Saturday, December 08, 2007

AAM Crashing (like the recent finacial markets)

Yes, there has been a rebound in the USA stock markets recently especially last week. I hope this trend continues.

There is no change to my reasoning from a week ago; hence some brevity is appropriate for this discussion. The spatial distribution of global tropical SSTs remain essentially the same, with minus 1-3C anomalies along the equator from 160E-west coast of South America. Warmest SSTs across the Indo-Pacific sector are located from the north coast of Australia to Indonesia with totals in excess of 30C in places. Increasing trades from the equatorial date line into the west central Pacific Ocean (anomalies ~5-10m/s) may add some re-invigoration to the on-going El-Viejo.

An oceanic Kelvin wave was generated by the westerly wind burst forced by the recent MJO. Per latest five-day averaged TAO buoy data anomalies in excess of 3C at ~150m depth in the region of the equatorial date line are associated with this Kelvin wave. The impacts of this and additional Kelvin waves onto our basin wide moderate cold event (in terms of SSTs) is a monitoring issue. However, I speculate that the early stages of a transition to El-Nino are in progress, especially given the biennial character of ENSO over the last several years.

The dynamical signal with the MJO is quickly returning to the Eastern Hemisphere. In fact, a back of the envelope calculation from near equatorial velocity potential Hovmoller plots gives me a phase speed of ~30m/s as this signal propagated through the Western Hemisphere. Interactions with the extratropics along with decoupling between the atmosphere and tropical convection allow for these kinds of fast phase speeds as the MJO response moves through the Western Hemisphere.

Full disk satellite imagery shows tropical convective forcing rapidly increasing across the equatorial Indian Ocean during the last several days, with enhanced rainfall still present across portions of tropical South America and South Africa. WH2004 phase space plots support this observation, suggesting that the MJO signal is currently in phase 1. Animations of upper tropospheric daily mean vector wind anomalies not only show twin anticyclones developing in the region of the Indian Ocean, but also easterly wind flow returning to the equatorial atmosphere. In the zonal mean equatorial wind anomalies are already negative, and what is left of the westerly wind flow anomalies has propagated poleward well into the subtropics and midlatitudes of both hemispheres (through eddy feedbacks).

The combination of frictional dissipation, eddy dynamical processes and the tropical forcing returning to the Eastern Hemisphere is rapidly putting the brakes on the global atmosphere (and speeding up the earth shortening the length of day by a few milliseconds) as I type. In other words, easterly wind flow anomalies are quickly being added back to the equatorial and subtropical atmospheres creating a momentum sink. Given our La-Nina base state this circulation response was expected once the MJO returned to the Eastern Hemisphere.

Per ESRL/PSD AAM plots updated through 6 December using the R1 data climatology, 3-day averaged global relative AAM tendency is at least minus 40 Hadleys. The surface torques as well as the Coriolis torque are contributing to this big AAM tendency. Actual global relative AAM will crash very soon, if it is not already doing so. The expected large circuit of the GWO is occurring, currently entering phase 1. During big subseasonal events such as what has been occurring over the last several weeks, both the MJO and GWO are probable to be in similar phases.

My feeling is that another large GWO orbit and respectable MJO are probable during the next ~50 days. Of course, uncertainties about timing, synoptic details, etc., are always an issue. The GWO should circuit to phase 3 (old GSDM Stage 1) by the end of week-2 and perhaps phase 5 (Stage 2) by the end of week-4 (~ 5 January 2008). Thus my USA outlook from a week ago remains the same. I think there will be a bit of a respite from the currently active western/central USA winter regime week-2 as troughs initially deepen along/off the west coast during that period. Most week-2 ensemble means now show this response. While the USA west coast (and Alaska at times) receives several rounds of possibly intense/severe precipitation during week-2, the rest of the country east of the Divide should be milder. More “Kona Lows” may also be probable for Hawaii.

Starting around Christmas and continuing well into January a cold and active regime appears probable especially if the tropical forcing initially couples to the warm SSTs north of Australia. Troughs would again be favored across the western states with an active southwest flow storm track across the Plains. Sparing details, this pattern may shift ~10-20 degrees of longitude farther east during mid-late January bringing possibly intense cold for particularly the northeast states while extending back into the central USA. At that time ridge amplification may occur from just off the USA west coast into Alaska.

Internationally, an increasing tropical cyclone hazard across the South Indian Ocean especially by week-2 is probable. Locations from the Philippines into the west central Pacific should enjoy some quieter weather at least week-1. Cold Arctic air will continue to build up across Siberia as well as Canada during the next several weeks.


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 try to post another discussion around next weekend or early week-2.

Ed Berry


John Pollack said...

Ed, your recent discussions have been doing far better than the financial markets. In particular, I was struck by your Nov. 18 posting regarding the evolution of the PNA sector in early December. It is an outstanding example of a successful sub-seasonal forecast, verified by recent events.

Ed Berry said...

Hi Jon,

Thank you for the kind feedback. It will be interesting to observe where the weather-climate situation goes during Jan 2008 per my recent post. A very interesting weather regime may focus on the Plains should the tropical forcing couple with the warm SSTs north of Australia.

Ed Berry