Friday, August 03, 2007

Update from the Slayer

The spatial distribution of global tropical/subtropical SSTs is similar to what was discussed on 31 July. There remains warmth across the Indian and west central Pacific Oceans with cool anomalies across portions of equatorial Indonesia. Impressive warmth remains around the South China Sea and northwest Pacific Ocean with SST totals in excess of 30C. The equatorial East Pacific cool anomalies are ~minus 1-2C while the subsurface continues to moderate.

Tropical convective forcing has become better organized centered ~10-15N/120E while extending in a band from India east-southeast into the west central and northwest Pacific Ocean. Three-day averaged OLRA are roughly minus 50-90W/m**2 with this enhanced rainfall, and there has been a slow east-northeastward shift of this activity toward the Tropical Northwest Pacific Ocean (TNWP). Other somewhat enhanced rainfall continues along the East Pacific ITCZ and Africa north of the equator.

Animations of upper tropospheric daily mean vector wind anomalies do give a nice signal of slow eastward movement across the tropics/subtropics. Since 31 July robust twin anticyclones have appeared ~140E having 150mb anomalies around 15-30 m/s while weaker anticyclones persist across Africa. There are downstream anomalous troughs just west of the date line with equatorial westerlies (15-25 m/s anomalies) across the Western Hemisphere to about South America. Rossby wave energy dispersions from both hemispheres are interacting with these features. Included is a Northern Hemispheric component across the Pacific Ocean to the first in a series of troughs about to dig along the west coast of the USA.

This circulation response working with other dynamical processes such as global frictional and mountain torques is adding westerly flow to the atmosphere. In fact, calculated relative AAM tendency (per R1 data) through 28 July which includes budget terms such as Coriolis torque and transports was ~plus 20 Hadleys. However, total AAM remains very low, roughly 3 AMUs (Atmospheric Momentum Units) below normal, and the Global Wind Oscillation (GWO) updated through 29 July is well to the left. The global circulation continues to behave La-Nina like, solidly in GSDM Stage 1. As the Eastern Hemisphere tropical forcing continues into the TNWP, interactions with the extratropics do include the possibility of a mountain-frictional torque index cycle. The latter would increase zonal mean westerly flow across the subtropical atmospheres of both hemispheres (ideally).

I remain unchanged in my feeling that this GSDM Stage 1 base state should be perturbed enough for the GWO to orbit to GSDM Stage 2. The magnitude and amount of persistence of this perturbation are unclear, particularly considering seasonal cycle issues. Overall, we may see GSDM Stage 1-2 for the next few weeks, then collapse to Stage 4-1 per reasons discussed last posting. Uncertainty remains huge about this specific subseasonal issue. However, at least synoptically for the USA the major circulation pattern reversal with an anomalous western USA trough and Deep South ridge is probable. Furthermore, Rossby wave energy dispersions arcing from the Indian Ocean tropical forcing into the higher latitudes will contribute to an anomalous deep cyclone covering much of western and central Canada. Hence a full latitude trough extending into the western USA is a good forecast meaning the polar westerlies should shift southward. Most numerical ensemble prediction schemes have now captured this change.

There has been some thought from at least one official forecast center based on model output the ridge may return to the Rockies during week 2. While this is an option, I could easily argue a “cold/wet regime” from the east slopes of the Rockies into the Plains weeks 2-3 should the TNWP become extremely active with several typhoons. Again, this emphasizes the uncertainty given the stochastic nature of the atmosphere. Right now there is little if any support from ensembles for a central USA cold/wet pattern weeks 2-3 (for example, like August 2004 as discussed in one of our weather-climate discussions on the ESRL/PSD MJO web site). Should this less probable scenario happen, it is currently outside the envelope range of uncertainty sampled any of the operational global models (multi-model ensemble spread). Again, this emphasizes the need for a weather-climate linkage diagnostic component (the GSDM) including daily monitoring to correctly making subseasonal predictions as part of a complete forecast process. Multi-model ensembles alone are not enough, particularly considering their biases, data assimilation issues, not predicting tropical convection correctly after about day 5, etc…………..

There is little change in my outlooks for weeks 1-3 (through ~24 August). The “wicked ridge of the west” will expand into the Deep South and eastern USA as the westerlies come southward into the Rockies and Plains downstream from the western USA trough. A reversal of the temperature and precipitation anomalies observed across much of the country for the last 4-6 weeks may occur. Severe MCS activity with flooding rainfall is likely from the Northern Plains into the Ohio Valley periodically through at least the middle of this month. This “storm track” should slowly shift south. Intense heat may focus on the south central into the mid-Atlantic states, along with high dewpoints.

Internationally, severe thunderstorms and heavy rainfall are probable from southern China into the Philippines at least weeks 1-2. Additional typhoon activity is probable for the TNWP weeks 1-3, possibly impacting locations such as China and Japan. Finally, tropical waves moving west from Africa have been recently intensifying. However, for numerous reasons I am now unclear about the risk of tropical cyclone development across the North Atlantic Ocean at least through week 2. The latter is yet another example why there is not a linear relationship between La-Nina basic states and an increased risk of North Atlantic tropical cyclone activity, at least subseasonally.


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

We call the behavior exhibited by this plot the Global Wind Oscillation (GWO), and one of its purposes is to extend thinking beyond the MJO.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, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

My next posting will not be until late next week since I have to cover a set of overnight shifts. In general, due to covering shifts and travel, my discussions will be irregular through at least the end of this month.

Ed Berry

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