Monday, June 04, 2007

Slowly Evolving and Painfully Non-Trivial (and to write about)

Reanalysis-1 (R1) data is still slow getting to ESRL/PSD meaning the AAM plots and GWO phase space diagram do not have the most recent information.

Global tropical SSTs remain generally above normal over the Eastern Hemisphere and portions of the Atlantic Ocean with cool anomalies across the central and eastern equatorial Pacific cold tongue. Magnitudes are ~0.5-1C with up to 2C warmth across the Arabian Sea (which supported the development of Severe Hurricane Gonu (Category 5) during the last few days), the southwest Caribbean and around the African coasts, with ~2C coolness just west of South America. The highest SST totals of ~30C are in the region of Indonesia into the west central Pacific north of the equator. SST anomaly tendencies have become negative across the western Indian Ocean due to recent rainfall, and “mixed” along the equatorial Pacific cold tongue due to recent surface westerlies. Upwelling of colder subsurface waters (~minus 2-3C down to 200m around 120-150W per TAO data) will be needed if La-Nina development is to resume.

In my posting last week I stated the MJO tropical convective signal was back. Since then, a convectively coupled Kelvin wave propagated downstream and weakened while strong thunderstorm clusters developed from Africa into the western Indian Ocean. From monitoring the upper tropospheric winds via animations of 150mb and 250mb daily mean vector wind anomalies and various Hovmoller plots, I think the MJO dynamical signal has stalled in the region of Africa. The 200mb velocity potential zonal wave number 1 spatial distribution with the negative center in the region of Africa also supports this notion, as does the Wheeler phase space plot.

Zonal mean easterly wind anomalies (~5m/s at 200mb) remain throughout the deep tropical and subtropical atmospheres flanked by above average westerly flow around 25-30 degrees north and south. There is still the above average zonal mean westerly flow across the higher mid-latitudes of both hemispheres with zonal mean easterlies in between the anomalous westerlies.

Getting into some more detail, anomalously strong twin upper tropospheric subtropical anticyclones remain across the Western Hemisphere (~10-20m/s wind anomalies per recent weekly mean) centered on the Atlantic with weaker pairs developing across Africa into the western Indian Ocean (which may have provided a favorable vertical wind shear profile for the development of Gonu) and ~140E. The latter pair of anticyclones is tied to Rossby wave energy dispersions (RWDs) from the northern and southern extratropics. In fact, there is currently a well defined RWD from the northern 140E subtropical anticylone which hooks up nicely with the trough that is about to dig into the western USA. Associated with twin upper tropospheric subtropical cyclones, westerlies (~5-10m/s at 150mb) have developed across the equatorial eastern Indian and central Pacific Oceans. Overall, there is a zonal wave number 2 (possibly 3) distribution to the circulation features across the tropical and subtropical atmospheres.

Summarizing thus far, I think we have inter-hemispheric symmetry of zonal mean wind anomalies, with the greater response in the Southern Hemisphere (austral winter). Furthermore, various time-longitude sections over different latitiude bands of fields such as 250mb meridional wind anomalies suggest at least a significant slowing of the eastward progressions of synoptic wave trains during the past week. Perhaps tied to the seasonal cycle, we may be evolving into a base state dominated by stationary rather than oscillatory behaviors. In this case, our base state is best described by GSDM Stage 1 with global relative AAM at least 1 standard deviation below the 1979-1998 operational data climatology. I can speculate that what we are currently seeing may be a slow feedback process to subseasonal variations that started back in December 2006.

So where does the atmosphere go from here? The upper tropospheric equatorial westerly flow anomalies across the central Pacific may be a signal that at least some eastward movement of circulation anomalies will occur during the next few weeks. The same should also occur with the tropical convective dynamical signal. My first thought would be to see the convective signal shift well into the Indian Ocean during weeks 2-3. I also have a thought the 140E twin anticyclones may shift into the warm west Pacific Ocean while another pair lingers around 60W. Hence we may see 3 regions of enhanced tropical rainfall by the end of week 2, with the most intense from the Indian Ocean northeast into Indo-China as part of the Eastern Hemisphere monsoon systems. I also have week-2 concerns of an increasing tropical cyclone hazard not only around the Philippines, but also the western Atlantic into the Caribbean.

The North Pacific Rossby wave energy dispersion discussed above I think is yet another 10-30 day rendition of a global mountain-frictional torque index cycle (currently headed toward loosely negative-positive). This variation is contributing to another negative global AAM tendency (inferred from the operational data plot) and the GWO phase space diagram should indicate an orbit toward GSDM Stage 4 when updated. However, in comparison to what was observed during much of April and May, this orbit is likely to be small, remaining around GSDM Stage 1.

As discussed previously in the face of many week-2 ensemble means showing ridge conditions across the western states valid this week, an anomalously strong trough is currently digging into the western USA. Per above, I think this synoptic pattern (with a strong ridge across the southeast states and from the central Pacific into Alaska) will persist for at least the next couple of weeks within our GSDM Stage 1 base state. Most operational models are now supportive. At some point this circulation pattern should shift far enough northwest to “allow summertime” across the lower 48 states (my focus for these discussions).

The weather ramifications of this type of circulation pattern across the USA should be apparent by now. For example, for at least the next 2 weeks the Central and Northern Plains into the Upper Mississippi Valley and at times Ohio Valley are likely to be targeted with several rounds of severe thunderstorms. Additional hazards will be tornadoes and heavy flooding rainfall. Intense summertime heat is probable for the Deep South. Western Atlantic/Caribbean tropical cyclogenesis may become probable weeks 2-3 which could impact the USA.


An experimental phase space plot of the GSDM utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts, particularly if an opportunity arises for us to have a dedicated web page effort. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I plan to issue a discussion this Friday (6/8).

Ed Berry

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