Tuesday, June 19, 2007

GSDM Stage 2 Coming – Summertime Version

Global tropical SSTs remain above average from the Bay of Bengal to the South China Sea on into the west central Pacific Ocean. As of 18 June per CPC, daily mean anomalies were ~.5-1.5C with totals over 30C along the equatorial west central Pacific and particularly the South China Sea. A horseshoe pattern of positive SSTAs still persists from the west central Pacific into the subtropics of both hemispheres. Recent intense convective rainfall has brought the SSTs close to normal across much of the equatorial Indian Ocean and Arabian Sea. Only weak cool anomalies remain along the equatorial Pacific cold tongue east of 120W (subsurface anomalies are relatively incoherent) while the SSTs are near climatology over the tropical Atlantic.

Development of La-Nina remains highly uncertain. In fact, similar to the boreal summer of 2004 and even nearly a year ago, the possibility exists of a surface westerly wind event on the equator across the west and central Pacific should the MJO (discussed below) come out into that region. Given the observed slow motion of the MJO so far (~2.5-3m/s phase speed from ~2-16 June), there could even be a robust downwelling oceanic Kelvin wave.

Full disk satellite imagery presents a very impressive signal of intense tropical convection across the Eastern Hemisphere, extending east-southeast from the eastern Arabian Sea into much of Indonesia. OLR anomalies have been at least ~minus 50-90 W/m**2 across this region for the past week. Numerous monitoring tools including the Wheeler phase space plot, the Global Wind Oscillation phase plot “derived” from the GSDM (see Appendix), coherent modes Hovmollers and AAM plots indicate this region of tropical forcing is at least a moderate MJO. The notion of a MJO developing across the Eastern Hemisphere was discussed on the 28 May posting. However, it took about 2 weeks longer to become coherent than what I thought at that time (not meaning I was "wrong" for 2 weeks since the notion was stated probabilistically).

This is the first MJO of this magnitude since the December 2006-January 2007 time period. There was a weaker MJO during much of February while 2 other coherent eastward shifts of tropical forcing occurred during approximately late March and late April. The latter were linked with extratropical variations including mountain-frictional torque index cycles (see past postings) which are captured by the GSDM (and Global Wind Oscillation) but not by “conventional MJO tools”.

As of this writing the core of the MJO is at ~5-10N/100E, and has shifted east-northeast of roughly 5 deg of latitude and 30 deg of longitude during the last week. Along the equator this means an increase of phase speed to ~5 m/s in contrast to the slower movement discussed above. The point is the speed of the eastward (along with the northward component) movement is increasing, which is typical for a MJO as it propagates across the Maritime Continent. Tropical thunderstorm clusters are becoming more numerous across the west central and northwest Pacific, while generally suppressed conditions exist across the Western Hemisphere.

The global circulation is responding to the MJO (understanding interactions with the extratropics and subsequent feedbacks). Animations of daily mean 150mb and 250mb vector wind anomalies show well defined but somewhat distorted twin upper tropospheric subtropical anticyclones ~60-90E with downstream cyclones ~120E and just east of the date line. This complex spatial pattern in some sense is a residual of past subseasonal variations now interacting with the MJO as well as wavelength differences due to austral winter and boreal summer.

Daily mean 150mb vector wind anomalies are at least 10-20m/s and even exceeding 40m/s with the subtropical cyclones ~120E. Strong upper tropospheric divergence is not only seen in the anomaly fields, but also in the totals. In fact, the 150mb daily mean vector wind total field from 18 June showed strong subtropical anticyclones ~90E supporting an intense Tropical Easterly Jet (TEJ) across equatorial Africa with wind speeds ~of at least 40m/s. Rossby wave energy dispersions from the extratropics of both hemispheres interacting with the MJO during the past 7-10 days have led to strong anticyclonic wave breaking across the deep tropics east of the date line. This wave breaking has allowed for a rapid increase of upper tropospheric westerly flow along the equator across the Western Hemisphere, with 150mb wind speed anomalies ~40m/s at 125W. This has contributed to an increase of zonal mean anomalous westerly wind flow approaching 10m/s at 200mb.

