Saturday, April 05, 2008


This will be a shorter discussion. There have been no significant changes to the global tropical and extratropical SSTs during the past week. Some warming to near climatology has occurred across the equatorial Indian Ocean while equatorial Pacific Ocean La-Nina SSTs remain ~minus 1-2C below normal. SST totals from the Indian Ocean into the Bay of Bengal are approaching 30C while exceeding 30C over much of the west-central and South Pacific Ocean. See links below for additional information: (note the initial projection) (link 18).

While I am not by any means surprised, the Eastern Hemisphere moist tropical convective forcing discussed a week ago did “unexpectedly” propagate eastward into the very warm southwest Pacific Ocean. Furthermore, tropical forcing increased considerably over the Western Hemisphere particularly from northern South America into central Africa. The latter included an enhancement of the Atlantic Ocean ITCZ.

While some may suggest that this Western Hemisphere signal is simply a convective coupled Kelvin wave, I disagree. Similar to early March, complicated Rossby wave energy dispersion (RWDs) dynamical processes linking the Eastern Hemisphere tropical forcing with the extratropics helped to force the current Western Hemisphere signal. To keep things simple, there has been robust tropical-extratropical coupling during the last few weeks, including a MJO component to the tropical variability (currently phase 8 of WH (2004)). I want to stress the MJO component (and other “things”) are significantly impacting the USA weather (more said below) as I type, and doing so in a manner not expected a week-ago. Any suggestion to the contrary is NOT scientifically defensible.

I am thinking that the Western Hemisphere tropical forcing is “taking over”, and will lead to a rapid intensification of deep moist convection across the currently suppressed Indian Ocean during the next couple weeks. The west-central to South Pacific signal may also persist, lending the possibility of our nemesis of 2 regions of moist tropical forcing for a period of time.

There is a zonal wave number 2 baroclinic circulation response across the tropics, including anomalous twin upper tropospheric anticyclones at ~140E. RWDs from the latter arc nicely to the developing western USA trough, and is consistent with the WB (2007, 2008) subseasonal composites for WH (2004) MJO phase 8. Dynamical processes involving the solid earth-atmosphere AAM budget (not discussed today) are starting to remove westerly wind flow from the global atmosphere. Updated through 2 April (ESRL/PSD R1 data plots), global relative AAM tendency is ~minus 20 Hadleys, with a large contribution coming from the Southern Hemisphere subtropical atmosphere. Zonal mean easterly wind flow anomalies remain well above average (~10m/s at 200mb) across the Northern Hemisphere subtropical atmosphere centered ~25N. These easterlies have actually shifted slightly southward during the last 10 days.

Summarizing the above, the WB (2007, 2008) Global Wind Oscillation (GWO) has orbited to phase 2 having a greater than 1 sigma projection. The MJO component has been “in step” with the GWO recently; however, whether or not that continues is unclear. I do think that more circuits around GWO phase 3 are probable for at least the next few weeks, particularly if our La-Nina base state becomes enhanced.

There are no major changes to my USA outlooks. However, RWDs tied to the MJO west Pacific Ocean signal discussed above is leading to greater meridional trough amplification across the western USA than I discussed a week ago. Many numerical models (ex., ECMWF) and their ensembles, having good initial condition information, suggested this notion at least as early as ~ 1 April. However, I should also mention that the NCEP GFS week-two ensemble mean prediction of 500mb height anomalies ending 5 April suggested an eastern Pacific trough and western USA ridge. I disagreed with this prediction suggesting essentially the opposite phase based on the GWO. However, like the models I did a poor job anticipating the tropical convective forcing to come out into the southwest Pacific Ocean. Hence the current trough digging into the western USA, while expected by me 2-3 weeks ago, is deeper (greater amplitude) than I thought.

As shown by the models for week-1, several synoptic events leading to potentially widespread and destructive high-impact weather are likely for the CONUS. Locations already hit hard by flooding and severe local storms may experience numerous rounds focusing on locations from Southern Plains to Iowa and the Ohio Valley. While bad news for much of the country, locations across the central and southern High Plains may finally get some beneficial precipitation next week. Of course, one or two late season snowstorms with blizzard conditions including intense thundersnow may occur from portions of the Rockies into the Northern Plains.

However, after week-1, status quo appears probable to return for the lower 48 states meaning a northward shifted storm track. As tropical forcing increases across the Indian Ocean, retrogression of existing circulation anomalies is probable week-2 suggestive of troughs just off the USA west coast and central states ridge. That would allow a break in the active regime. Weeks 3-4 are probable to again see more western USA troughs progressing into the Plains/Upper Mississippi Valley (weather ramifications should be understood) but leaving the southwestern High Plains dry. Perhaps the seasonal cycle and maybe even a west Pacific Ocean signal (leading to a strong subtropical jet into the Desert Southwest states) will mitigate some of that.

Severe weather internationally has remained relatively tame 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.

Intense to severe thunderstorm activity should decrease week-1 across tropical South America while spreading into central Africa and the equatorial Indian Ocean by weeks 2-3. The latter may occur initially south of the equator. By weeks 3-4 moist convection may be very active from the Indian Ocean into Indonesia. The Bay of Bengal may start to have an increased risk of tropical cyclones 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 will be a “wild card” for at least the next couple of weeks. Intense thunderstorm activity is probable to hammer portions of the paradise islands week-1 and I can see a scenario of the Philippines getting into the act week-2.


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, 12-13 April.

Ed Berry

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