Wednesday, April 26, 2006

What a Mess!

SSTAs across the equatorial central and east Pacific are close to normal with values varying from ~minus .5C around 120W to ~plus .5-1C across the southwest region of the equatorial date line. Somewhat cooler values were just off the coast of South America while plus 1-2C anomalies persisted across the subtropical South Pacific. Warm anomalies have recently emerged across the central Pacific near 30N. While La-Nina in regard to SSTAs across the equatorial cold tongue have “disappeared”, the general spatial distribution of warmth throughout the north and particularly the south subtropics of the Pacific are still consistent with a cold event. The latter is an important point I should have made clear in previous postings. The ramifications to the future of ENSO are unclear to me. Actual SSTs of 29C and greater continue from the IO into the South Pacific, as well as portions of the tropical Atlantic basin. The following are links to SST information and on the status of ENSO. I would expect the ENSO discussions to be updated within the next 2 weeks or so.

The EH tropical convective forcing continues to consolidate and maintain intensity with a centroid around 0/120E. The envelope of this enhanced tropical precipitation extended from the central IO across Indonesia into the extreme western Pacific. Two active regions within this envelope were northeast of Australia and the Bay of Bengal. At this time Tropical Cyclone (tropical storm intensity) Mala was occurring across the southern Bay of Bengal while a disturbed area of weather persisted northwest of Australia. Weekly mean OLRAs were on the order of ~minus 50-90 W/m**2 throughout this convective envelope. Perhaps linked to a weak convectively coupled Kelvin wave, at least an enhanced diurnal cycle of intense thunderstorm clusters has been occurring through most of northern South America as well as central Africa during the last few days (see links below).

My thought for about the last week has been for the EH tropical forcing to consolidate and then move east possibly projecting onto a MJO. However, Hovmoller plots of fields such as tropical surface wind anomalies (within 15 degrees of the equator) present a pattern of enhanced trades to the east of 120E and a WWE across the central IO that has persisted for at least the past 10 days. That same persistence was observed from Hovmoller plots of OLR/OLRA, including the time-filtered coherent modes (see links above). At this point I think it is a monitoring issue to see if the atmosphere wants to stay in a general persistent “La-Nina like regime” or shift to “something else”.

I think SDM Stage 1 currently best describes the global circulation; however, any objective signal of that pattern is not clean. Subtropical wave trains and STJs still continue across both hemispheres (a characteristic of Stage 4). Perhaps the most distinctive change in the global circulation during the past 7-10 days has been the increase of zonal mean upper tropospheric westerly flow throughout the tropical and subtropical atmospheres, with anomalies ~5 m/s. Accompanying this change has been an eastward shift of the upper tropospheric subtropical anticyclones to ~120E (where the convection is centered at) and development of downstream cyclones just east of the date line. This observation suggests to me the circulation has loosely transitioned back to SDM Stage 1, from SDM Stage 4 of about week ago (see AAM tendency link below – first one).

Anomalous zonal mean westerly flow also continues ~45-60N and S, with magnitudes ~10-15m/s. Elsewhere, zonal mean easterly anomalous flow dominates. Global relative AAM is still close to 3 standard deviations below the 1979-1998 climatology, but having gone up a bit during the last week or so. The point is that once again there is increasing meridional symmetry of zonal mean zonal wind anomalies which is similar to what was seen from ~mid February to early this month. That is a classic footprint onto the atmosphere of tropical convective forcing becoming dominate (keeping in mind the importance of the seasonal cycle).

So, the atmosphere may like the La-Nina base state that it has occupied since last fall. During my past writings I have at least implied that this cold event may be over, not only for the SSTs, but also for the atmospheric response as well (understanding lag relationships, the seasonal cycle (again), etc.). Have I been premature?

Of course there are other contributions to the characteristics of the circulation such as major north-south mountain barriers. As stated before, anomalous high pressure across the east Asian topography has recently exerted a positive torque onto the atmosphere, likely contributing to the current split flow pattern across North America.

Focusing on the PNA sector, strong anomalous westerly flow continues throughout the subtropical north Pacific contributing to the STJ now impacting the southwestern part of the country. Meanwhile, directly linked to the EH tropical convective forcing interacting with baroclinic wave packets crossing both north and south Asia (interacting with the topography), a complex Rossby wave energy dispersion (RWD) is currently impacting the CONUS, including the AWB trough just off the CA coast. The interplay of this RWD and the STJ has created serious sensitivity issues with many of the operational numerical models resulting in greater than climatology ensemble spread as early as day 3 (this Saturday). For the USA the issue is whether or not there will be a phasing of the northern and southern branches of the westerlies to lead to storm development on the Plains this weekend. Short answer is I think so, since this is what would be expected during a SDM Stage 1 situation. In addition, this evolution is the start of a process that the atmosphere dynamically has to do to begin the resolve the split flow across North America. Weather impacts would include severe local storms across the Southern Plains and Deep South with heavy rainfall over much of the central states into at least Ohio and Tennessee Valleys as the system moves east.

After the storm event this upcoming weekend, I think that as we go into week 2, it is probable that more troughs will start digging into the western part of the country particularly starting by next weekend (~May 6-7; again, timing is white noise). The SDM Stage 1 circulation should mature, especially once of the lobe of the polar vortex becomes displaced to the north of the 120E tropical convection. The latter will likely be observed as a northern high-latitude retrogression of circulation anomalies. Bottom line is the CONUS may have similar weather during this time as was observed most of March into early this month, except farther northwest. Given the warm SSTs of the South Pacific, a robust STJ may accompany this pattern. Unless the EH tropical forcing shifts east as a MJO (for example), the pattern of synoptic-scale western CONUS troughs lifting northeast into the Plains/Upper Mississippi Valley with a southeast states ridge may persist at least into week 3. I think the readers are quite familiar with the weather impacts of this pattern by now.

As an aside, below are some links to measures of numerical model performance, as well as some of the models (any search will find more). Notice the recent drop in skill about 10 days ago, with the longer-term general decline attributable to seasonal transition into spring. I suspect model performance will again decrease during the next few days.

For southwest Kansas, perhaps I should have kept some of my optimism I had a week ago for rainfall this coming Friday and/or Saturday. A lot will depend on where “everything comes together”. There may be a sharp gradient from northwest to southeast of very little to “a lot” of rain (and possible severe storms). In any case, still a great deal of uncertainty. There may be another opportunity for rain/storms about the middle of next week with a Pacific cold front. Starting around next weekend into the following week, very warm and dry weather would be most probable. However, a strong STJ along with less southwesterly momentum to mix down (climatology) may mean better opportunities for at least dry line storms.

I will try to do another update this weekend.
Ed Berry

No comments: