Nemesis

The spatial distribution of global tropical SSTs has not undergone any huge changes since a week ago. The La-Nina pattern is in place along the equator with ~minus 1-2C anomalies from about the date line to South America with the coolest around 125W (including Nino 3.4). These anomalously cool ocean waters extend to roughly 200m (TAO data) with magnitudes less than minus 3C near 100m/140W. Increased trades during the last week or so have intensified the negative SST anomalies along the equatorial Pacific Ocean cold tongue.

Across the Eastern Hemisphere I am getting a bit concerned about an emerging recent familiar spatial SST pattern, in this case featuring a warm Indian Ocean-cool Indonesia and very warm west central/northwest Pacific Ocean. SST totals across the equatorial Indian Ocean are ~29C and in excess of 30C over the Tropical Northwest Pacific (TNWP). This has some similarities to what was observed about a year ago, and may signal the return to 2 (at least episodic) regions of Eastern Hemisphere tropical convective forcing; the Indian and west central Pacific Oceans. As I have previously stated, this is a nemesis going back to the 2001-02 boreal cold season (global warming SST signal?).

During boreal autumn 2006 the Indian Ocean tropical convective forcing dominated and contributed to putting the kibosh on El-Nino ~ 1 December. Will the warm west central Pacific Ocean tropical forcing start to dominate going into 2008? My own speculation is to have that concern (read between the lines!). In any case, La-Nina SSTs and a coupled circulation response (~GSDM Stage 1.5) are probable the rest of this year.

Weekly averaged anomalies of outgoing longwave radiation (OLRA) present a nice signal of enhancement ~10N/60E and ~10N/140E, with suppression in between. The stationary component is the former while the latter has been forced by dynamical processes linked to the extratropics. Specifically, the Global Wind Oscillation (GWO) has done a reasonable job of capturing recent circulation impacts linked to a strong positive global mountain torque (~ plus 30 Hadleys; mostly Andes and East Asia) just prior to 1 September. Updated through 7 September the GWO was ~ GSDM Stage 1.5. Any MJO signal is weak and incoherent.

These complex interactions involved a chain of events leading to a strong inter-hemispheric symmetry of Rossby wave energy dispersions from the west central Pacific. Shown in weekly mean anomalies of 250mb vector winds (for example), strong ridges developed across eastern Alaska and west of South America. Wind flow anomalies were ~20-30m/s. This evolution led to cold regimes across the central USA and central South America, and the numerical models did not predict any of these behaviors very well for week 2.

Full disk satellite imagery and other monitoring tools have shown the TWNP tropical forcing to generally shift northwest with at least a couple of tropical cyclones during the last week. The Indian Ocean forcing has drifted to ~10N/80E and may consolidate with the convection to the east, typical of La-Nina. Global relative AAM remains ~minus 2 Atmospheric Momentum Units (AMUs, using the mks system) per R1 data, having decreased a bit during the past few days. I think the GWO is drifting back toward GSDM Stage 1, particularly since the global mountain torque and AAM tendency was weakly negative ~minus 10 Hadleys through 11 September, and the AAM transport signal is again directed poleward.

There is some evidence of the strong anomalies of deep zonal mean easterly wind flow to have propagated poleward from the equator into the subtropical atmospheres of both hemispheres since ~10 August. The recent TNWP convection did contribute, with, in fact, ~5m/s at 200mb westerly wind flow anomalies developing along the equator. A key circulation response to La-Nina is for these zonal mean easterly wind anomalies to propagate off the equator. Perhaps this process may occur with a series of subseasonal events. However, already starting to observe this response expresses a concern for this cold event to peak during boreal autumn.

Animations of upper tropospheric daily mean vector wind anomalies present a reasonable signal of twin tropical/subtropical anticyclones near 90E with twin cyclones near 150E. These circulation gyres are directly linked to both the Eastern Hemisphere tropical convective forcing and PNA/PSA RWDs discussed above. There has been a westward shift of all the above for the past few days, meaning the next trough to impact North America will maximize in the region of the west coast. This is consistent with the GWO, and all numerical ensembles forecast this general idea for weeks 1-2.

I think readers should be familiar with USA weather impacts due to a western states trough and eastern USA/Deep South ridge (understanding the seasonal cycle). More strong/severe baroclinic storms developing off of East Asia are probable to impact Alaska and the Gulf of Alaska periodically for at least the next couple of weeks.

Particularly with the recent meridional propagation of zonal mean easterly wind flow anomalies, our base state is very much susceptible (probabilistically) to lower and midlatitude anticyclonically wave breaking (AWB) lows. One of these has been limiting the development of Tropical Cyclone Ingrid across the western Atlantic (more said below). I think the models have the right idea of an AWB low along the USA west coast later week 1 only to eject into the Plains week 2 (linking with a subtropical jet emanating from the twin 150E tropical cyclones). I think seasonal transition to the boreal cold season will limit western USA troughs until the November-December period. Should our nemesis become the rule, perhaps GSDM Stage 4-1 may start out the boreal cold season leading to GSDM Stage 3-4 if an El-Nino starts to develop during winter-spring 2008.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. A week ago I tried to be favorable for the notion of Atlantic tropical cyclogenesis. Given what I have been monitoring and writing in these postings all summer, what was I thinking??? AWB, which has been a limiting factor for Atlantic tropical cyclogenesis all summer, has only intensified during the past week. The ~20-30m/s upper tropospheric equatorial westerly wind flow anomalies across the Pacific downstream from the twin tropical cyclones appear to heading into central South America, apparently having limited local impact across the tropical Atlantic Ocean. This situation of AWB needs to change if additional true tropical cyclones are to develop in the Atlantic basin (above climatology). For the USA, the concern may have to focus on hybids like Gabrielle and Humberto.

The TNWP is probable to continue with a tropical cyclone hazard week 1 hopefully relaxing week 2. Impact areas include the Philippines north into at least Japan. There is evidence that the Eastern Hemisphere convection is starting to drift south, meaning locations from India into Southeast Asia are probable to continue with rounds of intense/severe thunderstorms through week 2. Finally, I can also see some concern for 1-2 strong/severe extratropical cyclones to slam at least northern Europe weeks 1-2.

Appendix

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

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

http://www.cdc.noaa.gov/map/images/gcm/gsdm_40d.jpg

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 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 another discussion ~ 21 September.

Ed Berry