Saturday, May 06, 2006

What Goes Around Comes Around

Global SSTs remain little changed since my posting on May 3. Across the equatorial cold tongue anomalies are ~.5C with actual SSTs varying from ~26C at 120W to 29C and warmer around the date line. In general 29C and warmer SSTs extend from the South Pacific back into the IO with pockets across the east Pacific and from the Gulf of Mexico into the tropical Atlantic Ocean. The horshoe pattern of anomalous SST warmth prevails across much of the Pacific Ocean basin, especially for the southern hemisphere. This situation projects more on a La Nina than El-Nino. The future of ENSO remains unclear, especially since we are at the so called “spring barrier of predictability”. A contribution to this so-called predictability barrier comes from the seasonal cycle. Please see the following links.

The discussion on the MEI was updated on May 5, and I would expect the CPC advisory to be updated within the next week.

The MJO signal (again) remains very weak. However, there have been five ~30-day relatively coherent variations of the tropical convective forcing since December 2005. These have included eastward shifts (with MJO components) from the IO across all of Indonesia, generally along the equator, before stalling. Fast convective signals then generally did propagate into the WH during these events (in concert with other complex variations of the tropical convective forcing), usually in the form of a convectively coupled Kelvin waves (Kws).

Event number five came out roughly 10-14 days ago, then stalled ~5N/150E (it may be starting to move west), while Kws (VERY loosely speaking) propagated into the WH. These Kws have excited tropical convection along both the east Pacific (with the warmer SSTs) and Atlantic ITCZs, the SPCZ, northern South America and central Africa. Some tools to monitor these tropical convective variations can be found from the following.

The combination of the eastward shift of the EH tropical forcing and the WH signal has added a lot of westerly flow to the tropical and subtropical atmospheres during the last 2 weeks. For instance, upper tropospheric zonal mean westerly anomalies are currently ~5-10 m/s from roughly plus/minus 30 degrees of latitude of the equator.

On roughly April 24th the tendency of global relative AAM was a positive maximum of about 20 Hadleys, which signaled a transition of the global circulation to GSDM Stage 2. An observation I should have taken greater notice to is the relatively strong positive pressure torques due the Andes Mountains which may be linked to above Kws, only helping to add more westerly flow to the atmosphere. One positive Andes mountain torque occurred around April 19th and with another one currently in progress (seasonal cycle favors ~20-30 day regularity to these variations). Thus we are again seeing another positive maximum to the AAM tendency which may lead to a longer period of GSDM Stage 2-3 than I thought last week. The following are links to a few reanalysis AAM plots.

Cutting to the quick of all this, I do think the WH convective signal will come back around into the EH by the end of week 2 (see May 3rd posting). By that time the Andes mountain torque event should have gone though its life cycle. In addition, we cannot ever forget the East Asian topography. There already are lowering mean sea level pressures across East Asia, which may be helping to offset some of the anomalous zonal mean westerly flow present. Convection is also starting to increase across the IO, and the area from the IO into Indonesia may see intense tropical rainfall by the end of week 2.

The global circulation should mature to GSDM Stage 3 during week 1 (through next weekend), which will include retrogression of the anticyclonic circulation wind anomalies currently across northern Europe/Scandinavia. Given there are still feedbacks present from the lingering La-Nina base state, my thoughts would be for a transition to GSDM Stage 4 by the end of week 2. A signal of that would be a negative minimum of global relative AAM. During weeks 3-4 a transition to GSDM Stage 1 would be probable. After my poor assessment 4 days ago, my confidence in this evolution is low, mainly after week 1.

I think the models (please see past postings for links) look reasonable through at least the middle of next week for the CONUS. Typical of GSDM Stage 2, a trough will dig into the Rockies and Plains (with ridges off the west coast and across the Gulf of Mexico), while blocking retrogrades into Canada. The latter will enhance the split flow across North America, leading to a southward displaced storm track across the Rockies and Plains.

By next weekend into the following week, this pattern should de-amplify with the westerlies shifting north. In the presence of a STJ across the southern part of the country, by the end of week 2 into week 3, troughs may start digging along the west coast with ridging across the south central into eastern states. The blocking across Canada may shift into Alaska.

A cool and wet pattern looks like a good bet for much of the USA well into next week. Most robust cool anomalies are likely across the North and Central Plains into the Great Lakes States. Late season frosts and/or freezes may occur. Heaviest precipitation should focus from the Southern Plains into the Ohio Valley, including severe local storms. During week 2, much of the country will likely warm and dry out, perhaps very quickly. Cool and wet weather may shift back to the Pacific Northwest and perhaps linger across the Northeast. Above average heat may return to the Southern Plains by the end of this period. During week 3, perhaps finally that GSDM Stage 1 pattern for the CONUS will return. After all, I should get it right sometime!

For southwest Kansas, with our GSDM Stage 2-3 split flow pattern (with STJs), I am going to be cautiously optimistic about our rain chances well into next week. All things considered, a situation like this in July could lead to night after night of intense MCSs. Our best opportunity for rainfall should be ~Tuesday-Thursday. Temperatures look to stay below normal for about another week. Indeed, late next week at least frost could be a concern. Starting next weekend into the following week, I think temperatures will warm very quickly and rain chances will decrease. However, the STJ may still help with the rainfall opportunities.

I will try to do an update the middle of next week.
Ed Berry

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