Thursday, April 05, 2007

Rebirth of the MJO?

The most robust signal of global tropical SSTs are the cool anomalies east of about 150W along the equatorial cold tongue, with magnitudes as low as minus 3C. Negative anomalies of at least minus 4C remain down to ~200m extending to about the date line. The positive SST anomalies across the west central and South Pacific have cooled significantly during the past week due to recent intense rainfall and tropical cyclone activity. Anomaly magnitudes are only ~.5C with totals ~28-29C. Similar SSTs also exist across the equatorial Atlantic Ocean, and the equatorial Indian Ocean remains quite warm with SSTs ~29-30C. It is still unclear to me if the coupled ocean-atmosphere (and other processes) system is evolving toward La-Nina since May is a critical month.

The weather-climate situation remains very complicated meaning most (not all) signals are weak. Careful daily monitoring within the GSDM framework is a must for any week 1-4 predictions. Short course is the extended North Pacific jet of roughly 2 weeks ago contributed to an eastward shift of the most intense tropical convection from ~120E to 160E in a period of about a week. This forcing has since weakened with only “pockets” of convection hanging around central-eastern Indonesia into the central Pacific, northern South America, west central Africa and the central Indian Ocean. The latter is a component of the slow westward drift of the tropical forcing from the South Pacific which started around mid-January.

The most robust signals remain the global relative atmospheric angular momentum (AAM) and its tendency, keeping in mind the zonal mean contributions. Animations of upper tropospheric winds during the last week-10 days present some rapid but interesting behaviors. As the tropical forcing moved into the west central Pacific Rossby wave energy dispersion led to ridge amplification along the USA west coast and a deep central North American trough. Additionally, weak upper tropospheric zonal mean westerly wind anomalies (~5m/s or less at 200mb) have developed throughout the tropical and subtropical atmospheres, while exhibiting poleward propagation. The dynamics responsible for these processes have resulted in a global AAM daily mean positive tendency of ~40 Hadleys on April 2nd (reanalysis plots are 5-day averages). The magnitude of this tendency is roughly equal to the negative event observed back in mid-February.

Does this mean we have had a GSDM Stage 2 response and are headed for Stage 3, or are we seeing a perturbation in a slowly evolving GSDM Stage 1 base state possibly linked to a developing La-Nina? I continue to advocate the latter. While there is likely to be a “spike” in global relative AAM during the next few days (how much is unclear), that is all I think it will be. Several diagnostics including velocity potential and even the multivariate EOF Wheeler phase space plot tell me there is a developing dynamical signal across the Western Hemisphere. I think this signal will come back into the Eastern Hemisphere and it is probable that MJO tropical convection will appear across the warm Indian Ocean sometime week 2-3. This would argue for another negative AAM tendency and an overall persistence of a low AAM GSDM Stage 1 regime.

As most ensembles are capturing, another extended North Pacific jet episode is highly probable starting this weekend leading to strong troughs that will first slam into the USA west coast. This situation is likely to weaken coastal upwelling along the California coast, at least in the short-term. These troughs are then likely to move into the Rockies and Plains. This means the April cold regime now affecting the Upper Mississippi Valley into the Northeast should moderate early next week. Going well into next week and possibly through week 3, much of the lower 48 states may be in for an exceptionally stormy regime, centered on the Rockies and Plains. In fact, understanding the seasonal cycle, events similar to the period of the last couple of weeks of December 2006 and the last half of February 2007 may be probable.

Please note: These are probabilistic statements, which we will try to quantify in future posts. I continue to have travel. It is likely I will not be able to do another posting until around 4/11. The WB (2007) paper on the GSDM has been published in the February issue of MWR.

Ed Berry

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