Monday, February 06, 2006

The Atmosphere is Going into Reverse

Since we are working this week on a long overdue posting of a weather-climate discussion for the ESRL/PSD (formally CDC) MJO web site, this will be the last writing on this Blog until at least this Friday (2/10). The link to these discussions is

Slow but important changes are occurring across both the tropics and extratropics. SST anomalies remain below normal across the equatorial central and eastern Pacific, with values at least as cold as -2C and actual SSTs around 24C centered on 145W. Ocean surface temperatures remain slightly above average across the south and west Pacific, with anomalies ~ +.5-1C and SSTs from 29-31C centered near 10-15S/160E. This pattern is reflective of La-Nina.

After 4 months of little MJO variability, during the past couple of weeks or so there has been a coherent but relatively weak signal of the MJO propagating east from just north of Australia to roughly the southern eastern Pacific. One response from this MJO was very intense convection along the South Pacific Convergence Zone (SPCZ) over the warm SSTs this past week. The SPCZ convection added anomalous westerly flow to the tropical and subtropical atmosphere. In addition, feedbacks from the polar latitudes including the wave number 1 retrogressive transient (discussed in previous postings) added anomalous westerly flow to the northern extratropics (with similar behaviors perhaps across the southern hemisphere). These tropical/extratropical processes have ramped up the relative atmospheric angular momentum (AAM) to the highest values in at least 6 months, at about one standard deviation above the 1968-1997 climatology.

Monitoring of various indicies as well as statistical forecasts suggest this western hemisphere MJO signal will return into the Indian Ocean by late next week. Recent satellite imagery already shows tropical thunderstorm activity increasing across much of Brazil and the Amazon Rainforest, as well as South Africa and even the South Indian Ocean. Thunderstorm clusters are also shifting back westward into the eastern hemiphere from the SPCZ. Past experience would suggest a consolidation of these 2 areas of tropical forcing ~ 60-150E south of the equator late week 2 or week 3. This distribution of enhanced tropical convection would be consistent with La-Nina.

Since this change of tropical forcing would lead to above average easterly flow throughout the tropical and subtropical atmosphere, AAM may decrease to the low values that were observed a few weeks ago. Additionally, the retrogression across the north polar latitudes would also be expected to "hook up" with the tropical forcing, leading to SDM Stage 1 late week 2 or 3.

Most models are already predicting a positive phase of the PNA by later this week through this coming weekend (western North American ridge, eastern trough), with discontinous retrogression during week 2. The latter would result in a western North American trough/southeast USA ridge. Much of the anomalous signal the models are getting is likely coming from the high latitude retrogression, with little predictive input beyond about day 5 from the tropics. I do think the models have the correct general idea, but with little skill in regard to timing and important synoptic details. My preference continues to tilt slightly to the CDC ensemble, especially the week 2 calibrated probabilities. I would have a concern for a strong subtropical jet (STJ) impacting the at least the California west coast by next weekend. Also, the trough may shift back to the west coast, leading to cold/wet events for that region. Whatever the case, blocking across Alaska could set up cross polar flow of true Arctic air from Asia into North America, leading to a cold and wet regime that much of the CONUS has not seen so far this winter (remembering the seasonal cycle as we get closer to spring).

For southwest Kansas, other than the cooling to closer to normal temperatures, still the "same old, same old" dry regime through at least week 1 (~February 13). I have serious doubts of Arctic air making it this far west (not to mention modification due to higher sun angles and lack of snow cover across much of the High Plains). During week 2, we may start to see some changes, especially with better chances of precipitation by next weekend (~18 February). Real Arctic air may make it here (at least shallow) by late week 2 or 3 should there be a situation of a western USA trough and blocking across Alaska. Monitoring is critical, and we need to watch out for "surprises".

Ed Berry

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