Thursday, January 17, 2008

Another Injection due for La-Nina; Update

The spatial pattern of global tropical SSTs is generally unchanged from at least a week ago. Above normal waters remain across the Indian Ocean while negative anomalies, including those with La-Nina, extend from north of Australia into the equatorial Pacific to the coast of South America. Anomaly magnitudes are around 1-3C, with the warmest across the South Indian Ocean (totals ~30C) and coolest in the region of the equatorial date line to 120W. Latest TAO array buoy data shows that the colder than normal equatorial Pacific SSTs are deep, extending to around 200m. A pronounced SST horseshoe of warmth extends into the extratropics from the west Pacific, with ~2C magnitudes around 30N/180. The bottom line is that El-Viejo ocean conditions are well pronounced and mature. Several of the latest dynamical and statistical models suggest La-Nina may continue most of this year.

Full disk satellite imagery and other monitoring tools indicate the strongest moist tropical convective forcing is in the region of the date line/South Pacific Ocean, with 3-day averaged OLRA ~minus 70-90 W/m**2. The latter is part of a loosely organized region of tropical rainfall that extends from the Philippines east-southeast crossing the equator near 160E into the South Pacific Ocean along the South Pacific Convergence Zone (SPCZ). I would not have expected this tropical forcing to have propagated into the west central Pacific Ocean along the equator a couple of weeks ago. An equally important region of convective suppression is across the Indian Ocean, leading to the warming SSTs there. Other regions of enhanced diurnal convection are over tropical South America and South Africa.

The MJO signal remains strong per WH(2004) methodology, having ~2.7 standard deviation projection in phase 7 on 16 January. Circulation wind flow anomalies (discussed below) along with the Indian Ocean suppression are contributing. Unlike the late boreal fall 2007 MJO occurrence, during the last week the current MJO event has stalled. Several forecast tools indicate the MJO signal will redevelop across the Indian Ocean during ~week 2-3. The very warm waters of the South Indian Ocean support this prediction, as does daily monitoring of other diagnostics (more said below).

There is, however, some concern the west central Pacific Ocean tropical forcing may persist longer than was earlier thought, possibly another ~1-2 weeks. The latter is a monitoring issue, and does add uncertainty to making predictive statements about probable circulation anomalies to evolve during the next few weeks. Stay tuned.

Updated through 12 January (there are once again data issues) global relative AAM was slightly below the R1 data climatology while its tendency was ~minus 20 Hadleys. Much of that negative tendency was a response to the removal of westerly wind flow by the mountains, particularly East Asia. The GWO signal was weakly in phase 2 (5-day average), after its circuit into phase 5 roughly two weeks ago. Even though the global AAM signal is relatively “benign” (near climatology), zonal mean anomalies remain robust. Strong negative anomalies (~1-2 AMUs), due to intense easterlies, dominate the subtropical atmospheres flanked by positive midlatitude AAM anomalies. Positive zonal mean AAM anomalies are also present in the equatorial band with a contribution coming from above average upper tropospheric westerly wind flow (~5 m/s at 200mb). This type of inter-hemispheric meridional symmetry of zonal mean AAM anomalies is a signature of the on-going strong tropical convective forcing and their distribution is generally consistent with the La-Nina composite.

Animations of upper tropospheric daily mean vector wind anomalies for about the last week loosely show stationary twin subtropical anticyclones (cyclones) around 140E (east of the date line). The stationarity of these wind flow anomalies at least partly explains the recent MJO behavior indicated by the WH(2004) phase space plot. The expected amplification across the PNA sector discussed in our 10 January posting can be seen. There are both meridionally and zonally oriented Rossby wave energy dispersions (RWDs) associated with dynamical processes explained by both the MJO and GWO. However, unlike around 1 December 2007 and also different than earlier thought, the circulation amplification has occurred in MJO phase 7, not phase 8.

Animations of additional fields including mean sea level pressures and surface temperatures show a significant cold air surge coming off of East Asia into the northwest Pacific at the time of this writing (17 January). I think, when updated, there will also be a strong positive mountain torque as a response to a ~1045 mb anticyclone east of the Tibetan Plateau associated with this cold outbreak. In fact, the R1 data AAM plots updated through 14 January “moments ago” support the assertion of a positive East Asian mountain torque. One consequence of this synoptic evolution will be an intensification of the East Asian jet (250mb anomalies ~40m/s 17 January) that will lead to additional amplification of the ridge near 140W. I also speculate (which can be verified observationally) that feedback processes from this synoptic evolution will lead to tropical convective forcing returning to the Indian Ocean weeks 2-3.

Summarizing, the eastward shift of the MJO tropical convective forcing into the west central-South Pacific Ocean has destructively interfered with the La-Nina atmospheric circulation by adding global westerly wind flow. The onset of the lower 48 states cold regime is a direct response. Once the tropical forcing returns to the Indian Ocean-Indonesia region (~100-140E), constructive interference with La-Nina is probable as easterly wind flow anomalies are added. The GWO should then orbit back to phase 3 (GSDM Stage 1) and current circulation anomalies across the PNA sector are probable to retrograde. In fact, latest week-2 ensemble means from various operational weather centers are now generally predicting this response as their initial conditions become more representative of the on-going circulation. However, timing and details of synoptic evolution are always unclear, especially now given the west Pacific forcing discussed above. One can only offer probabilistic statements based on signals like those given by the MJO and GWO (which considers the MJO), as well as the numerical models.

In the face of increasing uncertainty, we are still staying the course in regard to the North American outlooks discussed 5 and 10 January. Amplification across the PNA sector including the east Pacific Ocean ridge has already occurred (this possibility was first discussed on the 15 December 2007 posting, well before the models “caught on”). Additional amplification of this pattern is likely during the next several days, and most models currently predict this. We think these anomalies are probable (synoptic details are unclear) to shift west by ~late week 2/week 3 possibly leading to Alaska blocking tied to an anomalously strong north central Pacific anticyclone and cold troughs that would impact the USA west coast, before coming inland.

Weather ramifications of this pattern for the lower 48 states have already been detailed in past discussions. The Aleutian Islands of Alaska may get hammered by the strong ridge building cyclones mainly week-1 while Kona lows may become a concern for the Hawaiian Islands after week-1. In general, a west-southwest flow storm track across the central part of the country downstream from western USA troughs and upstream from a deep southeast states anticyclone, consistent with GWO phase 3 “amplifying” La-Nina, is probable for the next several weeks (~3-6 weeks). Most of the country may feel significant impacts (severe local storms warm sectors, blizzards cold sectors, etc.) from what could be a severe winter weather regime at times.

Internationally, the current westerly wind burst across the South Pacific Ocean will maintain a tropical cyclone hazard (possibly severe) for the paradise islands most likely week-1. That same but much lesser concern cannot be ruled out for locations around the Philippines. Portions of Brasil and South Africa may also experience enhanced frontal thunderstorm activity weeks 1-2. As moist tropical convective forcing returns to the South Indian Ocean-Indonesia regions weeks 2-3, the risk of tropical cyclones is probable to increase around the region of Madagascar. This may be start of MJO #3 for the 2007-08 boreal cold season.


An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (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 t0 WH(2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.

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. Given travel, I am unclear when my next posting will be. Hopefully I will be able to post a discussion the weekend of 26-27 January. Please stay tuned.

Ed Berry

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