Saturday, February 09, 2008

Season of the Witch

The spatial pattern of tropical SSTs from Indonesia into the Pacific Ocean remains consistent with a mature strong La-Nina. Anomaly magnitudes are ~2-3C with the warmest north of Australia starting to extend into the South Pacific Ocean, and the coldest east of the equatorial date line. SST totals vary from ~31C north of Australia to less than 24C across portions of the Nino 3.4 region (latter NDJ value 1.4C per CPC). At depth per latest 5-day averaged TAO buoy data the cold anomalies extend to ~150-200m with positive anomalies around the equatorial date line (steep thermocline). Magnitudes are ~1-3C. The date line warmth appears to have weakened slightly during the last week possibly responding to a trade wind surge.

Cold continental air plunging south into the Arabian Sea in addition to past intense rainfall events has lowered SSTs considerably (totals less than 28C) in that region of the Indian Ocean. Additionally, positive SST anomalies have been slowly appearing along the coast of South America during the last several weeks. I will not speculate on the latter. However, the seasonal cycle favors warming of the equatorial Pacific Ocean waters, and it will be interesting to monitor any such impacts onto La-Nina. There are several modeling and statistical tools suggesting that El-Viejo is not going anywhere anytime soon. For example, please see (note the initial projection).

Full disk satellite imagery and the usual other diagnostic monitoring tools indicate the strongest moist tropical convective forcing centered ~5-10S/130E (with Western Hemisphere suppression), significantly farther east than when I wrote a discussion a week ago. In general, the region of strong-severe tropical thunderstorm activity extends from the central equatorial Indian Ocean east-southeast across northern Australia into a westward shifted South Pacific Ocean Convergence Zone (SPCZ). The SPCZ is loosely a graveyard of fronts from the Southern Hemisphere. At least 3 tropical cyclones are currently spinning across the South Indian Ocean, including category-4 Hondo.

In short, the Eastern Hemisphere tropical convective forcing is propagating east even faster than what I would have thought one week ago. In fact, my trusty back of the envelope calculation gives a phase speed of this forcing of ~8m/s south of the equator since about 20 January. However, the clusters of thunderstorm activity have slowed considerably during the last several days north of Australia (warm SSTs), and convectively coupled Kelvin waves have also been involved with the eastward movement. The WH (2004) methodology updated through 8 February gives roughly a 2-standard deviation MJO projection in phase 5. There are also other tools (coherent modes Hovmollers, Hovmoller plots of velocity potential, etc.) telling the world that MJO#3 for the boreal cold season is in progress.

While some brief coupling between the SSTs and large-scale convection may be occurring as I type, I think this tropical forcing will propagate into the Southwest Pacific Ocean along the SPCZ during the next couple of weeks or thereabouts (timing is always noise!!!). There are numerous reasons I now think so, and these may involve dynamical feedback processes that “linear-cookbook-shoehorn “meteorological reasoning”” may not explain very well (and difficult for the numerical models to represent).

I can only briefly elaborate here. One reason is we have had a nice SSTA perturbation of ~0.5-1C leading MJO variability much of the Austral warm season. As discussed above, while cooling has occurred across the western equatorial Indian Ocean, warming is now spreading into the Southwest Pacific Ocean east of Australia. Another reason is there has been extremely complex Rossby wave energy dispersions (RWDs) tied to the convection particularly into the northern extratropics during the last 2-3 weeks. The latter may be “pushing the MJO east” as a feedback into the tropics (one possible mechanism for this is discussed below).

Animations of upper tropospheric daily mean vector wind anomalies show the expected responses as the tropical convective forcing moves east. There are strong twin tropical/subtropical anticyclones centered ~90E and downstream cyclones just east of the date line. Anomaly magnitudes are ~20-40m/s with these features at 150mb, especially for the Western Hemisphere Pacific Ocean equatorial westerlies. At the surface anomalous south equatorial Indian Ocean westerlies extending into the SPCZ and date line trades complete the first order baroclinic response.

As already mentioned above, RWDs have recently been quite robust into the northern extratropics. In particular (see the ESRL/PSD animations), there has been a source from the west Pacific Ocean first tied to moist convection/twin anticyclones with MJO #2 (~mid January) then the twin cyclones with MJO #3 (not linear!). Responses have included strong synoptic-event anticyclones first across the North Atlantic Ocean around Greenland and recently Scandinavia. These features have guided RWDs southeastward across Asia keeping mean sea level pressures anomalously high east of the north-south mountain massifs in that part of the world. The baroclinic processes with the RWDs have been responsible for the earlier severe winter weather across China and most recently the intense Arctic air outbreak plunging into the Northern Plains as I type. Positive East Asian mountain torques (~10-15 Hadleys per ESRL/PSD R1 data plots) occurring roughly every 10 days has also accompanied these RWDs, adding westerly wind flow anomalies to the East Asian jet.

