Saturday, September 01, 2007

To be, or not to be La-Nina

Per NOAA/CPC SST plots and NOAA/PMEL/TAO buoy data; global tropical SSTs are slightly warmer than average across the Indian Ocean while significantly above normal across much of the west Pacific Ocean warm pool. Anomalies and totals are around plus 0.5-1C and 28-29C for the former and ~plus 1-2C and greater than 30C in region of 5-10N and 150E. Cooler than normal SSTs have become more coherent spatially across the equatorial central and east Pacific east of the date line, with anomalies as low as less than minus 2C ~100-120W extending to depths ~200m. Given that the seasonal cycle favors cooling, these anomalies are not trivial. The Caribbean into the tropical Atlantic Ocean remains near to slightly warmer than climatology.

So, is La-Nina becoming better defined? As stated previously, the global circulation is strongly La-Nina like with global relative AAM currently ~3-4 Atmospheric Momentum Units (AMUs ) below the R1 data climatology while the SSTs have only weakly supported that response. Since about January 2007 Hovmoller plots of OLR/A show a nice signal of tropical convective forcing consolidating across the Eastern Hemisphere ~90-120E. There were 3 weak northeastward propagating subseasonal (~20 to 30-day) variations (weaker in succession) of this tropical forcing since about 1 June. I even speculate that this whole situation started ~ 1 December 2006 when dynamical feedbacks from the atmosphere put the “kibosh” on what was supposed to be a basin-wide warm event. The atmosphere has since been trending toward La-Nina, seemingly followed by the tropical convective response and maybe finally the SSTs. There has been a surge of above normal trades during the last couple of weeks as a response to the Eastern Hemisphere tropical convection helping to cool the SSTs across the Nino regions.

At this point I think La-Nina circulation conditions (GSDM Stage 1) are highly probable through boreal fall into at least early winter (DJF). However, suppression across the west central Pacific Ocean is only going to favor expanding warmth west of the date line. These warm anomalies are also probable to remain very deep, to at least 400m. It will be interesting monitoring the tropical convective forcing this winter as the seasonal cycle makes its contribution. In the meantime, going into the boreal cool season the odds may slightly tilt toward a significant cold regime at times, focusing on the western 2/3rds of North America along with an active southwest flow storm track across the USA Plains.

GSDM Stage 1 continues to reign supreme. Full disk satellite imagery present a nice signal of tropical convective forcing centered ~10N/100E, with some enhancement across north equatorial Africa and flare-ups around the Americas and Tropical Northwest Pacific (on-going tropical cyclones understood). Animations of daily mean upper tropospheric vector wind anomalies show an emerging response of twin tropical/subtropical anticyclones ~90E with compensating cyclones just east of the date line. In fact, the combined distribution of anomalous tropical easterlies from the Indian Ocean into the Africa, westerly wind flow anomalies just east of the date line and OLRA have led to a decent MJO projection per Wheeler and Hendon (2004) phase space plots. A coherent Rossby wave energy dispersion from this tropical forcing and circulation response nicely hooks up with yet another sickening wicked Devil ridge across the western and central USA (even in a weekly mean).

As discussed above, global relative AAM is ~3-4 AMUs below the R1 data climatology. Much of that signal is still coming from strongly anomalous deep zonal mean easterly wind flow across the Southern Hemisphere tropical atmosphere with, for instance, ~5-10m/s magnitudes at 200mb. The last subseasonal event of tropical forcing did allow some of the anomalous zonal mean easterly wind flow to propagate off the equator into the northern subtropics to ~30N. In fact, even weak zonal mean westerly wind anomalies have appeared ~15N. I think what the RMM plots are showing as a MJO is in reality another 20-30-day subseasonal variation similar to the past 3. As of 30 August the global mountain torque has spiked to ~plus 30 Hadleys contributing to the plus 20 Hadley AAM tendency. I think there will be another orbit in GWO quasi-phase space just like that seen about 10-15 days ago. In other words, another perturbation onto the GSDM Stage 1 base state is probable during the next couple weeks.

For the Pacific-North American sector the above reasoning suggests an eastward shift of the coherent Rossby wave train already discussed. My own feeling would be for at least a couple of troughs to dig into the western USA, with the strongest during week 2. This pattern should then shift east and decay in amplitude, particularly in the presence of seasonal transition to fall. Hence a trend toward cooler and wetter is probable for the west and central USA, perhaps targeting the northern and central Plains week 2. Numerical ensemble prediction schemes from international weather centers are coming into better agreement on this notion, which was first suggested in my posting about a week ago for this forecast period. It is likely deep tropical moisture transport from both the east Pacific and Caribbean will be involved with these precipitation events. Locations around Alaska are going to be “close” to the amplifying ridge ~140-160W. However, storms coming off of East Asia are probable to impact that state weeks 2-3.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. I still see quite a bit of anticyclonic wave breaking activity across the North Atlantic Ocean allowing penetration of stable airmasses from the higher latitudes into the tropics. However, easterly wave activity is quite robust over Africa tied to an anomalously strong Tropical Easterly Jet, and there may be a couple of opportunities for hybrid systems to develop close to the USA during the next 2-3 weeks. In fact, what could become Gabrielle (Xena’s sidekick?) during the next few days may be currently festering midway between Africa and the Lesser Antilles.

Locations from the central and eastern equatorial Indian Ocean and India into China and Southeast Asia still appear likely to continue with the poundings of intense monsoon/frontal band rainfall this week into week 2. Perhaps the upcoming subseasonal event will be the last northward shift for this boreal summer. With the exception of Typhoon Fitow, much of the west central and northwest Pacific should have generally suppressed conditions for at least week 1. Equatorial North Africa should remain active for at least week 1 and perhaps a few more.


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 GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

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 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 a short update during roughly the middle of next week.

Ed Berry

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