Saturday, February 02, 2008

More Wraths From the Child of Darkness???

Indian-Pacific Ocean tropical SSTs (roles of global extratropical and Atlantic Ocean basin SSTs understood) continue to present a spatial pattern consistent with a mature strong La-Nina. Warmth remains across the Indian Ocean and particularly north of Australia while waters from the equatorial west Pacific Ocean to South America is generally still exceptionally cold. Anomaly magnitudes vary from plus 2-3C (totals ~30-31C) north of Australia to minus 3-4C from about 180-150W along the equator (totals ~24-25C).

Per 5-day averaged TAO buoy data the equatorial cold anomalies extend to about 150m depth with ~200m deep positive anomalies farther west centered on the date line. Magnitudes for the latter are around plus 6C and may be the early stages of an oceanic Kelvin wave. However, these deep positive anomalies were generated by a weakening of easterly wind flow anomalies on the equator forced by the previous MJO. The trades strengthening in that region once again may minimize any impacts from an oceanic Kelvin wave. However, that is speculation, and stay tuned. In any case, the very steep equatorial Pacific oceanic thermocline is also typical of a mature cold event.

Full disk satellite imagery and other diagnostic monitoring tools present a strong signal of tropical convective forcing centered ~5S/100E. This region of strong to severe thunderstorm activity loosely extends from South Africa to central Indonesia in a band roughly 10-20 degrees wide. Suppression is returning to the date line while the South Pacific remains sporadic, including Tropical Cyclone Gene.

The Eastern Hemisphere tropical convection does project onto a ~1 standard deviation MJO per WH(2004) methodology updated through 1 February. Whatever the case may be for the latter, I do think that projection will become more robust as the winds continue to respond to the tropical forcing (discussed below). My back of the envelope computation has components of this Indian Ocean forcing moving east from ~5-10m/s since about 20 January. I think this region is still getting its act together, and remember that it is important to understand the dynamics of the forcing and not shoe-horn it into a “name”. My feeling is the intense convection may soon stall north of Australia in the area of 120-140E given bath water SSTs there. As I discussed a week ago, the tropical convective forcing with what appears to be MJO #3 for the boreal cold season will likely amplify El-Viejo.

Animations of upper tropospheric daily mean vector wind animations show the expected response as tropical convective forcing intensifies across the Eastern Hemisphere. Strong twin tropical/subtropical anticyclones are developing across the Indian Ocean with down stream cyclones in the region of the date line. Anomaly magnitudes are ~20-40m/s with these features at 150mb, while at the surface equatorial Indian Ocean westerlies and anomalous date line trades complete the first order baroclinic response. Zonal mean easterly wind flow anomalies are returning to the tropical atmosphere while past subtropical westerly wind flow anomalies undergo dissipation involving “non-cookbook” dynamical processes.

There have been two recent interesting Rossby wave energy dispersion (RWD) events into the extratropics tied to the tail end of strong MJO #2 and the return of tropical forcing to the Eastern Hemisphere. I will only briefly discuss them here. Around 18-24 January a RWD occurred from the west central Pacific Ocean where tropical convection was still intense (GWO was orbiting into phase 5). A response was an anomalous anticyclone in the region of Alaska and northwest Canada. Courtesy of the Tokyo Climate Center (link below), zonal mean upward 100mb E-P fluxes ~60N became quite robust and 10mb temperatures rose ~60C in a few days toward the end of the month.

Temperatures have since dropped back to below normal and this event appears to have been a minor Sudden Stratospheric Warming (SSW). Tropospheric impacts are unlikely.

During this past week, while westerly wind flow anomalies were propagating poleward out of the subtropics, interactions of the synoptic eddies with the subtropical jet across the USA Desert Southwest led to a strong anticyclone in the region of Greenland. Whether or not this event initiates the road to a slightly negative phase of the NAO is unclear (models generally unsupportive). One important point (and there are others that can be made) is the latter is completely independent of the minor SSW discussed above.

