Saturday, December 01, 2007

Subprime Woes for the AAM Budget?

Other titles for this posting might have been, “The Horrors of Making Weather Predictions”, and “Spinning Wheel”. The latter is a song by Blood, Sweat and Tears a number of years ago, and it had a lyric something to the effect of, “what goes up must come down”. The point is we are in a global circulation state that is extremely difficult to understand (and write about), let alone forecast it. In other words, stochastic forcing rules. Hey, yet another title to use for later discussions. I offer below what some would simply view as an opinion (worth something, anyway).

The spatial distribution of equatorial and tropical SSTs across the Indo-Pacific sector remains consistent with a mature La-Nina (a.k.a. a cold event and El-Viejo). Negative anomalies are present in a wide equatorial band (~ +- 10 deg latitude of zero latitude) from 160E-west coast of South America, with anomalies lower than minus 2C in places extending to depths ~150m. Per 5-day averaged TAO buoy data ending 30 November, anomalies even less than minus -3C were observed ~150m/140W. Also noteworthy is that west of 160W a warm anomaly of ~plus 3C has recently developed at 150m. More said below about the possible importance of this feature.

The large scale character of these below normal El-Viejo SSTs was exasperated by a recent trade wind surge. Positive SST anomalies have become well defined from ~20S/180-north of Australia into the equatorial Indian Ocean, the latter a region of convective suppression during the last few weeks. Contributing to these warm SST anomalies has been the southward shift with the seasonal cycle, having SST totals in excess of 30C in places. Much of the equatorial and northern Atlantic Ocean basin remains above average, as does the North Pacific Ocean basin. In fact, there is still some signal of the warm extratropical horseshoe pattern of SSTs across the Pacific Ocean. Recent East Asian jet extensions and associated cold outbreaks and accompanying baroclinic storm developments across the North Pacific have led to cooling centered around the extratropical date line. The latter is an example of the atmosphere forcing the ocean, typical for the extratropics.

A strong signal of the MJO has emerged during the last several days. This was not expected by me, even though various statistical predictions of the WH2004 phase space diagrams did indicate this possibility. One reason I disagreed (and there were others) is that it is somewhat atypical during a moderate-strong La-Nina to have a coherent eastward propagating MJO globally. I should not be surprised given all the other “craziness” I have had to discuss in these postings during the last couple years. For example, behaviors I have termed as “The New World Atmosphere” and “Nemesis”.

The eastward propagation initiated ~80E along the equator during about mid-October. An intense tropical convective flare-up occurred ~140-160E during roughly 9-26 November, associated with the MJO signal along with extratropical forcing. Currently, the moist convective signal is centered ~15S/160W, but with other convection increasing across tropical South America and Africa. Full disk satellite imagery and coherent modes Hovmollers support this observation. My back of the envelop phase speed computation gives me ~3-4m/s eastward movement for the convective signal during about the last 45 days. Interestingly, there was a weaker but significant tropical convective flare-up ~150E during mid-late September, which is also on the time scale of ~50-60 days. The point is perhaps greater attention should have been paid to those “New World Atmosphere” SSTs across the west central Pacific Ocean.

The strong MJO projection onto the WH2004 RMM phase space plots has been coming from the second EOF, RMM2. Contributions to this ~3 standard deviation signal have been from the convectively suppressed Indian Ocean (positive OLRA) along with the equatorial vertical wind structure. The statistical RMM tools indicate the MJO dynamical signal to return into the equatorial Indian Ocean by the end of week 3. Hey, I have to go with it.

Now comes the portion discussing the AAM budget and GWO. The earth-atmosphere budget is about as complicated as it gets; however, I do think I can make some sense of it. I want to be relative brief about it.

Tied to the ~160E tropical flare-up discussed above along with a largely Southern Hemisphere forced friction-mountain torque (yes, the Northern Hemisphere also helped out) index cycle, a non-trivial amount of westerly wind flow was added to the atmosphere. Since early November ~3 AMUs was added to the global circulation, focusing not only across the equatorial regions, but also the northern and southern midlatitudes, in terms of the zonal mean. The midlatitude contribution was a response to eddy feedback dynamics that much research still needs to be done to understand it. There has been a recent peak of global relative AAM to slightly above the R1 data climatology. Just as the low AAM base state observed much of boreal autumn was an extreme weather event, this sudden increase was also the same.

