Wednesday, May 02, 2007


Weekly mean sea surface temperatures (SST) remain well above average across the Indian Ocean and weakly so over the west Pacific with magnitudes ~0.5-1C. SSTs greater than 30C are present from the Indian Ocean into the Bay of Bengal, decreasing to ~29C across the central and South Pacific. In response to increasing trades, negative SST anomalies have been strengthening around the equatorial date line while persisting west of South America. In the subsurface negative temperature anomalies of 3-4C remain at ~100 meters depth along the east Pacific cold tongue. There is still uncertainty about the intensity of any upcoming cold event, particularly since SSTs have also cooled to below normal around Indonesia. The latter is reminiscent of last fall, when we had a similar warm-cool-warm distribution of Eastern Hemisphere tropical SSTs.

The MJO signal remains incoherent. A few weeks ago I thought this mode of tropical convective variability would emerge across the Indian Ocean by now. Even though fairly intense convection is centered ~0/100E, MJO evolution appears less likely for the next 1-2 weeks. Full disk satellite imagery and other monitoring tools show that enhanced tropical rainfall has persisted across much of the eastern Indian Ocean for about the last week (OLRA ~minus 50-70 W/m**2). More recently a separate but weaker region of enhancement has developed over the west central Pacific. This mirrors the SST distribution discussed above. However, I think we are in a stationary or slowly evolving situation which is focusing the most intense forcing around 100E. Meanwhile, much faster tropical modes such as convectively coupled Kelvin waves and interactions with the extratropics are leading to flare-ups across other regions including the west Pacific.

Zonal mean easterly wind flow anomalies of ~5m/s continue over much of the equatorial and subtropical atmospheres helping to keep the global relative AAM low (~1.5 standard deviations below the reanalysis climatology). However, recently the global AAM tendency has become positive, reaching about 15 Hadleys. I attribute that to a positive East Asian mountain torque forced by a Rossby wave energy dispersion (RWD) and also an equatorial source forced by west Pacific tropical convection. This RWD has arced from north Asia into the central Pacific during the last 5-7 days while interacting not only with the tropical forcing at 100E, but also from the west Pacific. This RWD will lead to a strong western USA trough by the upcoming weekend, as shown by the numerical models. An enhanced probability of more western states troughs by this time was suggested in my April 20th posting.

The global circulation remains in GSDM Stage 1, and unless the character of the tropical forcing discussed above changes, we should roughly remain in that state for at least weeks 1-2. Stated another way, the global wind signal may orbit in phase space tilted toward GSDM Stage 1. If this situation persists through the rest of this month, I think it is more probable for a La-Nina to develop. For example, stationary tropical convective forcing across the Indian Ocean/Indonesia favors strong trades across the central and eastern equatorial Pacific, allowing upwelling of the colder subsurface waters currently present. For weeks 1-3, I think there will be another respite in the stormy regime across the USA by around the middle of next week, only to resume by week 3 but somewhat shifted northwest.


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

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. My next posting will not be until next week.

Ed Berry and Klaus Weickmann

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