Friday, December 15, 2006

Only a Matter of Time? Will There be Winter?

Global tropical SSTs generally remain above average. The greatest anomalies are across the Indo-Pacific region with magnitudes ~plus 1-3C especially along the equatorial cold tongue. The latter is associated with the basin wide warm ENSO, with the warm anomalies (up to 5C) extending to 150m deep per latest TAO buoy data. Interestingly, deep cool anomalies ~1-2C are appearing west of the date line and the 20C isotherm depth has shoaled to about 160W. Also, the warm-cool-warm SST distribution has become less distinct during the past several weeks. Perhaps our warm event may have already peaked. In any case, actual SSTs 29C and higher extend from the South Pacific into eastern Indonesia and over portions of the Indian Ocean.

The following are links to ENSO discussions:

Please also see the following CPC link (and others therein) for further ENSO, etc., insights, and remember that official USA information on anything related to ENSO comes from CPC.

Tropical convective forcing remains very complicated. Per full disk satellite imagery and monitoring tools (such as coherent modes and velocity potential Hovmollers) relatively fast dynamical signals moving through the Western Hemisphere are starting to emerge into Africa and the Indian Ocean. A fairly intense flare-up has occurred ~0/60E during the last week and may be evolving into a weak MJO per Wheeler phase plot. SST boundary forcing and Southern Hemisphere frontal activity has allowed another intense burst of convection to occur ~0/160E while intense diurnal activity persists across the Amazons of northern South America. At least 2 and arguably 3 significant regions (counting South America) of tropical forcing are impacting the extratropics. In fact, extratropical responses and feedbacks linked to some of this activity may have contributed to recent very poor week-2 500mb ACC skill scores for the NCEP GFS ensemble mean for North America (less than minus 0.3). Since early October the enhanced-suppressed-enhanced distribution of tropical forcing across the Indo-Pacific region has shifted roughly 40 degrees to the east, which may reflect a slow ENSO signal.

Zonal mean easterly wind anomalies have been increasing throughout the equatorial and particularly northern subtropical atmospheres since about the start of this month (~5-10m/s at 200mb). Anomalous zonal mean westerly flow flanks these easterlies suggestive of subtropical jets across both hemispheres. GSDM Stage 4 best describes the current weather climate situation (not all AAM plots from ESRL/PSD are available). Where we go from here remains very unclear. As I have offered before, my thoughts are to expect some consolidation of the tropical forcing from the west central into the South Pacific going into January 2007 (as it shifts south).

Nearly all the numerical models are suggesting closed low development across the Desert Southwest by early next week to then eject northeast affecting much of the country. Thus while winter storm conditions are probable from the central Rockies into the portions of the Plains, the Deep South may receive heavy rainfall along with severe local storms. True Arctic air will be lacking with this system.

After Christmas heading into the first couple of weeks in January 2007, I would almost be surprised if a robust GSDM Stage 2 situation did not evolve (bypassing Stage 1). That would mean ridge amplification just off the North American west coast into Alaska with a few digging synoptic troughs across the Rockies and Plains. Ramifications would include true Arctic air penetrating into the lower 48 states with lots of wintery precipitation to go along with it, especially for the eastern 2/3rds of the country. Afterwards the DREADED extended combined jet from Asia to the west coast of North America (with split flow across the continent) may appear which is typical of GSDM Stage 3 that is most probable during a mature global response to a warm ENSO.


The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006, in press).

Also, our latest weather-climate discussion was posted on November 29th at

In the beginning of Section 2 of that report we give a summary description of the Global-Synoptic-Dynamic Model (GSDM) of subseasonal variability.

I will try to do at least a short posting next week. As stated previously, while I am at ESRL/PSD we hope to post at least short writings on this Blog every other day in support of the Hydrometeorological Testbed Project (HMT -- please see

Ed Berry

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