of the North Pacific Ocean combined jet.

The earth-atmosphere angular momentum budget (AAM), which provides extremely useful diagnostic information about the real-time global and zonal mean circulation, remains more complicated than I would like. However, signals are clearer to me than a week ago, particularly taking into account all the above. Furthermore, I am actually fairly confident where the global circulation is headed during at least the next 2-3 weeks.

The screaming message is that dynamical processes are once again increasing zonal mean easterly wind flow anomalies throughout the tropical and subtropical atmospheres, especially north of the equator. In fact, since mid-late January strong zonal mean easterly wind anomalies (a good 10m/s at 200mb) have been shifting south from ~40N to 25N as a response. Prior to that, since early December 2007, there was the more typical poleward propagation. Why the recent southward shifts of the easterly wind flow anomalies is a good question, and I do have my thoughts. I will just offer here speculation that this all goes back to the RWD feedbacks from the South Pacific Ocean signal with MJO #2 including the role of positive East Asian mountain torque.

With surface frictional dissipation of extratropical westerly wind flow anomalies by the synoptic eddies (global frictional torque ~minus 10 Hadleys), updated through 21 February per ESRL/PSD using the R1 data/climatology, global relative AAM is nearly as low as early December 2007. The magnitude is ~minus 2 AMUs. In addition to the frictional torque, a strong recent negative global Coriolis torque (~20 Hadleys) has also been bringing global AAM down. Hence the global AAM tendency (calculated and computed) is ~minus 20 Hadleys, mostly from the tropics. In fact, there is already a signal of poleward propagation of this zonal mean negative AAM tendency off the equator.

Other terms such as the earth (mass) AAM are strongly positive (~1.5 AMUs) due to anomalous high surface pressure across primarily the tropical and northern subtropical regions of the globe. Finally, consistent with the GWO probable to orbit to phase 3 (more said below), while still complicated, a zonal mean AAM sink is again present across the Northern Hemisphere subtropical atmosphere with a source in the extratropics. This means (loosely) the eddies are again starting to flux AAM from out of the tropics into the extratropics, with the maximum transport ~30-35N. There is a weaker zonal mean signal of this flux divergence of AAM transport process across the Southern Hemisphere.

Given the low AAM base state and negative global tendency, the GWO has orbited to phase 2. This projection is well above 1 standard deviation, the strongest since December 2007 in terms of this quadrant of phase space. Hence we are observing more zonally oriented RWDs consistent with the GWO composites produced by Weickmann (paper in preparation by WB).

I do think there will be a circuit to phase 3 (legacy GSDM Stage 1) and beyond. Furthermore, having my reasons, I am going to be bold and offer that MJO #4 is probable. Knowing that timing is white noise, my thought is to be concerned about our El-Viejo base state being strongly amplified by early-mid March. Not only will this enhance an active March climatology, another episode of MJO variability “pissing off” La-Nina could be bad news for the USA and global locations already hit hard by severe weather (all types) since at least December 2007.

Issues discussed about the stratosphere last week still remain. Please see the following link for details.

Summarizing, I think there is an evolution involving the tropical convective forcing and global atmospheric circulation response which may lead to an enhancement of our La-Nina base state during the next few weeks. That includes the possibility of MJO #4 developing in the region of the Indian Ocean and the GWO orbiting to phase 3 and beyond. In earlier posts, given all the zonal symmetry concerns, I did not think constructive and destructive interference of La-Nina would still be an important issue. Obviously I have reconsidered that, and we will see what happens.

Probable weather impacts for the USA ~weeks 2-4 with more western states troughs should be easily apparent by now. This may literally be “the season of the witch!” As mentioned last week, higher latitude blocking may shift the central USA southwest flow storm track a bit south at times. Locations such as the Upper Mississippi Valley hit hard by severe winter weather may not only experience more of it, but also have “spring” delayed. Since it does not look like La-Nina is going anywhere any time soon, the track of what may be violent outbreaks of severe local storms is probable to shift northwest into the northern and central Plains/Upper Mississippi Valley heading into May and June.

Still unchanged, per WMO and other information, quite a bit of severe weather internationally continues to occur, tied to our on-going complicated weather-climate situation. I will leave it to the expertise of the appropriate weather centers internationally to alert the public of these risks.

It is less clear to identify the regions where tropical cyclone formation is most probable. I think locations along the westward shifted SPCZ into the South Pacific Ocean islands need to be concerned about that hazard mainly week-1. Since there already are surface westerly wind anomalies across the South Indian Ocean, the tropical cyclone hazard is probable to increase there week-1. That may continue weeks 2-3 impacting locations from Madagascar to the north coast of Australia. Strong-severe frontal thunderstorm activity is probable to continue cross portions of tropical South America especially Brasil week-1 spreading into South Africa by week-2


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 to the WH(2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.

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. In addition, a paper is in preparation by WB that will formally introduce the GWO. I will attempt another posting the weekend of 1-2 March.

Ed Berry

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