Friday, May 18, 2007

Stage 3 Orbit -- Update

Reanalysis-1 (R1) data woes continue meaning ESRL/PSD AAM plots have not updated since May 11th.

The spatial distribution of global tropical SSTs remain similar to at least a week ago. Notable anomalies include the 1-2C warmth across the western Indian Ocean/Arabian Sea and west central Pacific, the latter as part of a horseshoe pattern into the extratropics. SST totals are ~30C for the former and ~29C across the latter. The equatorial cold tongue still has ~minus 1-3C anomalies down to about 200m with the coldest of ~-4C at 140W/100m (per TAO buoy data). However, the recent west Pacific westerly wind burst (WWB) may have initiated an oceanic downwelling Kelvin wave since at least plus 1C subsurface anomalies (~100m) are spreading east across the date line as I type. This situation could disrupt an evolution towards La-Nina. In any case, we have a warm west Pacific and cold east Pacific, only adding complexity.

The coupling of the recent global positive mountain-negative (decreasing) frictional torque index cycle and eastward shift of tropical forcing into the west central Pacific not only contributed to the strong WWB, but also added more westerly flow to the subtropical atmospheres than expected. Physical mechanisms for this coupling include upper tropospheric Rossby wave energy dispersion into the equatorial central Pacific from the East Asian topography and an upwelling convectively coupled Kelvin wave interacting with the Andes Mountains.

200mb zonal mean zonal wind anomalies of ~5m/s are present ~15S and 25N with even stronger anomalous westerly flow across the upper midlatitudes of both hemispheres. At least for the Northern Hemisphere, the roughly 5-10m/s anomalous zonal mean westerly flow centered ~45N goes back to poleward propagation off the equator tied to the December 2006 strong MJO event. It is possible these anomalous branches of westerly flow will combine during the next few weeks.

Full disk satellite imagery shows tropical convective forcing slowly increasing across the warm Indian Ocean and Arabian Sea while weakly hanging around the Bay of Bengal into western Indonesia. Only 2 tropical cyclones remain of the once intense west Pacific forcing. There is no MJO signal. In fact, since early April convectively coupled Kelvin waves have been the only eastward propagating modes within loosely a base state having both East Indian and west central-South Pacific Ocean tropical forcing. Currently there is one moving through the Western Hemisphere with another (the one that hit the Andes mountains) helping to excite the Indian Ocean convection. Could this be the “wave of the future” in what I have dubbed as a “new world atmosphere” in past discussions?

I think GSDM Stage 3 best describes the weather-climate situation. If updated, I suspect the Global Wind Oscillation (GWO) phase space plot would be orbiting toward Stage 3 similar to early April. I do think the tropical convective forcing across the Indian Ocean into western Indonesia/Southeast Asia will become dominate during the next few weeks. One contribution is a probable early onset of the Indian monsoon. The GWO should orbit toward GSDM Stage 4 then 1 later this month into early June.

My broken record predictions of probable occurrences of stormy patterns across the Rockies and Plains, attendant temperature extremes, etc., shifting north with the seasonal cycle, remains unchanged. I suspect most models are a bit too progressive with the western and central USA trough for early-mid next week. However, per issues discussed it will not persist for ~5 days across the western states as I thought a week ago. However, recall that week-2 ensemble means from all operational centers valid next week were predicting strong ridge conditions across the USA, which is not likely. There will be widespread severe storms and heavy rainfall across the central USA with this trough. The magnitude is still unclear.

By next weekend much of the central and east may have cooler than normal temperatures while the west coast starts to heat up. I think it is probable for more unseasonably strong troughs to impact the Pacific Northwest into the Plains later week 2 and week 3 with the return of GSDM Stage 4-1. In fact, monitoring will be needed to see if this situation becomes persistent for more than a few days. Weather ramifications should be obvious by now.


An experimental phase space plot of the GSDM (depicting a global wind oscillation) utilizing normalized relative AAM tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to write an update sometime during Monday-Wednesday next week.

Ed Berry

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