Tuesday, February 27, 2007

MJO or not?

The distribution of global tropical SSTs remains similar to about a week ago. This includes general warmth from the Indian Ocean toward the central and South Pacific while cooling continues along the equatorial cold tongue east of 150W. However, recent tropical cyclone activity has led to a negative SST tendency across the South Indian Ocean while positive tendencies are present across the central and South Pacific.

After the recent burst of intense tropical cyclone activity across the South Indian Ocean, a renewed organization of the tropical forcing is occurring centered ~10S/120-140E. General enhancement is present in a west-northwest to east-southeast oriented band roughly 10 deg wide from ~5S/90E to ~15S/160E which does include the SPCZ. I continue to maintain that this forcing is tied to a decent MJO signal. Monitoring tools such as the Wheeler index, velocity potential and animations of 150mb, 250mb and surface daily mean vector wind anomalies support my contention. Care must be taken to remember that it is not only the convective signal which defines a MJO, but also the winds (the multivariate EOF Wheeler technique considers this). The atmosphere is a dynamical system that consists of forcing, response and feedback processes all the time, and the MJO is just one of those components. About 10-20% of the tropical variability can be explained by the MJO, which only adds importance to diagnosing that signal when it is present.Care must also be taken to understand the details of tools utilized to interpret various modes of subseasonal variability. For instance, the Wheeler technique can be unrepresentative when there are multiple regions of tropical forcing.

I think the dynamical signal with the MJO is centered ~10S/100E, which does not have to be at the same location of the enhanced tropical convection. The global circulation has strongly responded with features such as twin upper tropospheric subtropical anticyclones across Africa and the Indian Ocean and downstream cyclones just east of the date line. Circulation anomalies are starting to reverse in the lower troposphere; for instance, there are westerly wind anomalies just north of Australia. This is a signal of a tropical baroclinic mode. Finally, zonal mean easterly wind anomalies remain ~good 5m/s throughout the tropical and subtropical atmospheres and global relative AAM is at least 2 standard deviations below normal per reanalysis data/climatology. The circulation has been in GSDM Stage 4 for roughly the past week, and GSDM Stage 1 appears probable during the next 1-3 weeks.

A serious question for weeks 1-3+ predictions is how far east is this MJO signal going to propagate? Our thought has been for this particular event to be truncated (linked to a possible transition to La-Nina) meaning it may not get by roughly Indonesia. In the presence of periodic flare-ups across the central/South Pacific, I am going to continue that feeling. This means the East Asian Jet (EAJ) will remain retracted and more troughs are probable to impact the west coast into the Plains. Should the convection come out to the date line, then the EAJ would be expected to expand east.

Most models have now captured the recent EAJ retraction and I do agree with the notion of trough development from the Alaska into the East Pacific during week 1. That would favor a less active pattern for much of the USA after the upcoming Plains storm development, consistent with the “lull” I have discussed in past postings. I think that will change during week 2 and continue week 3. Initially, heavy precipitation should spread southward along the USA west coast, including the ARB region (for the Hydrometeorological Testbed Project). Troughs would then be expected to move through the Rockies leading to baroclinic storm development on the Plains with an Arctic cold air source. The latter has been somewhat lacking with our past active situation.

Please note: These are probabilistic statements, which we will try to quantify in future posts. My next 1 month period at ESRL/PSD with the HMT project will be from 3/4-4/3. I may not be able to post another discussion until early next week (~Tuesday) due to travel and shift work obligations. The WB (2007) paper on the GSDM has been published in this month’s issue of MWR.

Ed Berry

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