Friday, December 21, 2007

Typical El-Viejo??? - Update

There has been no significant change to the distribution of global tropical SSTs since my 15 December post, particularly across the Indo-Pacific region. All Nino regions have well below normal SSTs ~minus 1-3C extending to at least 100-150m deep, flanked by the warm Pacific Ocean horseshoe. An oceanic Kelvin wave with anomalies roughly plus 1-3C continues to propagate east (~2.5 m/s) through the region of 180-160W at around 150m depth per 5-day averaged TAO buoy data. While much of the equatorial and southern Indian Ocean has cooled to less than 28C (anomalies ~minus 1-2C) due to recent moist convection, the very warm waters from north of Australia to the South Pacific Ocean remain. Totals in excess of 30C are common across the latter region.

The Eastern Hemisphere tropical convective forcing has become somewhat better organized during the last few days. Full disk satellite imagery shows this large region of intense anomalous and at times severe tropical rainfall extending from the Arabian Sea to a concentrated area over central Indonesia (~0/120E) into the South Pacific Ocean. Various monitoring tools indicate there have been several components contributing to this tropical forcing. However, as shown by the WH2004 phase space and coherent modes Hovmoller plots, there is a significant MJO contribution (currently MJO phase 3-4). My own back of the envelop phase speed calculation has this convective region shifting east at ~4-5m/s since early this month. I am expecting a roughly 1-2 week period of coupling to the very warm 120E-180 SSTs by early January, before a MJO signal in all likelihood propagates into the Western Hemisphere. Again, I remind the readers there is always LARGE uncertainty about timing.

The global circulation is very much La-Nina like, which can be thought of as an enhanced climatological base state. Global relative AAM is roughly 2 standard deviations below the R1 data climatology (through 18 December) having well above average zonal mean easterly wind flow anomalies across the equatorial and subtropical atmospheres. The GWO is essentially orbiting around phase 3 (old GSDM Stage 1). However, there are always issues of non-linear dynamical feedbacks involving the extratropical eddies. For instance, per animations of daily mean upper tropospheric vector wind anomalies, while twin subtropical anticyclones (cyclones) have become established ~100-120E (~date line), there has been strong Rossby wave energy dispersions (RWDs) into the global extratropics. These RWDs have been very fast (~30m/s) not only leading to the barrage of western USA troughs, but also feeding blocking structures over Scandinavia and very recently northeast Asia. The latter will lead to a build-up of bitterly cold Arctic across Alaska during the next few weeks.

A strong zonal mean poleward AAM transport signal has become evident around 30N during the last few days, with flux divergence from the subtropics and convergence ~35N. In addition to the eddies, this transport signal has also been adding anomalous westerly flow to the northern extratropics (including the North Pacific Ocean). My point is the appearance of an AAM transport signal such as this adds support (through exchange processes involving the surface torques) to the predicted PNA synoptic evolutions I discussed a week ago.

I think there will be ~couple roughly 10-day circuits around GWO phase 3 until early-mid January when a large push to GWO phase 5 (GSDM Stage 2) may occur. That would be most probable after the period of atmosphere-ocean coupling discussed above. An evolution through GWO phases 6-8 (GSDM Stages 3-4) is possible afterwards (~weeks 4-6). Should the latter occur, more significant precipitation for locations such as California and the Desert Southwest may occur.

Hence there is no change to the predictive insights I offered in my 15 December discussion. Several troughs riding a strong North Pacific jet stream (yes, this can happen during a La-Nina) will continue to impact most of the country through early January 2008. I do think it is probable to see large ridge amplification from the east Pacific/off the USA west coast into Alaska by ~mid January 2008. Interestingly, the ESRL/PSD week-2 ensemble mean of 500mb height anomalies suggests this possibility while the NCEP GFS ensemble mean does not. However, even though the week-2 models have been “all over the place” recently, I think there will be upcoming better agreement on this GWO phase 5 scenario. If this happens, severe Arctic air may impact the Plains and eventually the eastern USA by ~mid January, after possibly initially spilling into the Pacific Northwest.

Internationally, severe rainfall and thunderstorms are probable to continue across portions of Australia and Indonesia and even into the Southwest Pacific during the next couple weeks. The north and northeast coast of Australia may become vulnerable to tropical cyclones weeks 2-3. Europe should become active as the Scandinavian blocking breaks down while bitterly cold air even by Siberia standards dominates northeast Asia. Again, some of the latter airmass may plunge into the USA via western Canada by ~mid January 2008.


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

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous the WH2004 “convention”.

Links to CPC and PSD ENSO discussions:

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us (soon) to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will not be able to post another discussion until late next weekend at the soonest.

Ed Berry

No comments: