Wednesday, May 03, 2006

Where Will the Westerly Anomalies Go?

Tropical Pacific SSTs have changed little since my last posting on April 29th. Anomalies along the equatorial cold tongue are generally within .5C of normal with weak cool conditions across the east Pacific and warmth across the date line region. Actual temperatures vary from ~26C near 120W to at least 29C southwest of the date line. The 29C and warmer SSTs (useful threshold for maintaining deep moist convection) generally extend from the IO into the South Pacific and have also recently appeared just south of Mexico (along the ITCZ). At depth there is some warmth of ~plus 0.5-1.5C down to about 200m around the date line, which suggests a slightly deeper than normal ocean thermocline. The horseshoe pattern of anomalous warmth still prevails across the subtropics and lower midlatitudes of the Pacific Ocean, as well as across much of the northern tropical Atlantic. Details can be found from the following links.

The ENSO situation remains unclear. However, there are monitoring tools which suggest the effects of La-Nina are still lingering. The following discussions should be updated within the next week or so to provide additional information.

Any MJO signal is still very weak. During roughly the last 10 days an eastward shift of the tropical convective forcing occurred (see April 29th posting), linked to a convectively coupled Kelvin wave (Kw). At this time the centroid of the tropical forcing is at about 5N/150E, and extended from central Indonesia into the west Pacific. Monitoring suggests this enhanced region of tropical rainfall has stalled. Additionally, enhanced rainfall was increasing across both northern South America and central Africa, with strong flare-ups across both the South Pacific (interacting with fronts) and even the eastern Pacific ITCZ.

The point is while the most significant tropical forcing still remains in the EH, there is a signal currently propagating through the WH tied to the Kw. My feeling is that the strongest forcing will remain across the EH, while the propagating signal comes back around into the regions of Africa and the IO during the next 2 weeks. I would expect to see perhaps a rapid enhancement of the IO convection within the next week or so, with possibly the Indonesian rainfall shifting back to the west. Similar to at least 5 previous events going back to December 2005, another consolidation of the tropical convection may occur late week 2 or 3. Please see the following links for details (as well as additional navigation to relevant links).

GSDM Stage 3 best describes the circulation response to the tropical convective forcing (in addition to other processes). Global relative AAM is still about a half a standard deviation below the 1968-1997 reanalysis data climatology. However, this is an increase from about minus 2 standard deviations from 2 weeks ago (see links below), and I think we are seeing GSDM Stage 3 in the presence of a weak La-Nina base state. Importantly, deep zonal mean anomalous westerly flow of at least 5m/s extends throughout the equatorial and subtropical atmospheres, and downward and poleward movement of these anomalies would be probable. An important question for prediction is just how much are these enhanced zonal mean westerlies going to impact character of the global circulation? Related questions also concern the impact of the both the tropical convective as well as topographic forcing interacting with RWDs within the enhanced westerly flow. These RWDs will be crucial for the synoptic evolution of midlatitude troughs and ridges.

My own feeling is to continue the thoughts offered on the April 29th writing. The enhanced mid/upper tropospheric westerlies are coming back around through north Africa and are headed toward subtropical Asia. Interacting with the Indonesian tropical convection, an extension of the polar jetstream (East Asian Jet (EAJ)) across the North Pacific ~40-50N is probable. This will be displaced a bit north of what is expected during these kind of situations due to the weak La-Nina base state. Meanwhile, a blocking anticyclone located in the Eurasian sector is probable to retrograde into Canada by the end of week 2. There is a tendency to see retrogression of existing high latitude anticyclonic circulation anomalies from Eurasia into North America during GSDM Stage 3 situations (complicated feedback issues). Starting sometime next week a well defined split flow pattern across North America looks plausible.

Focusing on the CONUS, where will be the most probable location of cyclonic circulation anomalies as the blocking anticyclone retrogrades through Canada? There have been differences in the numerical models (including large ensemble spreads) on resolving this matter. With the tropical forcing remaining well in the EH, I would expect to see closed lows developing across the Rockies and then lifting northeast into the Upper Mississippi Valley. More generally, much of the country may have a southward displaced storm track (regionally due to the Canada blocking; not to be confused with a northward displaced storm track in the zonal mean). With this pattern locations from the Pacific Northwest into the Rockies and Plains may experience the most active weather, including severe storms across the central part of the country. In terms of above average rainfall locations from the Upper Mississippi Valley all the way to possibly even the Mid-Atlantic States may be in line. Portions of the Southern High Plains look to remain dryer than normal.

I think this “GSDM La-Nina springtime version of Stage 3” will mature during week 2, possibly quickly transitioning to GSDM Stage 1 by the end of week 3 (~May 24th). At least for weeks 1 and 2, I do like the solutions offered by the CDC ensemble and ECMWF deterministic run (see links). However, all numerical models are generally starting to converge.

For Southwest Kansas, I wish I could offer more optimism for the decent periods of widespread rainfall that most would like to see. I hope that respectable rainfall occurs between now and this weekend. Opportunities for some showers and storms (perhaps linked to the dryline) should exist during the next couple of weeks, particularly with enhanced episodes of the STJ linked to flare-ups across both the South Pacific and eastern Pacific ITCZ. However, above average westerly flow coming across the Rockies starting next week is not at all favorable for repeated widespread rainfall events across southwest Kansas. In addition, should we start to see individual synoptic lows lift northeast toward locations such as Iowa from the Rockies, a couple of days of “hot southwest winds and some blowing dust” is not out of the question. If I were doing a week 2 prediction, it would read “above normal temperatures and below normal precipitation”. Should the circulation transition back to GSDM Stage 1 after this period, that may shift the storm track farther north (which may also not be good for decent rainfall).

For those interested on learning more about our GSDM, the Weickmann and Berry (2006) paper, “A synoptic-dynamic model of subseasonal atmospheric variability”, has just been formally accepted for publication into Monthly Weather Review (MWR). I will try to provide a link to a soft copy of our paper in a future posting on this web site.

I will try to do an update this weekend. We are working on another weather-climate discussion for the PSD MJO web page, which we hope to post within the next week or so ( ).
Ed Berry

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