Sunday, January 15, 2006

The Atmosphere has hit the Brakes

With respect to 1968-1997 climatology, the current global tropospheric relative atmospheric angular momentum (AAM) is about negative 2 standard deviations, perhaps the lowest we have seen for at least a year. This means that the global circulation is dominated by deep anomalous zonal mean easterly flow, much which is coming from the subtropics. Any above average westerly flow can only be found in latitude bands around 45N and 50S. Much of this low AAM has been forced by the tropical convection, with the primary region having shifted westward from roughly 130E to 80E over about the past 4 weeks. Currently the convection extends in about a 20 degree wide band from the central Indian Ocean to the south Pacific, centered just north of Australia.

One response has been a significant retraction and weakening of the east Asian/north Pacific polar jet stream since about January 4th, 2006. As discussed in previous postings, the jet was extended from north of India to the west coast of the USA, with wind speed anomalies in excess of 70 knots, at times. Currently the strongest winds are generally present from north Africa to east Asia, with much weaker anomalies. A time-longitude section (Hovmoller diagram) of 250mb wind speed anomalies between 25-40N demonstrates this point.

With the polar vortex becoming displaced toward Asia, a general east Asian trough, central/east Pacific ridge and western USA trough pattern, with embedded progressive synoptic systems, has evolved within the above discussed basic state. This is typical of a La-Nina situation, and is represented by SDM Stage 1 (see correct link below).

An important question to ask is if there will be any synoptic amplication? Monitoring tools would suggest roughly in about 10 days, meaning next weekend into week 2 (~January 22-29). The details of this amplification are unclear. Given the current location of the tropical convection, the possibility of a transition into SDM Stage 2 also needs be considered. Stated with very low confidence, after a Rockies and Plains storm event by about next weekend and maybe again week 2, a large amplitude ridge may develop just off the west coast of North America extending into Alaska by late week 2. This would favor transports of Arctic air particularly into the middle of the country by then. Some ensemble members of the models may be capturing this scenario (ex., GFS and Canadian).

For southwest Kansas, a record warm January is in progress, and I still cannot be optimistic about widespread significant precipitation given the progressive nature of synoptic features. However, temperatures should trend down this upcoming week, and the storm for about this Friday may slow down enough for measureable snowfall. We may see 1-2 other opportunities week 2. It must be remembered this time of year is typically dry for this part of the world. In fact, I can easily argue we are seeing enhanced climatology with the zonal mean westerly flow now shifting poleward of normal.

Ed Berry

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