Friday, March 09, 2007

Details are Important

Not much change has occurred with the tropical sea surface temperatures (SSTs) since my posting on Tuesday (3/6). While equatorial Pacific SSTs remain cool east of 150W (~minus 1-2C), warm anomalies are still present across the west central into the South Pacific. The latter are roughly 0.5-1C extending to depths of about 100m with actual SSTs varying from 29-31C. Only weak anomalies exist to the west.

There are at least 2 important regions of tropical convective forcing. One is nearly stationary centered around 10S/120E, and may be part of a slower process linked to a transition from El-Nino to La-Nina. The other is the dynamical signal with the MJO ~15-20S/160W. About 10 days ago these regions consolidated, and there were at least 2-3 tropical cyclones in the area of Australia left in the wake.

An important point is that a MJO signal remains. In fact, this signal can be traced back to late January 2007 in the near equatorial velocity potential Hovmollers courtesy of CPC. The signal is weakening especially in the convective field while it continues to evolve as expected in the upper level wind field (see below). The convective field now has multiple regions of tropical forcing with well defined anomalous twin upper tropospheric subtropical anticyclones near 120E and also near 160W (but mainly south of the equator).

At this time upper tropospheric zonal mean anomalous easterly flow continues to propagate poleward through the northern subtropical atmosphere, with magnitudes of ~10m/s around 25N (as a response to the MJO). This has contributed to a -2 sigma global relative angular momentum anomaly. Locally, there are weak upper tropospheric equatorial westerly wind anomalies starting to interact with the subtropics.

The MJO dynamical signal is moving east at about 10 deg long/day, and it may re-emerge into the Eastern Hemisphere by late week 2 or week 3. During this time zonal mean westerly flow should increase throughout the northern subtropics possibly allowing the East Asian Jet to undercut the recent blocking across the North Pacific. Convective flare-ups from the west central-South Pacific are also probable while a stationary convective signal may remain near 120E. Forecast uncertainty remains tremendously high after week 1 but I think we will see persistence of a GSDM Stage 1 anomaly pattern.

I think the models have a reasonable handle on week1 across the lower 48 states with ridge amplification just off the west coast and a central and eventually eastern states trough. This will keep the western states relatively warm and dry with cooler and wetter weather for the central and east USA. However, while I did underestimate in recent posts the impacts from the subtropical easterlies discussed above, I do think there will be more strong troughs to impact the western USA by late week 2 into week 3. The latter is most probable during GSDM Stage 1, and also during seasonal transition into spring. In fact, should the tropical convective forcing become quite intense across the Eastern Hemisphere during week 3 (leading to MJO #3 since December 2006?) with central Pacific flare-ups, an active regime for the USA may persist well into April.

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

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