Friday, April 11, 2008

You can't be Serious!

Given web server and time concerns, I am posting a very short update today (11 April). The spatial distribution of global tropical and extratropical SSTs along with their anomalies are generally the same today as they were 2-4 weeks ago. There are weak positive SST anomalies from the eastern Indian Ocean into Indonesia while the weakening El-Viejo SSTs are only ~minus 1C. Subsurface warmth to ~plus 5C anomalies at 160E/200m (per latest 5-day averaged TAO buoy data) continues. However, the latter has been drifting west. Given the character of the global circulation (discussed below), there is no observational evidence of a transition to El-Nino. If the on-going cold event survives boreal spring, related weather-climate ramifications may exist for at least several more months. Links below are to additional SST information. (note the initial projection) (link 18).

Since the eastward propagation of tropical convective forcing into the west central Pacific Ocean and evolution to a zonal wave number 2 pattern of tropical circulation anomalies, signals involving this type of variability have been weak. Before continuing, I want to emphasize the importance that the west Pacific Ocean tropical convective forcing (located ~0/140-160E approximately 5-10 days ago) had on the USA weather this past week.

First, the tropical forcing did have a MJO component (#4 for this cold season). Nevertheless, understand that the MJO only explains ~20 percent of the tropical convective variability, on average. Whether or not the west Pacific Ocean signal was a MJO is irrelevant. The point is that meridionally directed Rossby wave energy was dispersed (RWDs) by the west Pacific convection that arced across the PNA sector and then dynamically forced the western USA trough complex. There have been at least 3 synoptic weather events with this trough, all contributing to extreme USA weather since last weekend. The latter does not occur every week in April, hence that is greater than climatology.

Secondly, and this possibility was discussed on my 15 March posting (and followed up last week), there has been large week-2 forecast errors of the NCEP GFS ensemble mean ending 11 April. In fact, per ,

week-2 North American ACC scores of 500mb Z anomalies are nearly zero (5-day averaged). I do attribute these errors to not only the west Pacific Ocean signal, but also other dynamical processes explained by the WB (2007) GSDM and WB (2007, 2008) GWO. In any case, I strongly disagree with any statements from whoever or wherever minimizing the impact of the tropical forcing (as discussed above) on the USA (and global) weather during the last 1-2 weeks.

Full disk satellite imagery and all my other tricks indicate the tropical forcing is slowly increasing across the Eastern Hemisphere. Anomalous twin upper tropospheric tropical/subtropical anticyclones are spreading into the Indian Ocean with cyclones getting better defined around the date line. Given the baroclinic structure of these circulation anomalies along with the OLRA, the WH (2004) MJO index has orbited to phase 1. My own feelings are to expect tropical convection to increase across the equatorial Indian Ocean during the next 1-2 weeks while the west central into the southwest Pacific Ocean does “its own thing (the new world atmosphere is still hanging around)”. During weeks 2-4 there may be a significant increase of intense to severe tropical thunderstorm activity centered on western Indonesia.

Global relative AAM remains low (~minus 1.5 sigma through 9 April per R1 climatology) and is probable to stay that way for at least the next few weeks. The global mountain torque is ~minus 10 Hadleys including contributions from both East Asia and the Andes. That component of the earth-atmosphere AAM budget has led to a roughly minus 10 Hadley global AAM tendency. Hence the GWO is strongly in phase 2, and it is probable to do more orbits around phase 3 (GSDM Stage 1) in GWO phase space for at least the next few weeks. This translates to an increase of anomalous zonal mean easterly wind flow in the subtropical atmospheres. Zonal mean easterly wind flow anomalies are already quite large ~25N (roughly 10m/s at 200mb). Hence, keeping in mind the global-zonal mean-regional perspective I take in these discussions, the circulation remains strongly characteristic of La-Nina.

My USA outlooks remain a broken record. As discussed last weekend, circulation anomalies across the PNA sector are probable to retrograde during the next couple of weeks. Zonally oriented RWDs (typical response of a strong “GWO regime”) interacting with increasing Eastern Hemisphere tropical convective forcing will lead to this westward shift. I have been struggling recently with the details. Nonetheless, I do think what many of the latest model runs are showing predicting more western USA troughs then slamming the Plains is reasonable. In fact, given variations in amplitude along with the seasonal northwest shift, this pattern may continue well into May. Weather ramifications for the lower 48 states are “obvious” by now. However, week-1 (next week) should be fairly tranquil due to initially limited moisture transport from the deep tropics through the Gulf of Mexico.

Portions of the central and southern High Plains had some decent precipitation (rain and snow) this past week, particularly eastern Colorado and western Kansas. That was more than I expected. Still, my concerns of prolonged dryness remain for these areas. There are other factors that I can see which could mitigate some of this.

Severe weather internationally (including temperature extremes) appears to have increased a bit during the last week. I continue to leave it to the expertise of the appropriate weather centers internationally to alert the public of these risks.

As discussed above, tropical thunderstorm activity is probable to become quite intense/severe from the Indian Ocean into Indonesia during the next 1-2 weeks, perhaps centering on western Indonesia (roughly 0/120E) by ~weeks 3-4. Whether or not MJO #5 develops is unclear. Locations to be impacted may also include portions of Southeast Asia and the Philippines. There are already a couple of suspect areas for Eastern Hemisphere tropical cyclone development. That risk may increase especially for the Bay of Bengal by week-3. Climatologically, the Bay of Bengal has one peak period of tropical cyclones during May. The west central into the southwest Pacific Ocean remains a “wild card” for at least the next couple of weeks.


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:

The following is a link to information about the stratosphere:

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 two-part paper is in preparation by WB that will formally introduce the GWO along with subseasonal composites. Given shift work and upcoming travel, updates remain extremely difficult. I am planning on posting another discussion next weekend, 19-20 April.

Ed Berry

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