Wednesday, April 19, 2006

Tropical Convection Regrouping

In terms of the SSTs, La-Nina continues to fade. In the equatorial regions from at least the west central Pacific to the South American coast anomalies are less than 1c with actual temperatures from ~27C near 120W to 29C and higher from the date line on westward into the IO. At depth near equatorial SSTs are tilting to slightly above normal varying from ~plus 1c near 115W/50m to ~ plus 2C around 160E/150m. Very warm SSTs continue throughout much of the subtropical South Pacific with anomalies ~plus 1-2C and actual temperatures at or above 29C (ideal threshold for supporting persistent deep moist convection, for our purposes). Please see links below for details of global SSTs.

I gave some links in my April 15th posting for information on where ENSO may be going. Tools such as IRI and POAMA suggest a warm event by the end of this year. However, to me there is much uncertainty.The atmospheric circulation still continues to behave in a La-Nina like base state, with, for example, relative AAM more than 3 standard deviations below the 1979-1998 climatology (see link -- reanalysis plots not current). This is about what was observed in mid January and mid March, and possibly the lowest value observed for at least the past year (even with the stratospheric QBO apparently currently switching into the westerly phase). As stated previously, contributing to the vertically and globally averaged reduced atmospheric westerly flow is deep zonal mean anomalous easterly flow throughout the tropical and subtropical atmospheres. There is zonal mean anomalous westerly flow from ~45-60N, which I can attribute to an eastward shift of the tropical convective forcing from Indonesia to the west Pacific during late March and early this month.

Whether or not this is telling us there will be a "lag time" between La-Nina going away and a response in the atmospheric circulation (for that time-scale) is unclear. The eastward shift of the tropical convection in late March did contribute to a fairly coherent propagating upper tropospheric divergence signal (as seen from plots of 200mb velocity potential -- link below) into the WH. At this time that signal is re-emerging into the EH, and as seen from full-disk satellite imagery, intense tropical convective flare-ups are in progress across both the IO and north of Australia (more said below). The point is that this recent dip in the AAM and expected responses of other related indicies (including measures of subseasonal variability -- see SOI link below) would be expected given the tropical forcing. This forcing does interact with La-Nina, and in fact, may be currently contributing to its rapid demise working in concert with the seasonal cycle (recall the east Pacific equatorial westerlies discussed in previous posting).

As said so many times previously, the current MJO signal is weak. However, retrospectively from reviewing plots such as the time-filtered Hovmollers to isolate coherent modes of tropical convective variability, one could argue there may have been moderate MJO signals back in January and again during March (see link below). These were part of the 30-day variabilities discussed during past postings. Perhaps their time-scales were determined by the SST distributions and other behaviors that we have so much to learn about (ex., non-linear feedbacks from the extratropics).

Satellite imagery (and information which can be derived from it such as tropical rainfall) has been showing observational signals for the tropical convective forcing to consolidate around 0/120E. Flare-ups across the IO and especially north of Australia (latter involving Tropical Cyclone Monica; see continue.In general, this region of tropical forcing extends from the central IO eastward to just east of Australia, centered on the equator.Upper tropospheric divergence has been increasing from this region, and is currently interacting with a South Asian wave train and subsequent Rossby wave energy dispersion (RWD) across the North Pacific. This RWD is leading to a large upper tropospheric anomalous anticyclonic circulation across the central Pacific which is linked to a progressive but deepening trough off the USA west coast. The combination of the diverent outflow from the convection and a northeast-southwest tilted trough is contributing to an accelerating STJ from west of the date line aimed toward the southwest USA. The bottom line is globally the circulation appears to be starting a process of transitioning from SDM Stage 4 to 1 (low to high zonal wave number transitions across the extratropics), and the sequence of events just described for the PNA sector is consistent with that notion.

We have observed numerical models struggle performance wise with synoptic events during these types of transitions. However, today I saw the deterministic output from the ECMWF, GFS and Canadian models give signals of linkages of the northern and southern branches of the westerlies by early next week. The result would be for baroclinic cyclone development on the Plains involving the above mentioned moist STJ starting on about next Monday. While the details still need to be resolved, I think that solution is probable. Impacts would include not only severe local storms across particularly the Central and Southern Plains, but also much needed rainfall for drought stricken parts of the country (including the eastern part of the country from the downstream system).

In general, increasingly active weather for much of the country seems probable for week 1. Later this week through this upcoming weekend, disturbances moving along the southern branch of the westerlies (which will include the STJ) should give some welcome rainfall for Southern Rockies into the Southern Plains. Once storm development occurs on the Plains early next week, in addition to the possibility of severe local storms, heavy rainfall may also be a concern for the Mid/Upper Mississippi and Ohio Valleys. Colder air following in the wake of this system is likely to bring below normal temperatures including overnight freezes to at least much of the northern and central part of the country.

For week 2 and going into the first week in May, I would expect the split flow situation across North America to consolidate since the SDM Stage 1 circulation should mature. That would suggest a return to more amplified ridges across the central and east Pacific with energetic troughs entering western North America while ridging returns to the southeast USA. However, given the recent sudden changes in boundary forcing (tropical Pacific SSTs, for example) and the ongoing seasonal transition, my confidence with this notion is low. If the current EH tropical convective forcing evolves into a MJO, then a robust SDM Stage 1 response would be more probable. Impacts from this type of pattern would be similar to what was seen during much of March, but shifted a bit farther north and west. There is a climatology component to an active severe local storm situation across much of the Plains during roughly the first week in May.

For Southwest Kansas, I think we will have some opportunities for rainfall this upcoming weekend into the first part of next week (which may include severe thunderstorms). Our opportunities for rainfall for week 2 will depend on the tracks of the individual synoptic lows (critical whether we get "devil weather" and/or heavy rainfall and severe storms). I think there will continue to be an active STJ,which can be favorable for precipitation. Temperatures may cool to near normal for a week 1 average (including overnight freezing temperatures early next week), with a return to above normal readings for week 2. The latter may again include max temperatures at least well into the 90s.

I will try to post an update this weekend.
Ed Berry

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