Friday, June 29, 2007

Let's Focus on the Weather

At the time of this writing, ESRL/PSD AAM R1 plots contain data through 26 June and the GWO only through 25 June.

I wish I could answer all questions from recent emails. However, until we have a properly supported dedicated web page effort allowing a focus on efforts such as weather-climate linkage research and subseasonal forecasting; my timeliness to help folks will be significantly impacted. There have been numerous recent extreme weather events globally including across the USA (Texas floods, western heat, etc.), and it is unfortunate that improved predictive information cannot be provided to users who need to make weather sensitive decisions. I want (tried!) to make the following relatively short; including not repeating the issues discussed 26 June.

Global SSTs remain similar to earlier this week. Subsurface anomalies along the equatorial Pacific have cooled to ~minus 4C at ~160W/100m. However, these remain detached from the surface and are the result of the thermocline being raised by the recent anomalously strong (~10m/s) trade wind surge. The tropical west central and northwest Pacific (TNWP) SST totals remain ~29-31C, with a horseshoe pattern of positive anomalies extending into at least the subtropics of both hemispheres.

The main show of tropical convective forcing remains across the Eastern Hemisphere centered ~15-20N/100-110E while extending from the far northern Arabian Sea east-southeast into the extreme TWNP. At least one weak tropical cyclone developed across the Bay of Bengal (BB) during this week while Yemyin ravaged portions of Pakistan (WMO news). The envelope of this tropical forcing has been shifting east-northeast for the last couple of weeks, having an eastward component of ~3-4m/s. I think this is a weak MJO signal, and I also think this will increase tropical thunderstorm activity across portions of the TNWP during the next couple of weeks. In fact, full disk satellite imagery does present a signal of increasing tropical convection from around the Philippines to just west of the date line. There has also been a slight increase in convection over the tropical East Pacific while flare-ups remain along the western equatorial Indian Ocean. The former is tied to a convectively coupled Kelvin wave while the latter is linked to the South Asian Monsoon system.

GSDM Stage 1 still best describes the weather-climate situation. Animations of upper tropospheric daily mean vector wind anomalies through 28 June (FNL data) shows subtropical twin anticyclones ~120E with downstream twin cyclones just east of the date line while stationary twin anticyclones remain across Africa and weak twin lows over the western Atlantic. Wind speed anomalies are ~15-25m/s over the subtropical atmospheres with these features, with magnitudes still ~30m/s of the equatorial westerlies west of South America. Interacting with fast subtropical baroclinic wave packets and extratropical Rossby wave energy dispersions, I can see that at least “some” of the recent strong anomalous Western Hemisphere upper tropospheric equatorial westerly wind flow has propagated into the extratropics, particularly the South Hemisphere. The inability of these equatorial westerlies to nicely come back around into the Eastern Hemisphere may be tied to frictional dissipation. However, there has been recent anomalous flux convergence of AAM transports of ~.6 Hadleys ~40N (with Southern Hemisphere symmetry). In a very complicated way from watching the wind animations, I do think there is some linkage between these AAM transports and the anomalous equatorial westerlies.

Global relative AAM remains ~1 standard deviation below the R1 climatology while the global tendency and mountain torque are near zero. While decreasing, global frictional torque is still ~plus 10 Hadleys but with negative zonal mean anomalies ~40N and 30S. Finally, zonal mean anomalous easterly flow that was across the equatorial latitudes a few weeks ago has now propagated poleward to ~35N but with anomalous zonal mean westerlies to the north and south (magnitudes ~5-10m/s at 200mb).

My thoughts where the atmosphere is going remain unchanged from 26 June. I think tropical forcing will increase across the warm TNWP during the next couple of weeks, perhaps extending from Southeast Asia to ~5-10N/160-170E by the end of week 2. Meanwhile, thunderstorm activity is likely to remain periodically intense for locations centered ~India and the BB. Yes, there may be see 2 separate regions of tropical forcing per previous discussions. The future of any MJO signal is unclear. The Wheeler phase space plot indicates a weak projection moving toward Africa, which, for now, I think is unrealistic.

I would expect the Global Wind Oscillation to show a turn from GSDM Stage 1 to possibly Stage 2 depending on how intense any convection becomes across the TNWP. The added midlatitude westerly flow per above suggests the possibility of an intense trough ~160W of the North Pacific leading to a retrogression and intensification/amplification of the western North American ridge by the start of week 2. I also think there is a possibility for this wicked western USA devil ridge to shift back into the eastern Pacific while amplifying into Alaska weeks 3-4. In fact, there are even a few ensemble members from the numerical models that indicate this. Finally, going out on a limb per longer time scale variations discussed 26 June, I could even see a pattern reversal during August for the west and central USA while the Deep South ridge goes “no where”.

In the meantime, this scenario is not good news for the Interior West for at least weeks 1-2 given extreme/record heat, fire weather and drought concerns. Anticyclonically wave breaking lows across the Southern Plains will keep the flooding rainfall threat for especially Texas while the Northern Plains-Upper Mississippi Valley has generally cool and increasing wet conditions weeks 1-2. In fact, severe MCS activity may focus on states like Iowa starting week 2. Albeit optimistic by most, some relief from the heat may occur for the Interior West weeks 3-4.

Internationally, tropical cyclone threats may be a concern for weeks 1-2 across the East Pacific, TNWP including the Philippines, and even the BB. In fact, surface westerly wind flow anomalies are developing across the far west central Pacific suggestive of an increasing tropical cyclone threat in that region. The tropical Atlantic should remain suppressed. Flooding rainfall and severe thunderstorms show no let up for the areas across South Central and Southeast Asia already hit hard the last several weeks as does the onslaught of cold/wet conditions for especially Eastern Australia.


An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation (GWO)) utilizing normalized relative AAM time 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. We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion early next week. Due to covering shifts and travel, my postings on this Blog will be irregular through at least August.

Ed Berry

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