Sunday, April 23, 2006

SDM Stage 4 to 1 Transition -- Update

Please refer to previous postings for web links. I do want to keep this writing relatively short.

In terms of SSTs, La-Nina is ill-defined. Near equatorial central and east Pacific SSTAs are within .5C of normal. At depth anomalies as high as plus 1C exist down to 200m in the region of the date line. Actual tropical SSTs range from ~26C just west of South America (where weakly cool anomalies still exist) to 29C and warmer from just west of the date line back into the IO. Of interest are the very warm SSTs across the tropical and subtropical South Pacific with SSTAs at least plus 1.5C and actual SSTs at least 29C. This region has been supporting intense tropical convective flare-ups over the past several weeks.

The main news about the tropical convective forcing is the maturing consolidation centered around 0/120E, with enhanced tropical rainfall extending from the southeast portions of the Bay of Bengal east-southeast to all of Indonesia and northern Australia. The portion that propagated west during the last week or so contributed (which weakly projected onto a convectively coupled Rossby mode) to the formation of currently Severe Category 5 Tropical Cyclone Monica which is battering northern Australia as I type (see for details).

A WWE across the IO did accompany this consolidation. This and other tools tell me it is probable the tropical forcing should shift east at least into the western Pacific during the next few weeks. This may be our 4th event of ~30-day variations of tropical forcing we have been observing since December 2005. As discussed in my last posting, there have been MJO components to these variations. The impacts of the seasonal cycle on this behavior including circulation impacts adds uncertainty to any kind of subseasonal prediction.

AAM remains ~3 standard deviations below the 1979-1998 climatology. However, during the last few days there has been some increase, with much of it coming from the northern extratropics between 45-60N (which has shifted north during the past couple weeks). Zonal mean westerly flow has also been slowly increasing across the northern subtropics. For me to explain possible sources dynamically for the recent anomalous AAM variations we have been seeing is well beyond what I can write here. Suffice to say the recent increases in the anomalous zonal mean westerly flow across the northern higher midlatitudes may have had some contribution from the east Asian topography. The impact for North America has been the anomalous split flow pattern during roughly the past week.

I think SDM Stage 4 still best describes the global circulation. STJs are present, including the one impacting the southwestern USA. In concert with the positive east Asian mountain torque, the atmosphere transitioned to Stage 4 about 10 days ago. Week-2 numerical model performance was impacted, including the GFS ensemble (see link below). During the next couple of weeks, I think the circulation will transition to a mature SDM Stage 1, taking into account the seasonal cycle. SDM Stage 1 may persist into week 3.

In addition to the possible eastward shift of the tropical forcing, I think the lobe of the polar vortex (in terms of cyclonic circulation wind anomalies) will become displaced toward east Asia (instead of centered on North America at this time), directly north of the tropical convection. That will contribute to a lowering of surface pressures (and lead to negative mean sea-level pressure anomalies) across the east Asian topography (in a probable sense). Punch line is we should see the anomalous zonal mean westerlies come back south and contribute to more full latitude energetic troughs slamming into the western part of the country by the end of week 2 (weekend of May 6-7). Of course, timing is nothing short of very difficult, especially this time of year and in a complicated transitional circulation regime such as this. The only thing anyone can do with defensible skill (and accountability) is offer probabilistic statements rooted in solid science for predictions after about 3-5 days.

For the CONUS, week 1 looks generally anomalously cool and wet for the southern Rockies into the Deep South and eastern portions of the country while the Great Lakes into the Northern and Central Plains and Northern Rockies looks quite dry. Warmest anomalies look to be centered across the Northern Rockies. The Pacific Northwest may trend to cooler and wetter by about a week from now. This kind of temperature and precipitation response would be expected with split flow, going along with a moist STJ. Severe local storm activity should be reduced (considering climatology).

During week 2, the weather looks to get active for the western and eventually the central part of the country. This should be similar to what was observed during March into early April, only a bit farther northwest. Severe local storm activity across the Plains may become very robust by the end of this period (understanding the impossible predictability of details like quality of tropical moisture transport, instability, etc.) while generally cool and unsettled weather returns to much of the western states. The south central and much of the eastern USA may return to above average temperatures. As stated before, there is a climatology component to an active southwest flow storm track across the central CONUS during the first part of May. Perhaps this period will be enhanced, and may persist into week 3.

For southwest Kansas, I hope we get some measurable rainfall today or Monday (4/23,24). I am afraid I have (again) been getting too optimistic about our precipitation chances recently. I am concerned that the next STJ, etc., closed low for later this upcoming week may stay too far south to give us any rainfall as the northern branch starts to come south. That situation will have to be watched.

During week 2, once troughs start digging into the western part of the country and then lift northeast toward Iowa and the Northern Plains/Upper Mississippi Valley, the probability of "devil days" (anomalously warm and dry days with "strong" southwest winds/blowing dust) will go up. However, the seasonal cycle should mitigate some of that meaning at least some possibility of dryline type storms. Also, given the warm SSTs across the tropical South Pacific (discussed above), we may also get some opportunities from a moist STJ ahead of these western states troughs. Understanding variations of temperature (perhaps freezing temperatures Tuesday and Wednesday this week), the above normal tercile looks like a good bet for a mean temperature the next couple of weeks.

I will try to post another update about the middle of next week.
Ed Berry

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