As was expected, per ESRL/PSD AAM plots updated through 31 January, global relative angular momentum (AAM) is decreasing rapidly, ~minus 20 Hadleys computed tendency. The will decrease the length of day by ~5 milliseconds during the next few weeks, and I hope this is not a bad sign for the financial markets next week! Contributions are coming from the global mountain torque, particularly the tropics (East Asia should soon become negative), local frictional dissipation by the midlatitude eddies of those poleward propagating subtropical westerly wind flow anomalies, and a zonal mean sink developing ~20N. Global relative AAM itself peaked too slightly above the R1 data climatology during the last week of January. That was roughly 50-60 days after the last peak around 1 December 2007, which is consistent with both the GWO and MJO (loosely equals the GSDM) recurrence time scales.

Although still somewhat unclear, I think a strong zonal mean poleward AAM transport signal is developing ~30N (NE-SW tilts of the synoptic eddies support this notion). A response appears to be southward propagation of zonal mean westerly wind flow anomalies from the higher latitudes. Combined with anomalous zonal mean easterly wind flow anomalies ~35N (see past postings for thoughts about their evolution), once again ridges are well pronounced across the Northern Hemisphere midlatitudes (lesser Southern Hemisphere). Where these ridges set up regionally has a lot to do with who has feast or famine in terms of precipitation.

Updated through 31 January the GWO has orbited into phase 1 (GSDM Stage 4), and I expect that it will soon be at phase 3 (GSDM Stage 1). While the time scale is always difficult to estimate, I speculate phase 3 should be the case by the end of week-2, roughly in phase with both the MJO and La-Nina. The last occurrence of this kind of superimposition was around the first week in January 2008, when the “storm on steroids" slammed the USA west coast on about the 3rd.

Summarizing, it is probable that MJO #3 will amplify La-Nina during the next few weeks. While the repeatability has been remarkable this boreal 2007-08 cold season, seasonal transition soon into spring alone will add uncertainty to any subseasonal outlooks. Also adding uncertainty is if this current MJO will have periods of moving and stalling like the previous event. Finally, timing is playing with white noise!

Zonal mean westerly wind flow anomalies propagating southward (per above) suggest the continuation of progressive synoptic features across the USA for at least the next 7-10 days, with limited Arctic air. Recent model runs from various global operational weather centers support this reasoning, as would be expected.

However, there are large differences in the week-2 ensemble means of, for example, 500mb height anomalies. Some of these differences can be attributed to systematic biases and not predicting well the on-going evolution of the tropical convection in terms of forcing-response-feedbacks, etc. Based on DJF composite analyses done by Weickmann of both the GWO and MJO (WB paper is in preparation), I think it is probable to see more strong cold troughs hit the west coast later week-2 leading to a southwest flow storm track across the Plains roughly week-3 (~16-23 February). Arctic airmasses are likely to get involved, and risks of the high-impact weather detailed in past postings will again increase.

Finally, I have to express some concern about developing dryness across the High Plains from Kansas and Colorado on south. The current progressive regime is bad news for that region in terms of precipitation. This is a La-Nina signal, and a strong event like that going on suggests worsening drought. However, given the MJO and other variability observed the past several months, and that I am not a “recipe-cookbook” meteorologist, there is certainly reason to speculate improvement particularly going into spring. I hope that is not “wishing” on my part. If this dryness persists and worsens, southwest winds with blinding blowing dust and high fire danger conditions may occur relatively soon (Hell on earth?).

Per WMO and other information, there has been quite a bit of severe weather internationally during the last several weeks. Included are record cold and snow for portions of this Middle East and China, and severe flooding thunderstorms for portions of South Africa and Indonesia. While attributable (and not surprising) to the circulation behaviors discussed during past postings, I generally leave it to the appropriate weather centers to alert the public of these risks.

Locations from South Africa into much of Indonesia will likely continue to deal with severe thunderstorms and flooding week-1 then focusing on the Maritime Continent to the north coast of Australia weeks 2-3. The risk of intense tropical cyclones should shift to around Australia by roughly week-3. The Southwest Pacific Ocean remains our wild card, and Tropical Cyclone Gene should hang around for at least a few more days.


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.

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 9-10 February.

Ed Berry

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