Recap on the Complexity

Tropical west Pacific SSTs still remain above average in a horseshoe pattern centered on 155E with anomalies ~0.5-1.5C and actual temperatures from 29-31C. There has also been some warming across portions of the equatorial Indian Ocean with positive anomalies ~0.5-1C and SSTs near 30C. Below normal SSTs remain from the north coast of Australia to the equator, with anomalies ~1.0C. With the exception of the equatorial Pacific ~120W, eastern hemisphere tropical SSTs are weakly positive. While the negative anomalies along the central equatorial Pacific cold tongue are currently minimal, colder subsurface anomalies remain down to roughly 200m. The latter are important for possible La-Nina development.

Largely forced by the warm SSTs, favorable upper tropospheric winds and Rossby wave energy propagation from the extratropics, tropical convective forcing remains loosely organized centered ~0/140E. This area extends from the central Indian Ocean into the South Pacific. However, the convection in this region is not a large departure from climatology. There is still evidence from various fields such as animations of daily mean 150mb vector wind anomalies and velocity potential Hovmollers that a weak dynamical signal is moving through the Western Hemisphere. I think it is centered at roughly 0/60W and moving east at about 10 deg long/day. We expect this signal to come back into the Eastern Hemisphere sometime during week 2, and allow the tropical convective forcing to increase from Africa to the warming SSTs of the Indian Ocean. We may see the third MJO since late last fall evolve in that region by late week 2 or 3.

GSDM Stage 1 best describes the current weather-climate situation. Relative atmospheric angular momentum (AAM) remains well below the 1968-1997 reanalysis data climatology at ~1.5 standard deviations. The global tropical and subtropical atmospheres are dominated by zonal mean easterly wind anomalies with magnitudes of 5-10m/s. The fast dynamical signal discussed above should reinforce what is already a low AAM regime during the week 2-3 time frame. Synoptically, the most probable response across the Pacific North American sector would be for a discontinuous retrogression of existing circulation anomalous by at least 20-30 degrees of longitude. This means the ridge currently over the western North America should redevelop ~150W.

Even with the above reasoning, forecast uncertainty for weeks 2-3 remains very high. If a coherent MJO develops across the Indian Ocean by week 3, forecast confidence in subseasonal outlooks may improve. Most models forecast a fairly strong trough to impact the west coast early next week and then shift into the Rockies and Plains afterwards. It is possible this first trough may “split” with northern and southern stream developments during the course of its evolution. In fact, recent signals of AAM transports would support this possibility. Afterwards, a more “classic” GSDM Stage 1 springtime active pattern for the western and central USA may emerge for weeks 2-3. This would argue for late season snowstorms across portions of the Rockies and northern/central High Plains with a greater than climatology risk for severe local storms across the Plains and Mississippi Valley.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I am on TDY at ESRL/PSD with the HMT project until April 3rd. I will try to post another discussion next Tuesday. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry and Klaus Weickmann