Finally, for those who watch the stratosphere, there may be another minor warming in progress linked to the RWDs. There is another recent spike in 10mb temperatures and upward directed 100mb zonal mean E-P fluxes around 60N. Please see the following link for details.

Loosely, after 2 strong going into 3 MJOs and various other sources for large-scale wind anomalies the last few months, the global circulation currently exhibits strong zonally symmetric circulation wind anomalies. For example, there are strong anomalous ridges across the subtropical-midlatitude atmospheres. There are also meridionally symmetric zonal mean zonal wind anomalies about the equator with broadly extratropical westerlies, subtropical easterlies then equatorial westerlies. That is why global relative AAM is near climatology given there apparently exists an equilibrium between processes wanting to add and remove westerly wind flow from the atmosphere. This may also offer some explanation for the recent weak signal of the Global Wind Oscillation (GWO). The point is that just as the SSTs exhibit mature La-Nina conditions so does the circulation response in the presence of unusual MJO variability (for a strong cold event).

Now it is a matter of rigorous daily monitoring to understand where the global circulation goes from here, especially given the upcoming transition seasons for both hemispheres. The notion of La-Nina becoming amplified or otherwise may no longer be relevant. Global AAM tendency (through 7 February) has dipped to ~minus 20 Hadleys with a contribution coming from the global mountains. Hence the GWO has weakly orbited to phase 1. A decent zonal mean poleward AAM transport signal (roughly 5-10 Hadleys) at ~35N has developed since the start of this month. The gist is that I would expect the global mountain torque to become positive soon particularly if the tropical convective forcing shifts into the west central-South Pacific Ocean per above.

Summarizing, there are a lot of complicated forcing-response-feedback (and subsequent interactions) dynamical processes going on as I type, leading to what some may view as a “simple global circulation”. I do expect the tropical convective forcing to shift into the SPCZ during the next couple weeks, adding westerly wind flow anomalies to the atmosphere. Interactions with the global mountains and extratropics may lead to a large AAM tendency soon allowing the GWO to possibly strongly orbit to first phase 5 (GSDM Stage 2), then phases 6-8. Whether or not MJO #4 develops during March is unclear. However, I have my reasons to speculate that AAM tendency may become strongly negative in a few weeks forcing the GWO to phase 3 (GSDM Stage 1).

There is general agreement among the numerical model ensembles from various weather centers of a trough-ridge-trough pattern across the PNA sector week 1 into week 2, with the ridge ~140W. This forecast is not unreasonable per above, and it does have some support based on DJF composite analyses done by Weickmann of both the GWO and MJO (WB paper is in preparation). Uncertainty becomes HUGE afterwards, mainly for timing. Numerical models are probable to struggle during the next several weeks not only because of seasonal transition, but also due to the issues already discussed.

A week ago I was thinking a GWO phase 3 response for the USA by ~16-23 February. By the end of that period there may instead be a strong North Pacific Ocean jet slamming the west coast linked to GWO phases 7-8. By weeks 3-4 (~23 February-8 March) phases 1-3 of both the GWO and MJO may be probable, leading to USA west coast cold troughs (extending from Alaska) and a southwest flow storm track across the Plains.

With the possibilities discussed in past postings, the ramifications of at times a severe winter weather regime for the USA unfortunately unfolded for much of the lower 48 states this past week. I do think there will “less intense weather” overall next week, keeping in mind model predictions of more closed lows developing across the Southern Plains. Any high impact weather should be focused east of the Rockies during the next 1-2 weeks, possibly including Arctic air. Attention may then turn to the USA west coast and Desert Southwest afterwards.

Per above, the risk of the types of high impact weather experienced across the lower 48 states this past week may again increase weeks 3-4 in similar locations (winter storms including intense-severe thundersnow Northern Plains and Upper Mississippi Valley, severe local storm outbreaks across the Deep South-Ohio Valley, etc.). I continue to have concerns about the developing dryness across the Southern High Plains from western Kansas into the Texas Big Bend. Enhanced subtropical jet activity at times extending into the Desert Southwest and Southern Plains may mitigate drought in these areas during this upcoming Spring.

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 appropriate weather centers to alert the public of these risks.

Tropical cyclone activity is probable to diminish across the South Indian Ocean while increasing north of Australia week-1. During week-1 and particularly ~weeks 2-3 severe tropical cyclones may impact the coasts of Australia given the warm SSTs. That risk should shift into the region of the South Pacific Islands afterwards. While suppressed week-1, strong-severe frontal thunderstorm activity may impact portions of tropical South America especially Brasil by week- 3.


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 16-17 February.

Ed Berry

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