Note: The R1 data ESRL/PSD AAM plots now present the “complete” budget (updated through 11 August as of this writing), and will be referred to.
This posting will be relatively short. The spatial distribution of global tropical and subtropical SSTs is similar to what was discussed on 9 August. There has been cooling across the equatorial East Pacific cold tongue loosely in the Nino 3 region. Recent 5-day averaged anomalies (per TAO buoy data) are ~minus 2.5C at the surface extending to depths ~150m at 140-160W with values ~minus 4C. While the global circulation is strongly La-Nina like, perhaps the equatorial East Pacific may be starting to respond.
The strongest tropical convective forcing remains well entrenched across the Eastern Hemisphere. One anomalous region has been festering across the equatorial Indian Ocean centered ~5S/90E with another focused across the Tropical Northwest Pacific Ocean (TNWP) just east of the Philippines. The South Asian monsoon system has also intensified during the last week. There is also minor enhancement of tropical rainfall around the Americas, and easterly wave activity across northern equatorial Africa has become quite intense.
Some evidence indicates another ~40-day mode of tropical rainfall variability maybe organizing across the equatorial Indian Ocean. However, recent full disk satellite imagery already suggests a quickening of eastward movement of the thunderstorm clusters. We will see if this possible episode organizes into a MJO particularly if it propagates northeast. In any case, we are still stuck in a generally stationary GSDM Stage 1 (more said below) base state having generally weak subseasonal variations. A difference is that our nemesis of 2 regions of Eastern Hemisphere tropical convective forcing may be starting to appear as seasonal transition to the boreal cold season approaches.
Total and relative angular momentum is probably the lowest in at least a year. Both are ~minus 2.5-3 AMUs below the R1 data climatology, and much of the contribution is coming from deep zonal mean easterly wind flow anomalies throughout the tropical and subtropical atmospheres. The largest zonal mean magnitudes are in the Southern Hemisphere (SH), having 200mb easterly wind anomalies ~10m/s. There is also a strong poleward transport signal from the SH tropics into the lower midlatitudes, with a weaker counterpart developing north of the equator. Global relative AAM tendency is ~minus 25 Hadleys as of 11 August and the Global Wind Oscillation (GWO) is as far to left as I have seen it, in the GSDM Stage 4-1 quasi-phase space. The global signal from the surface torques is still weak (for now).
There are currently strong twin upper tropospheric anticyclones with extensive cross-equatorial flow centered ~80E tied to the Indian Ocean (IO) tropical forcing. A weak compensating set of cyclones is present just west of the date line. With the exception of this date line cyclone pair, the entire tropics is dominated by upper tropospheric easterly wind flow, with anomalies ~15-25m/s from Africa into the Atlantic Ocean (favorable for convectively active easterly waves). Rossby wave energy dispersions (RWDs) across the Northern Hemisphere (NH) extratropics interacting with both the IO and TNWP forcing are already impacting North America. Combined with what I expect to be strengthening zonal mean westerly flow across the NH midlatitudes, my notion offered 9 August of the trough-ridge-trough pattern across the USA shifting southeast looks probable. In fact, the western USA may see a return to a barrage of troughs similar to last spring going into September, possibly interacting with a cold air source from Canada.
Week-2 ensemble means from several international operational weather centers are offering varying solutions for North America; however, there has been a recent trend toward a western USA trough with a southeast states ridge. As before, when I see the numerical models trend toward what diagnostic reasoning suggests (per GSDM), my confidence increases. The latter should be the case since I am utilizing a proposed more complete forecast process for subseasonal prediction. Nevertheless, uncertainty is still huge given issues such as the future evolution of the tropical forcing and seasonal transition. The general synoptic pattern across the PNA sector this fall will have some dependency on the GWO. For example, will there be a return to large circuits to GSDM Stage 2, etc. (especially if the warm TNWP stays active)?
Cooler and wetter weather is the most probable trend for particularly the northwest half of the USA during the next 1-3 weeks. The southeast is likely to continue with above normal heat and humidity. It is obviously unclear to say where any possible land falling tropical cyclone will go. However, the need to monitor this situation is likely to increase (see latest statements from the NOAA/NWS/Tropical Prediction Center). The center part of the country is also likely to become increasingly stormy, including severe thunderstorms and flooding rainfall.
Locations from India into China and Southeast Asia are likely to continue with the poundings of intense monsoon rainfall possibly for several more weeks. The TNWP including the Philippines are also probable to stay active with heavy rain and tropical cyclone activity week 1, with perhaps a respite across the west central Pacific for at least week 2. Northern equatorial Africa is likely to stay active through at least week 1, and I think the Atlantic Hurricane Season “is about to get underway”.
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 component of the GSDM.
These are probabilistic statements, and work is ongoing to quantify in future posts. 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. My next update should be early next week. In general, due to covering shifts and travel, my postings will be irregular through at least the rest of this month.