Tuesday, August 15, 2006

The New World Atmosphere – Update

The following is a link to our recently accepted paper by MWR which discusses the GSDM (Weickmann and Berry 2006).


From taking into consideration the interactions of 4 different subseasonal time scales, a sequence of maps depicting a coherent set of repeatable events has been derived for the Northern Hemisphere cold season from November-March. This set is broken up into 4 stages, referred to as GSDM (for Global Synoptic-Dynamic Model) Stages 1-4 in the text of my Blog. Figure 13 in our paper presents a schematic of the GSDM. Ideally it would be advantageous to post our weather-climate discussions (link at the bottom) with greater frequency to provide additional detail while having a more complete weather-climate record of attribution and prediction. In these discussions I adapt the GSDM for the warm season. Our list of work includes a seasonally adjusted rendition of the GSDM.

Please see previous posting for links. This writing will be very short, and it is unlikely I will do one this weekend due to covering operational shifts and work to get the next weather-climate discussion out for posting on the ESRL/PSD MJO web site. I should be able to do a complete Blog discussion next Wednesday, 8/22. What I posted on August 12th is still quite valid.

Tropical Ocean SSTs remain above average across most of the Western Hemisphere and the Indian Ocean, with cooler than normal values centered on Indonesia. Actual SSTs in excess of 30C have been observed along and just north of the equator ~160E, with 29C and warmer over the TNWP as well as from the East Pacific, Caribbean into the Gulf of Mexico. Latest TAO data suggests the oceanic Kelvin wave is located around 160W, and that surface westerly wind anomalies (~5m/s) have once again returned to the equatorial date line region.

There continues to be little signal of a MJO, and there is some evidence of weak coupling between the tropical convection and warm SSTs west of the date line. An eastward propagating dynamical signal has emerged into the Western Hemisphere, moving along the East Pacific ITCZ as I type (phase speed ~8-10m/s). Finally, consolidation of tropical forcing is taking place centered ~10-15N/110E (including the monsoon systems), with an intense flare-up just north of the equator centered ~60E.

Consistent with previous events during the last several months, I would expect Indian Ocean flare-up to propagate northeast and also merge with the on-going convective activity from India to Southeast Asia. This whole region of enhancement could easily shift into the TNWP weeks 2-3 maintaining the tropical cyclone concern for that region.

While tropical convection should persist west of the date line (SST boundary forcing), general suppression may be the case around Indonesia, all during weeks 1 and 2. The TNWP forcing could easily reinvigorate the SST boundary forced convection west of the dateline afterwards. Finally, the Western Hemisphere dynamical signal may propagate though the Americas into the Atlantic by weeks 2-3, creating a tropical cyclone hazard for the East Pacific and perhaps the North Atlantic later on. The latter will need careful monitoring, especially for hybrids, particularly if the environment of the deep tropical Atlantic remains unfavorable.

Zonal mean westerly wind anomalies to continue to increase from 15S-25N, with magnitudes at 200mb ~5m/s at 15N. Animations of 150mb and 250mb daily mean vector wind anomalies shows the appearance of twin upper tropospheric anticyclones around the date line within the tropics, with distorted twin subtropical anticyclones ~120E. These anticyclones (with lower tropospheric cyclones – baroclinic processes) are the result of the divergent outflows from the tropical forcing discussed above. The pair at the date line is a response the SST boundary forcing (and perhaps an ENSO signal due to an evolving possible warm event; a slower process than subseasonal), and are contributing to the recent increase of zonal mean anomalous westerly flow throughout the tropical and subtropical atmospheres. I can link the large anticyclonic circulation anomaly just southeast of Greenland to at least once recent rapid Rossby wave energy dispersion (RWD) from the date line anticyclones forced by the SSTs. Some of the anomalous westerly flow is already impacting not only the Tropical North Atlantic, but even enhancing moisture transport into the Desert Southwest of the USA (interacting with the extratropics).

GSDM Stage 1-2 best describes the global circulation (see links to AAM plots). A transition to GSDM Stage 2 would be most probable during week 2, possibly persisting into week 3. Afterwards, offering anything statistically useful is unclear. This would suggest retrogression of the ridge back into the Great Basin and perhaps into Alaska, with a central and eastern North American trough. Responding to some of the initial conditions discussed above, most ensemble numerical models support this notion for week 2. However, uncertainty continues extremely high, and I have been suggesting this option for at least 2 weeks (which has not been a total bust). We have been dealing with an atmosphere having somewhat unusual forcing for at least the past few months, and trying to apply “conventional (linear) thinking” to it. For example, trying to apply MJO reasoning to dynamical processes that are not MJO. Expected weather has already been discussed in previous postings.

Ed Berry

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