Monday, May 14, 2007

Like Dealing with the Devil (It's complicated!)

The spatial pattern of Eastern Hemisphere tropical SSTs remains relatively unchanged with warmth across the Indian Ocean-cool Indonesia and warm west central Pacific. Largest anomalies and totals persist over the west central Indian Ocean into the Arabian Sea with magnitudes of plus 1-2C and greater than 30C respectively. Cooling continues along the equator east of the date line with values of at least minus 2C west of South America. Negative subsurface anomalies of at least minus 3C exist down to ~100m but positive anomalies greater than 1C down to ~300m are present west of the date line. A strong surface westerly wind burst (WWB) centered ~150-160E on the equator has been in progress for at least the past 5 days. There is some possibility for a downwelling oceanic Kelvin wave to develop as a response and transport some of this warm water to the east during the next few months. This is an example of why development of La-Nina is still a bit of an open question.

The Eastern Hemisphere tropical convective forcing loosely mirrors the SST distribution. One intense region remains relatively stationary ~0/100E while extending into the Bay of Bengal (including Tropical Cyclone 01B) and Southeast Asia while another area is over the west central Pacific. Overall, since the strong MJO event back in December 2006-January 2007, these 2 regions have been the dominate source of tropical forcing (equatorial Africa and tropical South America have been episodic). In fact, this behavior goes all the way back to the winter of 2001-2002. With the exception of the weak-moderate MJO during February, we have observed ~10-30 day excursions of tropical convection into the west central and South Pacific while forcing has loosely persisted ~90-120E.

Possibly related to the seasonal cycle, since late February I think there has been a close relationship between the eastward excursions of tropical convection into the west Pacific and a global mountain-frictional torque index cycle. The latter is a component of the GSDM having a broad periodicity of 10-60 days and is related to familiar teleconnection patterns such as the PNA (see WB (2007) for details). The point is while west Pacific tropical forcing has added anomalous westerly flow to the subtropics there has also been an additional source from the East Asian topography (for our purposes). These contributions have allowed extensions of the East Asian/North Pacific jet within an overall GSDM Stage 1 base state having tropical forcing across the East Indian Ocean. The jet extensions have contributed to the barrage of anomalously strong troughs into the western USA and associated extreme weather impacts. Numerical models have struggled predicting these situations especially for weeks 2-3.

The orbits around GSDM Stage 1 seen on the phase plot of the global wind oscillation (hopefully the most recent plot will be available soon) have been related to the above. Per reanalysis data plots through May 11th the global mountain torque is ~plus 30 Hadleys with the largest coming from the Andes Mountains (linked to a recent convectively coupled Kelvin wave) with a lesser component from East Asia. Global AAM tendency is ~plus 20 Hadleys meaning once again our overall GSDM Stage 1 base state is being perturbed with anomalous subtropical westerly flow. In fact, zonal mean westerly flow at 200mb is at least plus 5m/s throughout the subtropical atmospheres of both hemispheres.

I suspect the global wind oscillation signal is currently ~GSDM Stage 1.5. I do think there will be another orbit perhaps close to Stage 4 before going back to Stage 1. In any case, AAM transports have become strongly poleward (due to the tilts of the synoptic eddies) across the Northern Hemisphere since ~May 1st. A sink has developed ~35N with a source ~55N. The transport signal is largest contribution of the AAM budget, and in this case will maintain ~GSDM Stage 1. In fact, the only real difference between now and back ~December 1st, 2006, is the northward shift of this signal with the seasonal cycle.

During the next few weeks I think the tropical convection will increase in coverage across the Eastern Hemisphere including the Indian Ocean from ~40-120E (with a contribution from the currently very active convection across Africa). An early onset to the Indian monsoon is probable. Flare-ups will continue across the west Pacific. I remain firm on my predictions of more strong troughs to impact the western USA starting next week. The models are “all over the place” on catching up to the atmosphere and synoptic details are unclear. I think this situation may persist into June shifting northwest with the seasonal cycle. Probable weather ramifications include exceptional outbreaks of severe storms again across the Plains and early summertime heat across the Deep South and portions of the East.


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

Recent data issues have caused availability problems with this plot. My apologies.

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 post an update this Friday.

Ed Berry

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