Thursday, November 23, 2006

New World Atmosphere Winter Before El-Nino???

Please see past postings for web site links. In this writing I am going to post a version of a draft of Section 2 (Predictive Insights) which will appear in our next weather-climate discussion for the ESRL/PSD MJO web site.

Global tropical SSTs remain well above average across the Indian Ocean (IO) and from around the date line to the west coast of South America, with weekly mean magnitudes of 1-3C along the equatorial cold tongue. Warmth also persists across the North Atlantic, particularly from the Caribbean to the west coast of Africa. At depth the positive anomalies along the equatorial east Pacific extend to about 250m with values to around 5C at 125m at 120W meaning a deeper than normal thermocline assisted by the Kelvin wave currently propagating eastward along it. Anomalies south of Indonesia remain slighly below average while their spatial distribution decreases. During the past week equatorial SST tendencies have generally been positive with magnitudes ~.5-1.0C. Part of this SST distribution is from a well defined and currently strengthening warm phase of ENSO.

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.

Per full disk satellite imagery two prominent areas of tropical convective forcing are evident. One is located over the warm SSTs around the equatorial date line with the other larger region centered on the IO (including central and southern Africa), with latest 3-day averaged OLRA ~minus 70-90 W/m**2. The recent Western Hemisphere tropical forcing linked to a convectively coupled Kelvin wave has lost coherence. From animations such as 150mb and 250mb daily mean vector wind anomalies, the IO forcing appears to be organizing into an MJO. There is now the quadrapole of upstream twin subtropical anticyclones (and low level westerlies) with downstream cyclonic anomalies, and these features are interacting with the extratropics through wave energy dispersion processes. In fact, the latest plot on the Wheeler diagram shows a projection well above 1 sigma and there is also a projection onto the coherent modes Hovmollers. Finally, a loose phase speed computation has a movement of roughly 5m/s or about 20 deg of longitude during a 5-day period from November 15-20.

The extratropics have been strongly impacted by this complex tropical forcing, with subsequent feedbacks. Since late October zonal mean westerly wind anomalies (~5-15m/s at 200mb) have propagated from the tropical into the subtropical atmospheres (to ~25-30N) while being replaced by easterlies. During early November an anomalous combined jet did extend across the North PacificOcean ~35N meaning a GSDM Stage 3 situation was briefly present, and this was about the time excessive rainfall across the Pacific Northwest commenced. Some of this westerly flow did make it to the surface augmenting deep tropical moisture transport from the date line region to the northwestern USA. Additionally, upper tropospheric divergent outflows from the African and IO convection through interactions with baroclinic wave packets led to the current blocking structure near Kamchatka. Per animations of 150mb and 250mb daily mean vector wind anomalies from ESRL/PSD, feedbacks from this blocking assisted with the dynamic suppression of convection along and to the north of the equatorial date line (where SSTs are warm) until about a week ago.

Careful examination of the animations shows that a recent Rossby wave energy dispersion linked to the MJO arcing into the Southern Hemisphere extratropics contributed to the flare-up of the South Pacific tropical convection. This region of forcing has been expanding back to the west-northwest into northern Indonesia during the last few days. Moreover, convectively coupled Kelvin waves emanating from the MJO along with the expanding date line/South Pacific convection have allowed some filling in to occur meaning a band of tropical thunderstorm clusters across northern Indonesia linking the 2 regions. Twin upper tropospheric anticyclones supporting cross equatorial flow from the Southern Hemisphere are also present around the date line. The latter are supporting the linkage of a subtropical jet with the trough currently moving into the western USA.

Global relative AAM is about 1 sigma below the 1968-1997 reanalysis data climatology as of November 20th. Contributions to this low AAM are coming from the zonal mean easterly anomalies across the deep tropics and the Southern Hemisphere subtropical atmosphere as well as ~40N. The latter can be linked to the recent blocking episodes from Greenland to Kamchatka. Global relative AAM tendency reached a negative minimum of ~minus 20 Hadleys around November 16th only to be approaching plus 20 Hadleys on the 20th. Positive zonal mean tendencies of around 2 Hadleys were appearing from the equator to around 20N with weakly negative values across the Arctic (latest AAM plots here), with the former believed to be linked with South Pacific tropical convective flare-up discussed above. The contributions of the mountain and frictional torques to the global AAM budget were ~plus 10-15 Hadleys each. However, there were significant zonal mean variations. For instance, the mountain torque had magnitudes of ~2 Hadleys and a minus-plus-minus distribution from 30-90N.

GSDM Stage 4 has best described the weather-climate situation for roughly the past couple of weeks. However, given the MJO signal, careful interpretation of the AAM plots and critical daily monitoring, a transition to GSDM Stage 1 may be in progress. For instance, animations do show the recent Kamchatka blocking becoming "dislodged"and is tied to a Rossby wave energy dispersion from the IO convection. As shown by most models (but with large differences in the details) the trough which has been present across the Gulf of Alaska is about to deepen into the western USA (leading to a greater projection onto the negative phase of the PNA). Arctic air has been building up across Alaska and northwest Canada for the last several weeks, and this trough will have that airmass as a cold air source. The latter has been discussed as a possibility in previous postings.

However, where we go from here remains tremendously uncertain, especially given the seasonal cycle (and other issues such as high latitude blocking). There are also going to be impacts from tropical convective flare-ups until if and when "El-Nino kicks in". For instance, the positive AAM tendency we are now seeing I think is directly attributable to the current "MJO forced flare-up". A probable thought may be for MJO forcing to consolidate with the date line thunderstorm clusters leading to a large region of intense tropical rainfall extending from near the Philippines to the South Pacific centered ~150E by late week 2 or week 3. This region may then stall before shifting east-southeast toward the central Pacific/SPCZ by early January 2007. Thus perhaps GSDM Stage 1 with subtropical jets may be the case for the weather-climate situation through week 2, followed by GSDM Stage 2 for much of (mid-late?) December. Afterwards, GSDM Stage 3 may appear, which is "typical" during the warm phase of ENSO. In the USA outlooks that follow, confidence is below average for week 1, then as low as it gets for weeks 2-3.

Week 1 (24-30 November 2006): GSDM Stage 1 with a moist subtropical jet is most probable. This situation generally favors an active weather regime for the Rockies and Plains, with a southwest-northeast storm track across the central USA. An important issue is how soon deep tropical moisture transport through the Gulf of Mexico can resume after the recent surges of cool dry air. Latest observations suggest low level moistening is occurring across the Gulf of Mexico. Most models do predict baroclinic development on the Plains by around the middle of next week, but with still serious phase and amplitude issues. They are "catching-up" to the changing tropical convective forcing and other processes. A thought to offer would be a slower and more amplified solution for trough development, and the models are now trending there. In any case, bitterly cold Arctic air is likely to penetrate into the Rockies and Plains by the end of this period while the Deep South warms up. Depending on the details, portions of the Rockies and Plains may have severe winter weather conditions while severe local storms become a concern for the locations such as the south central states to the Ohio Valley.

I also want to make special mention of the still on-going tropical cyclone risks across the central Pacific and possibly other regions. Satellite pictures show evidence of development trying to occur west-northwest of the equatorial date line. Locations such as the Philippines may be impacted later this period. Tropical cyclone Yani is already in progress across the South Pacific islands, and more development may follow. Finally, locations across the Indian Ocean may become at risk for tropical cyclone development as the MJO slowly moves east.

Week 2 (1 - 7 December 2006): Same as week 1, but with the usual synoptic variations in amplitude. Perhaps another episode of baroclinic development across the Rockies and Plains may occur toward the end of this period. I do not think the week 1 storm system will be the "last western/central USA" trough. In fact, the stronger and slower moving troughs may not be until the last half of December, particularly if a transition from GSDM Stage 1-2 occurs.

Week 3 (8 -14 December 2006): See week 2. We may have a period during weeks 4-6 of extremely cold air covering particularly the central USA should a mature GSDM Stage 2 evolve. That may allow a snow pack to build across locations such as the Upper Mississippi Valley and Great Lakes. Given the magnitude of our warm ENSO (and other factors) I would be surprised not to see an anomalously strong combined jet ~30-35N extend from East Asia into the western USA (with split flow across North America) by ~ the middle of January 2007, which would be GSDM Stage 3. This would significantly increased the probability of high impact weather (heavy precipitation, high winds, etc.) for the USA west coast perhaps affecting California the most (other regions for hazards and weather understood).


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

From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM. Our latest weather-climate discussion dated August 18th, 2006 (and updated September 9th), has been posted on the ESRL/PSD MJO web site at

I will try to do another posting before my trip to the STISS (THORPEX) in Germany.

Ed Berry

1 comment:

Bill Ludwig said...

Ed, thanks for your insight. We have been searching for an explanation for the unusually prolonged kamchatka/bering sea blocking ridge that 'plagued' Alaska for most of November (Anchorage will likely finish in the bottom 5th percentile for November precip). We figured it was due at least in part to persistent tropical convection, but nobody else seemed ready to state it as such, and location was another problem. Using your GSDM paper, we roughly tagged a "stage 4" on it, but not a complete match, due to what for the first half of the month was a fairly weak MJO signal. Anyway, thank you again - we will be checking your blog regularily, so don't forget Alaska!