Thursday, August 09, 2007

Still Slaying -- Update

Note: The ESRL/PSD AAM plots now present the “complete” budget (updated through 6 August as of this writing), and will be referred to.

Per CPC and other data sets, global tropical SSTs are generally above average across the Eastern Hemisphere while cooler than normal across the equatorial central and east Pacific and about climatology across the Atlantic Ocean. Anomaly magnitudes vary from ~0.5-2C with the greatest across portions of the equatorial Indian Ocean and west of South America ~110-120W (even ~minus 3 on 8 August). The latter is in the Nino 3 region, and recent enhanced trades have contributed to some cooling during the last week. In fact, subsurface anomalies are as low as roughly minus 3C at 160W/150m per latest 5-day averaged TAO buoy data suggestive of the thermocline being raised. Warmest SST totals remain across the west central-northwest Pacific Ocean and the Caribbean-Gulf of Mexico regions with values ~29-30C. While the global circulation is very much La-Nina like, the SST pattern is only weakly representative of a cold event.

Weather-climate linkage signals continue to be extremely difficult to interpret. The MJO has virtually collapsed. All Wheeler-Hendon type phase space plots from international weather centers have projections less than 1 standard deviation. There is some “tilt” back toward the Indian Ocean, and attention needs to be paid to that for predicting upcoming subseasonal evolution.

As part of a consolidation with an equatorial Rossby mode, about a week ago a northeastward propagating component weakly projecting onto a MJO came out into the Tropical Northwest Pacific Ocean (TNWP). Per coherent modes Hovmollers the signal made it as far east as ~160E. A couple of tropical cyclones have developed in that region since. At this time the most significant tropical forcing remains well back in the Eastern Hemisphere centered at about 15N/120E. There is loose evidence of a tropical convective component moving into the Western Hemisphere. Recent 3-day averaged OLRA present ~minus 70 W/m**2 and lower around the Philippines. There are also negative anomalies north of the equator at the date line (a response to the weak signal moving into the Western Hemisphere) and back across the equatorial Indian Ocean. Notable positive anomalies of OLR are centered on the region of Indonesia.

Full disk satellite imagery nicely shows the convection around the Philippines while extending west and southwest into the Bay of Bengal. This is a change in the orientation discussed late week. Equatorial Africa into the equatorial Indian Ocean is getting more active. I have speculation that in the wake of our recent northward propagating event from the equator into the Southeast Asia, another one may be getting organized. It is possible this may be a form of 20-30 day variability, linked to the extratropics, and within what has been a stationary GSDM Stage 1 base state given boreal summer (discussed below).

Typical of a cold event, global total and relative atmospheric angular momentum (AAM) continues very low, roughly minus 2 Atmospheric Momentum Units (AMUs). The mass term has increased somewhat during the last week. The last MJO event did not add as much zonal mean westerly wind flow to the atmosphere as I previously thought. In fact, the Global Wind Oscillation (GWO) updated through 5 August (5-day average) objectively shows the global circulation solidly in GSDM Stage 1. There was only a pathetic circuit when the MJO came out.

Zonal mean easterly wind flow anomalies have been rapidly increasing throughout the equatorial and subtropical atmospheres over the last few days. At 200mb magnitudes are ~5m/s, flanked by zonal mean westerly wind flow anomalies at the mid and higher latitudes. A meridional distribution of zonal mean zonal wind anomalies such as these supports anomalously strong subtropical ridges from the Devil including the one impacting the USA. Hence global relative AAM tendency is negative (~15 Hadleys) and the surface torques is also trending toward less than zero. Finally, the coriolis torque is also ~minus 10-20 Hadleys when loosely it should be positive at this time.

Animations of upper tropospheric daily mean vector wind anomaly fields suggest twin tropical/subtropical anticyclones redeveloping across the Indian Ocean while the pair across the west central Pacific weakens. There are weak twin cyclones along and just east of the date line. The upper tropospheric Western Hemisphere Pacific Ocean equatorial westerly wind flow anomalies also discussed last week have weakened significantly and are going into the Southern Hemisphere. It was Rossby wave energy dispersions (RWDs) arcing across Asia interacting with the west Pacific anticyclones that contributed to eastward shift of what was the “wicked ridge of the west” (western USA for international readers).I can already see the RWDs coming out of the Indian Ocean that will lead to some retrogression of the Deep South ridge back toward the Plains. This will not be as far west as some numerical ensemble solutions were indicating a week-10 days ago.

Uncertainty remains huge for making any subseasonal forecast, particularly as we start to head into fall. During the last couple of weeks I think have had somewhat better success with what I have believed to be the most probable options. I am going to be bold here and speculate that the MJO dynamical signal, having interactions with the extratropics (forcing-response-feedbacks, etc.), is going to get its act back together across the Indian Ocean during the next week or so and then propagate east-northeast into the TNWP by roughly weeks 3-4. How intense this event may be and its impact on the GWO is unclear. I would like to see the Wheeler-Hendon plots come out of their “1 sigma” circles “soon” across the region of the Indian Ocean. Should this occur, any model, numerical, statistical, etc. is unlikely to predict this possibility right now. It would be interesting to see what “brute force stochastics” might give us right now about MJO and anything else dealing with making predictions in our current lazy GSDM Stage 1 regime. For weeks 1-4, let’s stick with Stage 1 through week 2 possibly getting perturbed a bit to Stage 2 during weeks 3-4.

I think the ramifications for the USA are obvious by now. For much of week 1 deadly intense heat is a good bet centered from the Plains into the Deep South. Severe MCS activity will occur on the northern fringes of this ridge from Hell, likely from the Northern Rockies to the Great Lakes into portions of the eastern USA. This general trough-ridge-trough pattern across the USA should shift southeast weeks 2-4 if the scenario discussed above is realized.

Some respite for portions of India and Bangladesh from the severe storms and flooding may occur during a portion of week 1. However, locations from India into China and Southeast Asia are likely to continue with the poundings of intense monsoon rainfall possibly for several more weeks. As I type a tropical cyclone is trying to develop across the far northern Arabian Sea, which should weaken as it gets inland (but not before causing “problems”). While northern equatorial Africa is getting active, dry stable air from the North Atlantic Ocean has been inhibiting tropical cyclone development of the waves as they move off the West African coast. While I can offer some attribution to this, suffice to say this situation needs to change if there is to be an active Atlantic tropical cyclone season (understanding the Caribbean, etc.). Finally, more strong troughs appear on the horizon for Europe for at least the next couple of weeks.

Stay tuned if we start to observe 2 regions of Eastern Hemisphere tropical convective forcing this fall, the Indian and west central Pacific Oceans. If the former dominates, GSDM Stage 4-1 would be most probable.


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 (general link)

We call the behavior of this plot the Global Wind Oscillation (GWO), and one of its purposes is to extend thinking beyond the MJO.

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. I will try to do an update early next week. In general, due to covering shifts and travel, my postings will be irregular through at least the rest of this month.

Ed Berry

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