Saturday, September 08, 2007

Are we Sure???

The spatial distribution of global tropical SSTs has not changed too much since a week ago. There is a slightly anomalously warm (cool) Indian Ocean (Indonesia region) to significantly above average Tropical Northwest Pacific Ocean (TNWP) pattern across the Eastern Hemisphere. A horseshoe shape of anomalous warmth still extends into the extratropics from the latter. Starting around the date line all Nino regions appear to be cooler than normal per CPC 7 September 2007 daily mean SST analysis, with a tilt toward warmth from the Caribbean into the deep tropical Atlantic Ocean.

SST totals and anomalies from the west central into the northwest Pacific Ocean are in excess of 30C and plus 1 C over a large area. Negative SST anomalies of 1C and lower extend from ~160W to less than minus 2C ~110W per recent 5-day averaged TAO buoy data. All these anomalies are quite deep to at least 200-400 meters meaning a steeper than normal equatorial oceanic thermocline. So, at least for now our La-Nina has become an essentially basin wide event, having a pronounced westward shift since ~ 1 August forced by enhanced trades. In my past postings, I suggested the cool SSTs would be confined to the equatorial east Pacific Ocean. So far that appears to have been a poor assessment.

However, I do have what I think are fair concerns in regard to the future our developing cold event given ongoing subseasonal behaviors (discussed below). It is unfortunate that the resources are not (yet) presented to this effort which would allow weather-climate linkage risk assessment and probabilistic information (etc.) that may provide additional and useful predictive information. From speculation, I am concerned this La-Nina will peak during boreal fall; in a sense similar to El-Nino about a year ago (there has been a biennial character since ~2001). I will leave this path by again typing how interesting the weather-climate situation may become should the 30C and warmer waters of the west central Pacific Ocean become strongly convectively active by the start of 2008.

From full disk satellite imagery and other tools, there are 2 regions of tropical convective forcing across the Eastern Hemisphere. North equatorial Africa remains broadly active with periodic enhancement around the Americas. For the Eastern Hemisphere, one area is the stationary tropical signal centered ~10N/60-80E linked to the evolution of La-Nina discussed in my Labor Day Weekend postings. The TNWP forcing I think is another manifesting of the 20-30 day variability observed since around early June involving east and northward shifts. Even though recently updated RMM WH2004 phase space plots indicate some MJO projection with predicted eastward movement, I do not think the current TNWP flare-up is a result of MJO variability.

Since early June, the first couple were very weak MJOs while the last 2 I think have had an extratropical component of forcing. In fact, the most recent has involved a relatively strong positive global mountain torque of ~plus 30 Hadleys leading to a relative AAM tendency of similar magnitude just before 1 September. A circulation response has been for an increase of anomalous zonal mean westerly wind flow loosely from the equator to about 30N with magnitudes of 2-5m/s. There has even been a weak poleward propagation of anomalous zonal mean easterly wind flow from the equator to ~40N since about 10 August.

The most significant contribution to the most recent positive global mountain torque may have come from East Asia, but having nice dipole symmetry with the Southern Hemisphere due to the Andes Mountains. From daily monitoring of fields such animations of upper tropospheric daily mean vector wind anomalies, I can link extratropical Rossby wave energy dispersions (RWDs) tied to the Indian Ocean forcing to the positive global mountain torques. Subsequent dispersions into the TNWP enhancing upper tropospheric divergence led to the current observed flare-up in that region. These same animations suggest RWDs from the TWNP enhanced convection impacting the PNA sector as I type. A response has been for the GSDM Stage 2 response leading to a digging Rocky Mountain trough.

From a somewhat broader perspective, around 20 August we saw a poleward AAM transport signal maximized ~45N only to recently become directed equatorward ~35N. The interactions discussed above were involved in this reversal. When updated, the ESRL/PSD Global Wind Oscillation (GWO) quasi-phase space diagram should show an even larger circuit toward GSDM Stage 2, but still on the left hand side of the plot. Global relative AAM still remains low, ~ minus 2 Atmospheric Momentum Units (AMUs) below the R1 data climatology. Much of that contribution is still coming from the Southern Hemisphere equatorial and subtropical atmospheres. I think this will be the largest GWO circuit since the 2 large events during March and May 2007, having a contribution from non-MJO tropical convective forcing.

Finally, other roughly 20-30 day GWO circuits leading to at times 2 regions of tropical forcing across the Eastern Hemisphere may be probable going into boreal fall. These would likely involve both the Indian and west central Pacific Ocean basins. This is not a new concern on my end for numerous reasons, and any longer-term impacts on our La-Nina situation are unclear. However, monitoring is critical since numerical coupled ensemble prediction schemes are unlikely catch complicated subseasonal behaviors like GWOs until their impacts have already occurred.

For the USA week 1, the models have generally caught the GSDM Stage 2 response of the Rockies and Plains trough. Much of the country will have the first surge of autumn cool air, with the largest negative temperature anomalies generally focused on the Northern Plains and Upper Mississippi Valley. Heavy precipitation is likely to occur across locations such as from the Southern Plains into the eastern states. During weeks 2-3, the east Pacific Ridge-central states trough pattern should loose amplitude, with more troughs from the east Pacific into the western states becoming probable. The GWO should quickly orbit into the GSDM Stage 4-1 phase plane. Another “strong trough” situation may be probable for the Rockies and Plains ~ week 3. Again, predictability can be very low during transition seasons. Stay tuned if much of the western and central USA has a “real winter (assuming the Arctic Ocean freezes back over!)” with perhaps many GSDM Stage 4-1 situations per above.

Internationally, generally from the Indian Ocean into the TNWP periods of intense rainfall are probable week 1, including tropical cyclone activity across the latter. The focus should then shift back into the region of India into Southeast Asia and Southern China at least week 2, with still a tropical cyclone concern for the region around the Philippines. Northern equatorial Africa should stay convectively active for at least the next couple of weeks.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. As discussed a week ago, Gabrielle developed as a hybrid system. I can see a scenario favoring tropical cyclone development perhaps even above climatology across the deep tropics of the Atlantic Ocean week 1 and possibly continuing week 2.


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 GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

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 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 be on travel during the week of 10-14 September, and I am unclear when my next update will be.

Ed Berry

No comments: