Low AAM

Tropical Pacific SSTs are relatively ill defined as compared to the past several months of La Nina. Along the equatorial cold tongue anomalies are less than .5C (actual SSTs at least 27-29C east of date line), and at depth anomalies are less than 1C. Interestingly, during the past week or so, linked to dynamic interactions with extratropical wave trains of both hemispheres that were tied to the eastward shift of the Indonesian tropical convection, anomalous surface meridional flow toward the equatorial east Pacific occurred. The anomalous convergence near the equator lead to westerly wind anomalies of at least ~5 m/s, which may have initiated a weak downwelling oceanic Kelvin wave (at least "of sorts") starting near 160W-170W. This process may have been responsible for the sudden weakening of the cool SST anomalies there (considering the seasonal cycle). Links below will lead to additional global SST information as well as animations.

http://www.pmel.noaa.gov/tao/jsdisplay/

http://www.cdc.noaa.gov/map/images/sst/sst.anom.gif

http://www.cdc.noaa.gov/map/clim/glbcir.anim.shtml

The point is there have been significant changes to the SST boundary forcing during the past 2-4 weeks. This has lead to a breakdown of the coupled atmospheric response involving relatively persistent EH tropical convective forcing and SDM Stage 1 circulation (and a rapid loss of numerical model skill recently), which has been observed from about mid February to early this month. The latter has been responsible for several mid-latitude troughs deepening initially across the east Pacific (as well as so-called Kona lows), then slamming the CONUS leading to a very active weather pattern (impacts have already been discussed). In general, the winter of 2005-2006 has been extremely interesting presenting many challenges to the SDM particularly with non-linear feedback issues. Our weather-climate discussion dated February 15th gave some attention to these matters (see link below).

http://www.cdc.noaa.gov/people/klaus.weickmann/disc021506/weather_climate_discussion_10Feb06.html

Perhaps all of the above, working with the seasonal cycle, is one mode of how La-Nina dissipates. Whether this is the case or not, is unclear. The following links give some attention to the state of ENSO.

http://www.cdc.noaa.gov/people/klaus.wolter/MEI/

http://iri.columbia.edu/climate/ENSO/currentinfo/SST_table.html

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.html

http://www.bom.gov.au/bmrc/ocean/JAFOOS/POAMA/exproducts/poama_v10/fc_rt_nino_l30.htm

We will see what happens.

At this time, from a weather-climate monitoring viewpoint, most signals are very weak, including the MJO. There is some evidence of a coherent eastward moving tropical convective signal across the WH, per link below, about to re-emerge into the EH.

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ir_anim_monthly.shtml

I do think this notion is reasonable. Full-disk satellite imagery offers support, including some evidence of tropical convection starting to slowly intensify from the IO-Indonesia.

http://www.ssec.wisc.edu/data/geo/mtsat/

Finally, there is a robust relative AAM signal, with current values nearly 3 standard deviations below the 1979-1998 climatology (link below). This value is getting to levels we saw back in both mid January and mid March. Much of this low AAM is coming from increasing deep zonal mean anomalous easterly flow throughout the subtropical and tropical atmospheres. I suspect the tendency of AAM is also negative (reanalysis plots contain data only through April 9th at the time of this writing at
http://www.cdc.noaa.gov/map/clim/aam.rean.shtml ).

http://www.cdc.noaa.gov/map/images/aam/glaam.gif

The punch line is that I do think SDM Stage 4 best describes, loosely, our current weather-climate situation. Complications include what may a positive east Asian mountain torque leading a northward displaced extension of the EA/North Pacific jet toward North America as I type (which will contribute to the next western states trough this upcoming week). I also suspect tropical convective forcing will robustly re-emerge into the EH in the region of the IO-Indonesia during the next 2 weeks, and that the circulation will revert back to SDM Stage 1 by early May. In the meantime, SDM Stage 4 favors STJs (and at least weak southern branch storm tracks) globally, including the southwestern part of the USA.

For the USA and North America, I think we will see a lot of split flow for the next 1-2 weeks. Most operational NWP and their ensembles have been picking up on this for the past few days. For instance, there has been a trend to take the next western USA trough/closed low evolution around Tuesday-Wednesday next week farther south. By week 2 (~April 23-29), I think this split flow will mature with lots of high pressure across Canada and low pressure across the southwest and south central part of the CONUS (vertical structures and dynamics understood). Later during week 2 and week 3, I suspect more ridging may return to the central and east Pacific leading to more energetic western USA troughs (there is a climatology component to this, which may be enhanced in this case).

After relatively active weather for parts of week 1, there should be some respite at least from severe local storms for portions of the northern USA week 2. Meanwhile, many of the dry regions of the Southwest and Southern Plains may get some needed rainfall week 2, which may also spill into the mid/upper Mississippi Valley, for example. Most of the country will probably see warmer than normal temperatures both weeks, although perhaps closer to normal week 2. Locations such as the Northeast and southwest deserts may see below normal temperatures week 2. The first week or so in May may be a delight for storm chasers on the Plains/Upper Mississippi Valley.

For southwest Kansas, there may some chance for a showers and thunderstorms around late Tuesday or early Wednesday. Starting around next weekend through week 2, even after yet another day of "devil weather (southwest winds and blowing dust, very warm temperatures, extreme fire danger, etc.)" I like our rain chances. I am probably being too optimistic, but that is fine. In fact, rainfall may even average above normal for week 2. I hope that it does, because some of that "devil weather" may return starting in May. Even with shots of cool air, temperatures should tilt toward the above normal tercile for the next 2 weeks.

I will try to post an update the middle of next week. Also, we do need to start work on a weather-climate discussion for the ESRL/PSD MJO site, which will take time.

Ed Berry