Saturday, February 16, 2008

Shoe Horns and Recipes will not Work on this Witch

Edited 18 February to clarify some points.

The spatial pattern of tropical SSTs from Indonesia into the Pacific Ocean is still consistent with a mature strong La-Nina, with negative anomaly magnitudes ~2-3C (totals ~24-25C) from the equatorial date line to about 110W. Slight positive anomalies have been appearing along the coast of South America for the past several weeks. One can speculate various reasons on the latter. However, with the seasonal cycle favoring warming, the cold equatorial anomalies cited above are not trivial.

The combination of persistent intense-severe tropical convective rainfall (including tropical cyclones) south of the equator and cold continental air plunging into the North Indian Ocean and South China Sea has led to substantial SST cooling across those regions. SST anomalies are ~minus 1-2C across much of the Indian Ocean to near Australia, having totals ~27-28C. The only positive (but weaker) anomalies of ~1-2C are from northeast of Australia into generally east of Philippines with totals ~28-29.5C. The spatial horseshoe pattern of positive SST anomalies extending into extratropical Pacific Ocean of both hemispheres continues to be very well defined, typical of a mature La-Nina (and emerging negative PDO signal, for folks who study that?).

At depth per latest 5-day averaged TAO buoy data recently trade wind intensified cold anomalies of ~minus 3-6C extends to ~150-200m east of 150W with slightly deeper positive anomalies to the west. The latter extend to at least 140-150E with magnitudes ~1-3C, and is the most recent attempt of an oceanic Kelvin wave. In terms of 20C degree isotherm depth, this Kelvin wave is the strongest so far this boreal 2007-08 cold season. Enhanced trades do appear to be weakening it. In any event, the equatorial Pacific Oceanic thermocline remains much steeper than climatology.

Worth repeating from a week ago, 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).

Also, the WMO has released a nice update dated 11 February on La-Nina, and can be accessed from the following link.

Please remember that MJO tropical convective variability (3 MJOs) has been strong since November 2007 and have acted to constructively and destructively interfere with ocean-atmosphere La-Nina coupling (latter especially in the zonal mean). Specifically, MJO variability is the strongest since at least the 2004-05 boreal cold season, and is atypical of a strong La-Nina. Working within the La-Nina base state, the MJO variability has significantly impacted global weather with numerous occurrences of severe high impact storms. That has included the USA, and will continue for at least the next few weeks (more said below).

Full disk satellite imagery and other tools show the strongest moist tropical convective forcing centered ~10S/130E while still extending from eastern Indonesia and the Philippines southeastward into the SPCZ. The SPCZ has been shifted west, typical of La-Nina, and has impacted portions of Australia with severe thunderstorms and flooding rainfall per WMO. This region of enhanced rainfall strongly projects onto a MJO utilizing the WH (2004) methodology (#3 for the 2007-08 boreal cold season; ~1.5 sigma, with the ENSO signal removed), currently in phase 7 through 15 February. Most models (ex., the NCEP GFS) have been underestimating both the strength our current MJO and its rate of eastward propagation for at least the past 2 weeks.

Existing circulation and SST anomalies tied to La-Nina coupling along with atmospheric responses from the previous 2 MJOs appear to be affecting the behavior of the current one. For instance, since early this month a westward shifting component along the equator weakly projecting onto a Rossby mode per Wheeler and Kiladis (JAS 1999; WK(1999)) coherent modes Hovmollers has been present. Interactions with the northern extratropics may have also contributed to the westward shift. In any case, well south of the equator (need to remember the seasonal cycle) from northern Australia into the southwest Pacific Ocean eastward propagation is present. For example, surface westerly winds anomalies ~15m/s from the South Indian Ocean into the SPCZ has been present for at least the past 7-10 days. The latter have provided a source of background cyclonic relative vorticity for the development of several tropical cyclones including category-4 Ivan that is about to impact Madagascar.

My point is evaluation of any MJO must consider other fields such as the SSTs, tropospheric wind structures, etc., in addition to regions of precipitation enhancement and suppression. Besides the seasonal cycle, propagation of this current MJO is south of the equator because of the cold El-Viejo SSTs in the region of the equatorial date line. I do think a difference between our current MJO and the previous event (MJO #2) is that it is less probable for extremely intense convection to persist (~2 weeks) across the South Pacific Ocean. The SSTs in that region are certainly warm enough to support tropical convection (and are), ~28-29.C, but not 30-31C like that seen during mid-January.

A Western Hemisphere dynamical signal is also steadily emerging, with eastward moving anomalous twin upper tropospheric subtropical anticyclones loosely straddling the date line (discussed below) and divergence increasing aloft over South America forced by the downstream cyclones. A valid suggestion (per WK(1999) coherent modes Hovmollers) is that the latter is a response from a convectively coupled Kelvin wave. However, the wind animations suggest MJO circulation anomalies are responsible for the increasing upper tropospheric divergence across tropical South America (as part of the Western Hemisphere decoupling process). Full disk satellite imagery has shown an increase in convection across that region during the last few days.

This MJO already has significantly impacted and is probable to continue impacting the USA for at least the next couple of weeks. One cannot simply look for wave-1 signals in the upper tropospheric velocity potential field to assess the strength of a MJO. There is no cookbook to understanding the dynamics of MJO variability particularly in terms of the non-linear forcing-response-feedback atmospheric processes currently going on.

A week ago discussion was given about our global circulation exhibiting zonally symmetric zonal mean zonal wind anomalies meridionally symmetric about the equator. I will not repeat all that here. There is some evidence from animations of upper tropospheric daily mean vector wind anomalies that the zonal symmetry may be changing. I want to keep the elaboration that follows as brief as possible (a properly dedicated web page effort would greatly facilitate communication and understanding).

Well defined twin tropical/subtropical anticyclones (wind flow anomalies ~20-30m/s at 150mb including cross-equatorial flow) are present ~160E tied to MJO #3 (16 February), and continue to steadily move east. Downstream cyclones also with large wind flow anomalies exists ~160W. Consistent with phases 7-8 of the DJF MJO composites produced by Weickmann (paper by WB in preparation), strong meridional Rossby wave energy dispersions (RWDs) arc from the anticyclones across the PNA sector leading to anomalous ridge across western Canada. In fact, this is (ONLY) one reason why the short-term global models have struggled with the evolution of the baroclinic storm on the USA Plains that is about to develop (at the time of this writing on 16 February). The latter serves as yet another example why an understanding of the dynamics of slower evolving large-scale processes needs to part of any forecast process for making predictions for days 1-7. Weather and climate are linked and using numerical models as a stand alone forecast tool is another cookbook technique. All the statistical model bias correction in the world is not a scientifically acceptable solution!

The global atmospheric AAM budget remains non-trivial. Loosely, there are competing dynamical processes wanting to both add and remove westerly wind flow from the atmosphere. Perhaps we are observing a La-Nina equilibrium in terms of the earth-atmosphere angular momentum budget. Updated through 14 February courtesy of ESRL/PSD, global AAM is slightly below the R1 data climatology with the tendency ~minus 10-20 Hadleys. That is why the Global Wind Oscillation (GWO) is orbiting in phase space toward phase 2. However, zonal mean zonal wind anomalies remain very strong (at least ~5-10m/s at 200mb), with equatorial westerlies, easterly wind flow anomalies dominating the subtropical atmospheres (propagating south in the Northern Hemisphere) and westerly anomalies across the Southern Hemisphere extratropics. However, zonal mean easterly wind flow anomalies have recently appeared ~50N. So, what could be going on?

Recall last week I talked about the East Asian mountain torque remaining persistently positive since ~mid January having roughly 10 day variations. This apparently started as a feedback to west central and South Pacific Ocean tropical convective forcing from MJO #2. RWDs, interacting with MJO #3 currently heading into the Western Hemisphere, have continued to force at times intense anticyclonic circulation wind flow anomalies across the Northern Hemisphere polar latitudes. That includes the western Canada ridge discussed above.

RWDs arcing from the western Canada anticyclone through the North Atlantic Ocean into Asia have forced another spike in the East Asian mountain torque of ~plus 15 Hadleys. However, this time there is a strong global component that has involved MJO #3, a poleward directed zonal mean AAM transport signal ~35N especially earlier this month (roughly 4-8 Hadleys), and a compensating frictional torque currently ~minus 10 Hadleys. The recent frictional-mountain torque index cycle variation is why the PNA index (Pacific-North American teleconnection) has become weakly positive. The point is that these highly complex non-linear interactions involving multiple time-scale processes has led to the extension of the East Asian jet (EAJ) that will slam the USA west coast, which the numerical models finally start to catch onto about 5 days ago.

Reading the above brings back memories of the recent “storm on steroids” that severely impacted the California coast ~3-4 January 2008. However, the former occurred while the MJO and GWO were evolving through phases 3-5 while what is about to occur may be with phases 7-8 of the WH (2004) MJO plot. Furthermore, the GWO may already be collapsing into phases 1-3 given other processes tied to, for instance, a negative Coriolis torque of ~minus 10 Hadleys. Again, there are no cookbook rules of thumb!

Animations of various wind fields and several tools clearly tell me that upper tropospheric divergence is increasing across tropical South America (per above) and will soon do so across South Africa. This is why it is probable for a reasonable dynamical MJO signal to propagate rapidly through the Western Hemisphere possibly returning to the Indian Ocean by week 3. In fact, WH (2004) phase space RMM predictions from European meteorological agencies support this notion as suggested by the GSDM reasoning given here. The zonal symmetry issues discussed above have made good GWO signals tough to come by lately; however, I think a circuit to phase 3 (old GSDM Stage 1) is probable during the next few weeks. Whether or not we get MJO #4 is unclear.

Finally, for those who watch the stratosphere, as discussed above, there have been numerous RWDs leading to bursting anticyclones across the Northern Hemisphere polar latitudes. Another such event is currently in progress, and at least a minor warming may occur. Upward directed 100mb zonal mean E-P fluxes around 60N have become quite robust during the past week. It is not unusual to see a major SSW going into March, which can then impact the troposphere by raising sea-level pressures across the Arctic. I will leave it to the reader to speculate on the predictive details. Again, stratospheric issues are only another feedback and trying to shoe horn them into AO/NAO etc., does not go well with me. Please see the following link for details.

Summarizing, the MJO dynamical signal is moving into the Western Hemisphere. While impacts from severe tropical convective rainfall are likely to be across the Southern Hemisphere, global weather including the USA will be affected by the MJO variability during roughly the next 1-3 weeks. Additional complicated processes explained by the GWO will also contribute. In terms of the tropical forcing and global circulation behavior, I speculate by weeks 2-3 the GWO will be orbiting around or near phase 3.

Most, if not all models (ensembles), have now captured the notion of the USA west coast focusing on California getting slammed by a southward displaced enhanced North Pacific combined jet stream by the end of week 1. Ramifications for those folks may be “similar” to the “storm of steroids” of early January, keeping in mind the magnitude of that event.

Troughs then moving into the Rockies leading to an intense southwest flow storm track across the central USA are quite probable. Should blocking start to develop across locations such as from Alaska into the Arctic, the storm track across the Plains may be shifted a bit farther south. In any case, the risk of the types of high impact weather experienced across the lower 48 states a couple of weeks ago may again increase weeks 2-3, perhaps enhancing climatology going into March (winter storms including intense-severe thundersnow Northern Plains and Upper Mississippi Valley, severe local storm outbreaks across the Deep South-Ohio Valley, etc.). Hopefully locations such as southwest Kansas into west Texas will get some of this precipitation instead of dry intrusions and even “southwest winds and blowing dust”.

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.

Tropical cyclone activity should diminish week-1 across the South Indian Ocean and northwest coasts of Australia. Intense tropical cyclone activity may focus along the SPCZ into the South Pacific Ocean islands weeks 2-3. Strong-severe frontal thunderstorm activity is probable across portions of tropical South America especially Brasil by week-2 and South Africa by week-3, if not sooner (timing is always white noise!). Enhanced moist tropical convection may return to the South Indian Ocean 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.
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 23-24 February.

Ed Berry

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