The GWO presents this signal nicely, very weakly in the phase 7 (legacy GSDM Stage 3) plane through 28 November (5-day averaged). Even though the GWO projection is near zero, the variation is also a 2-3 standard deviation event. So, we have experienced 2-3 sigma events of both the GWO and MJO. The GWO signal can be seen in terms of broad zonal mean westerly wind anomalies from ~30N-30S with magnitudes ~5m/s. A true MJO would be more equatorially confined before propagating poleward. The punch line to all this will be to see if global relative AAM goes down as fast as it came up during the next few weeks. Now you know where analogies to the recent subprime woes of the financial markets come in, etc.

Currently the global surface torques have become weakly negative (through 29 November) as seen by lowering mean sea level pressures along north-south mountain ranges and anomalous surface westerly wind flow across the tropical and extratropical Pacific Ocean. The latter represents frictional dissipation of the westerly flow. In fact, a strong westerly wind burst at ~150E accompanied the MJO convection discussed above, and I think a down welling oceanic Kelvin wave may have been initiated leading to the subsurface warming also mentioned above.

The AAM transport signal is incredibly complicated. However, there is a decent poleward transport signal at centered ~45N and 50S (roughly 10 Hadleys for Northern Hemisphere). There is evidence that these transports (flux divergence of AAM transport) are leading to meriodional propagation of zonal mean zonal wind anomalies. For instance, zonal mean anomalous easterlies (tied to La-Nina) are shifting into the midlatitude atmospheres while the equatorial westerlies also come off. The midlatitude westerly anomalies are also shifting poleward. Again, there is strong inter-hemispheric symmetry of both zonal mean and regional-scale circulation anomalies due to the strong tropical forcing.

Cutting to the chase of this mess, should the tropical convective forcing shift into the Indian Ocean/Maritime Continent during weeks 2-3, while anomalous ridging may be favored at the higher latitudes, anomalous westerly wind flow may be probable from the subtropics-lower midlatitudes. The latter would be a response to anomalous easterly wind flow anomalies returning to the equatorial regions of the atmosphere (more said below).

Animations of fields such as upper tropospheric daily mean vector wind anomalies present one of the best Rossby wave energy dispersions (RWD) across the PNA sector I have seen in a while. Linked to twin subtropical anticyclones along/west of the date line, a low-high-low-high pressure great circle arc including the deep western USA trough can be seen 1 December 2007. A large anticyclone is also bursting into the Arctic, which could help build up more bitterly cold air. Wind anomalies in excess of 50m/s have occurred with this RWD during the last several days. This situation serves as an example of an extreme synoptic weather event linked to the slower processes involving both the GWO and MJO per above. Another point is that we have experienced an extreme event of subseasonal atmospheric variability that already has and will impact multiple time scales until further notice. For instance, are we beginning the slow process of transitioning from El-Viejo to El-Nino?

Let’s move onto insights for weeks 1-4. I can only wish for the day others can observe and appreciate the variations from the GWO (GSDM) perspective that we try to share. As stated above, phase 7 of the GWO best describes the current weather-climate situation. Any predictive information I offer here is likely to going to be “wrong” anyway (low confidence, seriously), so I may as well “go for it”. Again, stochastic forcing rules, and any objective prediction schemes will likely suffer more than usual during the next few weeks.

Most models already suggest some form of amplification weeks 1-2, including the possibility of Plains storm development ~10-14 December. This is a response to the likely collapse of the strong North Pacific jet. In any case, my feeling would be expect a situation of a deep east Asian trough-central/east Pacific ridge (into Alaska and polar latitudes at times) leading to at times an anomalous trough focused across the western/central USA by weeks 3-4. That option would be an opinion to respect should AAM crash and the GWO strongly circuits to ~phase 3 (old GSDM Stage 1) particularly if westerly flow remains strong across the subtropics. Ramifications for the USA may be sporadic episodes of “winter” weeks 1-2 (Pacific Northwest wetness, cold across the northern states, etc), followed by a significant cold and wet regime weeks 3-4. The most intense cold and storm track activity by that time may focus on the Rockies and Plains; however, locations such as the northeast states may be impacted as well. I think other USA concerns are apparent.

Internationally, in addition to an increasing tropical cyclone hazard across the South Indian Ocean weeks 1-2, there should be some concern for this hazard across the South Pacific Ocean paradise islands as well. Much of Europe is likely to remain active while portions of tropical South America get beneficial rainfall. Finally, at least seasonable cold is probable for Siberia (to come into the USA by around Christmas?).


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 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 WH2004 "convention”.

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. I will try to post another discussion around next weekend or early week-2.

Ed Berry

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