Stationary?

With the exception of the cool equatorial East Pacific (negative anomalies ~1-2C just west of South America), global tropical SSTs remain generally above normal as of the 11 June 2007 daily mean analysis from CPC. Largest positive anomaly magnitudes of at least ~1-2C were present over the far western Indian Ocean, the South China Sea (SCS), portions of the Caribbean and eastern Atlantic. SST totals of at least 30C remain across the west central Pacific along with a recent appearance across the SCS. A recent trade wind surge across the equatorial date line region has locally cooled the ocean waters. However, positive anomalies extend to at least a depth of 100-150m meaning the west Pacific cooling will be minimal. Development of La-Nina remains unclear, particularly since subsurface anomalies (per TAO data) have warmed considerably during the past couple of months along the east Pacific equatorial cold tongue.

Full disk satellite imagery has shown a significant increase in tropical convection across the Indian Ocean during the last week, centered ~0/80E as of this writing. This large region of enhanced tropical rainfall (OLRA at least ~minus 50W/m**2) extends from east of Africa into western Indonesia as well as portions of the Arabian Sea and India (enhancing the local monsoon system). There is a “fanning out” of “convective debris” as well as intense rainfall into both hemispheres, particularly the Northern Hemisphere. In fact, interacting with Rossby wave energy dispersion dynamics across Asia, an intense frontal band of severe damaging rainfall and thunderstorms has persisted for several days from the northern Bay of Bengal into southeast China on to the east of Taiwan. These type of interactions has been responsible for yet another enhanced North Pacific jet having recent weekly mean wind speed anomalies ~25m/s at 250mb. There is general convective suppression across equatorial Africa and the west central Pacific while weak enhancement remains around the Americas.

So, do we have a MJO? My feeling is we do. Loose phase speed calculations have this area of enhanced convection coherently moving east at ~3.5 m/s (2-3 deg long/day) since the start of this month across the western Indian Ocean. Animations of 150mb and 250mb daily mean vector wind anomalies from ESRL/PSD not only show intensifying twin anticyclones across the Indian Ocean, but also a rapid increase of anomalous equatorial westerly wind flow at 150mb east of the date line. As of 11 June 2007, as part of a cross equatorial flow response to Rossby wave energy dispersion (from both hemispheres) and subsequent anticyclonic wave breaking activity, 150mb westerly wind anomalies were ~30m/s north of the equator at 160W. This is much stronger than observed a week ago. Furthermore, from plots such as 200mb zonal mean zonal wind anomalies, there is weak evidence of easterly anomalies propagating off the equator into the subtropics (strong westerly anomalies still remain ~30N and S) with westerlies replacing them. Finally, there is also evidence from animations of tropical mean sea level pressure anomalies of an upwelling atmospheric Kelvin wave already hitting the Andes Mountains. In fact, the latter has contributed to another strong negative global mountain torque of ~25 Hadleys (AAM tendency ~minus 20 Hadleys) as of 9 June per reanalysis (R1) data. These and other indications from detailed daily monitoring within the GSDM framework as well as statistical information from plots such as the Wheeler index give me plenty of evidence for a MJO.

There are thoughts that since our recent zonal wave number 1 spatial pattern of 200mb velocity potential is now wave number 2 due to a weaker center around the Americas, one should be skeptical about a MJO. The Rossby wave energy dispersion/anticyclonic wave breaking processes only briefly discussed above has also been occurring around the Americas leading to a low latitude trough just west of Mexico. This is a feedback from the extratropics which has led to the enhanced divergence in that region. It is an option this Western Hemisphere signal may persist longer than the MJO time scale.

GSDM Stage 0.5 best describes the current weather-climate situation. In fact, the GWO has orbited back into the Stage 4-1 phase space quadrant as of 9 June. Understanding uncertainty, my thoughts are for the enhanced convection with the MJO to propagate east and northeast during the next few weeks, with the center reaching perhaps ~10-15N/120-130E by the end of week 2. I also think there is a mountain-frictional torque index cycle linked with this MJO, all meaning (sparing more details) that by around the start of week 3 the global weather-climate situation may be in GSDM Stage 2.

Any outlooks for weeks 1-3 must always incorporate diagnostic information from weather-climate monitoring, especially right now. I expect particularly for weeks 2-3 model performance from many international global operational ensemble prediction systems to suffer, perhaps greatly. Given the slowly evolving character of this MJO and subsequent response, feedbacks, etc., the models should have enough initial condition information to do "well" for ~5-7 days.

For the USA, the northwest shifted active pattern from this past spring is probable to continue for at least the next 7-10 days. Afterwards, going into week 3, a fundamental circulation (at least transient) change to a regime across the PNA sector similar to the summers of 2003 and 2004 may occur. This would suggest ridge amplification west of North America into Alaska, possibly linking up with anticyclonic gyres already present across the Arctic (not discussed to save space). An anomalous trough would then be probable~100W with a downstream ridge across Florida and the Caribbean. There is a hint of this type of regime from some week-2 ensemble means, especially the ESRL/PSD. However, what is depicted by these models I think will be farther west and more probable week-3. Of course, timing is always uncertain.

Weather ramifications would be anomalous cold/wet focusing on the Plains with warm/dry along the west coast and Deep Southeast. International ramifications for the period of weeks 2-3 may include an enhanced risk of tropical cyclone activity across the northwest and east Pacific (understand current conditions across the latter) with a reduced chance of tropical cyclone development across the Atlantic. The evolution of this MJO is unclear to me; however, it may stall in the Eastern Hemisphere and loose coherence after week 3. Should a strong dynamical signal from this MJO propagate coherently into the Western Hemisphere, we may see more than just the transient phase space orbits into GSDM Stage 3 like those observed this past spring.

Appendix

An experimental phase space plot of the GSDM utilizing normalized relative AAM time tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg

These are probabilistic statements, and work is ongoing to quantify in future posts, particularly if an opportunity arises for us to have a dedicated web page effort. The WB (2007) paper on the GSDM has been published in the February issue of MWR. Due to travel it is unlikely I will be able to write another discussion until early next week.

Ed Berry