As shown by the Global Wind Oscillation as of 13 June and the R1 AAM plots through 16 June as well as the above discussion, the weather-climate situation is in GSDM Stage 1. Per R1 data global relative AAM remains at least minus 1.5 standard deviations below the 1968-1997 climatology due to anomalous zonal mean subtropical easterly wind flow flanking the equatorial zonal mean westerly flow mentioned above. There are also anomalous zonal mean easterly winds across mid-higher latitudes of both hemispheres flanking strong midlatitude westerlies (see plots). Again, we have a very complicated meridional but somewhat symmetrical distribution of zonal mean zonal wind anomalies tied to subseasonal behaviors for at least the past couple of months.

The signal from the time tendency of global AAM as well as the torques and transports is still fairly weak. However, as of 16 June global AAM tendency has increased to ~plus 10 Hadleys while the global frictional torque is ~plus 20 Hadleys. The increase in the frictional torque has not only come from the strong trades currently moving into the Western Hemisphere, but also from the extratropics. In any case, I do expect AAM tendency to become strongly positive during the next week-10 days and the Global Wind Oscillation to do a large orbit to GSDM Stage 2 by that time.

Specifically, I am expecting the MJO to continue propagating east-northeast into the Tropical Northwest Pacific (TNWP) by the end of week 2 into week 3. It is probable the most intense tropical rainfall will extend from Southeast Asia and China across the Philippines into the west central Pacific. It is unclear to me what will happen afterwards. However, I would expect a signal to propagate into the Western Hemisphere weeks 3-4 while perhaps a stationary component hangs around the TNWP given the exceptionally warm SSTs there. Hence it is probable for the global circulation to be in GSDM Stage 2 by late week 2 and/or week 3. GSDM Stage 3 as part of a large orbit in Global Wind Oscillation phase space (similar to the past 2 in terms of amplitude) afterwards is certainly an option.

International weather ramifications include not only a slight possibility of Bay of Bengal tropical cyclogenesis by week 2 (against climatology), but also a continuation of severe thunderstorms and rainfall across areas already hit hard across Southeast Asia (per WMO). In addition, there will be a growing typhoon hazard across TNWP week 2 and beyond (and perhaps “eventually” the tropical East Pacific). The upper tropospheric Western Hemisphere anomalous equatorial westerly wind flow discussed above not only increases the probability of precipitation suppression across the tropical North Atlantic, but also decreases the probability of tropical cyclone formation in that region. Understanding all options, it will be interesting to see how this situation works out for the 2007 Atlantic Hurricane Season.

For the USA, GSDM Stage 2 suggests anomalous ridge amplification from just off the west coast into northwest Canada and Alaska (~125-140W) with a downstream trough ~100W and a ridge across the Deep Southeast and Florida. Up until 2 days ago, nearly all week-2 ensemble means from various global weather centers were predicting ridge conditions across the western and central USA with the phasing of anomalies nearly opposite to what I am suggesting. More recent runs are now trending toward a more probable solution such as mentioned above which is derived from a detailed diagnostic understanding of the weather-climate system.

Weather ramifications would be anomalous cold/wet focusing on the Plains with warm/dry conditions along the west coast and Deep Southeast, starting during week 2 (the models should do “okay” through ~days 5-7). These temperature and precipitation anomalies would be loosely similar to that observed during the summer of 2004. In fact, there are other tools incorporating the recent anomalously high soil moisture content across portions of the Plains which also support this notion for at least July. However, short story is soil moisture feedbacks are only secondary since any final outcome of temperature and precipitation anomalies primarily comes from the dynamical forcing onto the atmosphere. It is an option for GSDM Stage 2 not only to persist through July, but also into boreal fall.


An experimental phase space plot of the GSDM (which we call a Global Wind Oscillation) utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at


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 objectivity. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to do another posting this Friday.

Ed Berry

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