La-Nina on Steroids; Severe USA Winter Weather Regime Coming???

Per latest 5-day averaged TAO buoy data, SST anomalies along the equatorial Pacific Ocean from 160E-South America remain strongly negative with magnitudes ~1-3C extending to ~100-150m depth. While significant cooling of SSTs has occurred during the last 1-2 weeks north of Australia due to recent intense convective rainfall, comparable warming has been in progress for the suppressed South Indian Ocean. Regardless of recent subseasonal weather events, well-pronounced mature La-Nina SSTs continue, as part of the global tropical SST system.

Although weakened, moist tropical convective forcing remains loosely consolidated across the Eastern Hemisphere centered ~0/140E. Per WH(2004) phase space plots; there is still a significant MJO projection, ~2 standard deviations located in phase 7. However, the eastward movement has again slowed. In fact, OLR/A Hovmoller plots suggest a portion of this tropical forcing has started to drift back to the west. Whether or not a weak dynamical signal will propagate into the Western Hemisphere is unclear. Recent full disk satellite imagery does show some enhancement of diurnal/frontal thunderstorm activity across tropical South America and perhaps South Africa. Although it is probable the MJO signal will re-emerge across the South Indian Ocean weeks 2-3, its exact path through phase space per WH(2004) methodology is unknown.

The MJO variability observed since boreal autumn 2007 is the strongest in at least 2 years per time series plots of the WH(2004) RMMs. What is so remarkable is to see this kind of tropical convective variability during a moderate-strong La-Nina, as is currently the case. Specifically, there have been 2 moderate-strong MJOs, including the on-going event. Depending on the MJO’s phase, these variations have alternately weakened or strengthened the La-Nina tropical convective forcing. As part of a shift by tropical convection to the west Pacific Ocean in late November, away from the typical La Nina location over Indonesia, a large amplification of the circulation occurred across the PNA sector. The response was an intense ridge from the east Pacific Ocean into Alaska and a deep western USA trough. These anomalies then retrograded as zonal mean tropical easterly wind flow anomalies developed, linked to a negative mountain torque and tropical convection returning to Africa and the western Indian Ocean.

Most recently, strong poleward fluxes of relative AAM ~35N during the last week of December dramatically increased the zonal mean westerly wind flow across the Northern Hemisphere midlatitudes. In fact, wind speed anomalies were ~70m/s at 250mb on 31 December 2007 north of a central Pacific Ocean anticyclone, the latter a response to the phasing of the MJO and the preferred region of La Nina convection over Indonesia. The point is through cooperative dynamical interactions involving the La-Nina base state, MJO variability and Rossby wave energy dispersions, the global circulation during the first week of January 2008 can be thought of as an “amplified La-Nina response”. Thus we attribute the west coast storm of ~3-5 January 2008 to “El-Viejo on steroids”!

Updated through 7 January, global relative AAM is near normal. In addition to the MJO, GWO processes such as possibly the strongest North American positive mountain torque of (~30-40 Hadleys) on record which occurred around 3 January also led to the AAM increase. Synoptically, the large and intense ~1045 hPa anticyclone that plunged across the Southern Plains and Mexico contributed to this torque. The rapid moisture return in the Plains in its wake was a needed ingredient to the severe local storm outbreak in that region a few days ago. Through eddy exchange processes linked to a negative global frictional torque (~10 Hadleys) and what will soon be a negative global mountain torque, AAM tendency is probable to become strongly negative again. In fact, the negative AAM tendency may be comparable to that observed in early December. We do expect global AAM to again drop substantially during the next few weeks, especially as subtropical zonal mean easterly wind flow anomalies remain strong. After a brief circuit through GWO phases 5-8, phase 3 is most probable by ~week 2. Again, timing is always an issue.

There is no change to the North American weeks 1-3 outlooks discussed 5 January. Similar to December 2007, a large amplification across the PNA sector is probable within the next 10 days. As more models are showing, ridge amplification from the east Pacific Ocean into Alaska with a downstream central/eastern USA trough is a likely outcome. We think these anomalies will shift west leading to Alaska blocking and a western states trough weeks 3-4. This will allow the delivery of Arctic air into the lower 48 states, along with increasing southwest flow storm track activity across the Plains. Later weeks 2-3 the west coast may again be hammered by cold troughs as circulation anomalies retrograde. Again, timing and details about the upcoming circulation evolution are still unclear. 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.

Internationally, at least enhanced possibly severe diurnal thunderstorm activity is probable weeks 1-2 for locations such as tropical South America and west central-South Africa. Tropical cyclone activity (including Elisa) should continue across the South Pacific Ocean mainly week-1. We think consolidation of tropical forcing should occur by about week-3 from the South Indian Ocean into western Indonesia. This may be start of MJO #3 for the 2007-08 boreal cold season.

Appendix

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

http://www.cdc.noaa.gov/map/clim/gsdm.shtml

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:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml

http://www.cdc.noaa.gov/people/klaus.wolter/MEI/

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. Please stay tuned.

Ed Berry and Klaus Weickmann