Wednesday, February 01, 2006

Regime Transition Madness and our Baby Girl

This posting is to update the current weather-climate situation as discussed during the recent January 25 and 30 postings. The issues written about then are still relevant. Also, please note that data on the CDC web site has not been updated since January 27. The problem is being worked on.

On our January 30 posting concern was expressed about predictability being unusually problematic including the issue of the models prematurely amplifying ridges off the west coast of North America. Two behaviors (discussed below) that we have been monitoring have become robust enough not only to add some credence to a ridge (finally) developing of the west coast, but perhaps signal the initiation of a major regime transition. Most numerical models including their ensembles are coming into better agreement of this evolution as their initial conditions more correctly sample the anomalies contributing to the transition. However, uncertainty of timing and details of this regime change particularly beyond day 3 remains very high.

The first behavior is the high latitude retrogression. During the last few days we have observed the wave number 1 retrogressive transient become more coherent, with the deep low that has been located around Alaska shifting back toward Asia. Feedbacks from this process should allow the EAJ/north Pacific jet to retrace a bit. Thus while the current jetstream remains anomalously strong slamming the Pacific Northwest with several storms, we think that situation may abate starting early next week.

The second observation concerns the behavior of the tropical convection. While stationary forcing remains ~5s/140E, there is a slowly propagating component that has allowed a significant increase of thunderstorm activity to extend east-southeast toward ~ 12s/170W across anomalously warm waters of the tropical south Pacific Ocean. The previous convectively coupled Kelvin wave has lost coherence and decayed. SST anomalies are roughly 1-1.5C above normal from about 10S/160E- 15S/150W with actual SSTs of at least 29C. The tropical convection appears to be located along a gradient of SSTs, and has displaced the SPCZ a bit farther northeast of normal. This SPCZ convection continues to add westerly flow to the subtropical atmosphere of both hemispheres.

Since the propagating convective component has become involved with the southern extratropics, it is difficult to speculate about its future evolution. However, there may be an increase of thunderstorm activity over South America and South Africa, and perhaps the South Indian Ocean, if the signal continues moving east during week 2. This can occur
either through the tropics or midlatitudes via Rossby wave trains and associated jet streak dynamics.

So not only does ridge amplification off the west coast now appear a reasonable scenario by early next week, but there may also be a strong subtropical jet (STJ) undercutting it as well. In fact, should the tropical convective forcing finally evolve into a pattern more typical of La Nina (which continues to strengthen), we may see a "Rex blocking - like" structure of the polar and STJ winds that shifts northwest during weeks 2 and perhaps 3. This would suggest a blocking ridge in the Gulf of Alaska into the Arctic and strong westerly flow at least as far north as California.

The weather impacts would suggest colder and wetter for much of the CONUS particularly week 2 (February 9-15) and perhaps afterwards. In fact, calibrated probabilities from the CDC ensemble are quite supportive of this notion, and can be found at

Compared to the predictions above, we would increase pops for above normal precipitation slightly for California and the central/southern Plains. The Pacific Northwest may continue to catch a break. Furthermore, the change toward much colder temperatures would be centered on the north central USA.

Ed Berry and Klaus Weickmann

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