Sunday, February 19, 2006

Back to Order in the Tropics

Just an update to previous Blog issuance on February 16. Again, please see our recent weather-climate discussion dated February 15, 2006, on the ESRL/PSD MJO web site at

for a more in-depth look.

SST anomalies remain cooler than normal across much of the central and eastern equatorial Pacific, ~ minus .5-1C. These magnitudes have decreased during the past 7-10 days, which may be related to the seasonal cycle. Similar anomalous warmth continues from the western Pacific back into the SIO. Anomalously warm SSTs are also present across both the subtropical south and north Pacific Oceans east of the date line, somewhat flanking the cool SSTs in a "horse shoe" manner. With actual SSTs of ~25C near 140W in the equatorial cold tongue and in excess of 30C across the warm pool, the trades have been slowly re-strengthening. Low-level easterlies have also been increasing across the SIO during the past week.

The MJO is very weak. Tropical convective forcing has significantly re-emerged across the eastern hemisphere (EH) during the past week, currently centered ~5-10S/140E. Intense thunderstorm activity extends westward well into the SIO and to the east along the SPCZ to 140W. The latter is a response to the warm SSTs in that region. What MJO signal that remains from a few weeks ago is believed to be contributing to this EH intensification.

All of the above is consistent with expected behaviors from the tropics during a cold event. The extratropics are also responding accordingly. The EAJ has retracted, with poleward displaced twin anticyclones near 140E and downstream twin subtropical cyclones now around the date line (leading to a STJ into the southern CONUS). A trough-ridge-trough pattern extends from Asia-North America, projecting onto the negative (or reverse phase) of the PNA (a.k.a. RNA when negative), with split flow across the central and eastern Pacific. Split flow is also becoming present across the Atlantic. Finally, with zonal mean easterly anomalies over the high and subtropical latitudes and still westerly anomalies near the equator and midlatitudes for both hemispheres, relative tropospheric AAM is decreasing. Tools such as time-latitude sections of zonal mean zonal wind anomalies do give a signal of the westerly anomalies starting to move off the equator into the subtropics, with easterlies replacing them.

Similar to what was observed about a month ago, I would expect the most significant lobe of the polar vortex to become re-established across central and eastern Asia. It would not be unreasonable to see a trough dig into the east Pacific in about a week, as the extratropics continue to respond to the re-intensification of the EH forcing. That is also similar to what happened nearly a month ago. As most models show, that would lead to a transient synoptic ridge amplification along the west coast and allow a trough to deepen across the central and eastern states. A surge of Arctic air would accompany this trough.

I would think with the zonal mean westerly anomalies coming off the equator the EAJ would extend in roughly 7-14 days. This would result in a downstream progression of synoptic features, in the overall "RNA/Pacific (and Atlantic) split flow pattern" discussed above. As the CDC ensemble shows for week 2, a deep trough would be probable for the Pacific Northwest and northern Rockies with ridging across the south central and south east states. The storm track would likely be from the from the west coast into the Northern Plains. In general, by week 2 (2/27-3/5) in terms of the circulation SDM Stage 1 would be most probable (currently Stage 4 with the STJs).

Week 1 impacts would include another surge of an Arctic airmass into much of the central and eastern states, but not as cold as its predecessor. The wettest locations should include some precipitation for the Pacific Northwest as well as the southeastern states. By week 2 locations from roughly northern California in the Pacific Northwest may see significant precipitation. Those locations would also be expected to remain cooler than normal. Colder than normal weather may also linger across the northeast states as blocking develops across the North Atlantic during week 1. Similar to a recent high latitude retrogression (see past postings), that North Atlantic block may shift west into northern Canada during week 2. Finally, warm and dry conditions are probable for the central and southern Plains into the southeast. The northern Plains may also become active with possibly 1-2 events of baroclinic cyclogenesis.

Nothing new for southwest Kansas. Still looks like little or no precipitation for the next 2 weeks. Temperatures should warm this upcoming week to above normal, followed by cooler than normal temperatures for about next weekend. Above normal temperatures are then probable for the following week going into the first part of March.

Ed Berry

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