Saturday, November 08, 2008

"Season of the Witch Part Deux", for Real?

The views expressed are those of the author and do not necessarily represent those of the National Weather Service.”

Please keep in mind the ESRL/PSD GSDM web link, below, while reading this discussion.

The 91-day signal to noise ratio (snr) anomaly composites are now updated daily, centered on the date shown. Please see product descriptions. WB (2008), part-1 of a 2 part paper, where the GWO is formally introduced is in press for MWR publication. There is a link in the Appendix to download the manuscript.

As expected from any slow boundary forcing on the atmosphere, global tropical and extratropical SSTs have changed little from a week ago. The warmest ocean waters have shifted south of the equator to the region of Australia and west of New Guinea, ~140E. Totals are ~30C, with 29C SSTs still covering much of the TNWP. The anomalous warmth across the Indian Ocean has increased during the past week, with totals roughly 29C and anomalies ~plus 1C. The equatorial Dateline back to 150E has ~minus 2C SST anomalies, responding to trade surges.

An equatorial trade wind surge from the Dateline to Indonesia has resumed, with anomalies ~10m/s. Further, per latest 5-day averaged TAO equatorial depth sections, negative subsurface anomalies 3C and larger centered ~200m/160W have evolved, presumably due to an upwelling oceanic Kelvin wave. The bottom line is I think the “Nino SSTs” are now responding to the global La-Nina base state. This is further evidence that “ENSO neutral” is an inaccurate description of the global weather-climate situation. It is not a question whether “La-Nina” has “resumed”, but simply how intense “round two” will be. In fact, many well posed numerical and statistical models support the latter, as do more complete indices than “Nino 3.4”. Please see links below. (note the initial projection) (link 18).

I expanded a bit on the SSTs in an attempt to keep the rest of this discussion shorter. While in the minds of many the current weather-climate situation may appear confusing with (understandably) a significant noise factor, I do think non-trivial signals are coming back. My attribution comfort level remains okay. While the details during the last couple of weeks have not played out as “expected”, I do think the previous probabilistic predictive statements offered are loosely on track. However, my confidence in any outlooks discussed below has decreased, and also serves as an example that any good numerical model agreement does not imply high forecast confidence (yes, insert the Rottweiler!).

The MJO component of the Eastern Hemisphere tropical convective forcing came out to about 160E, then “fell apart”. Full disk satellite imagery and other tools show the tropical forcing consolidating around 0/120E while extending from the South Indian Ocean to the west central Pacific. Having my reasons, I think we are seeing a response to our La-Nina base state, including the trade surge per above. In fact, the latest WH (2004) measure of the MJO retaining ENSO shows ~1.5 sigma projection in octant 3 of phase space. Animations of upper tropospheric daily mean vector wind anomalies show tropical circulation anomalies that are reasonably consistent with ~phase 4 of the 250mb snr composite psi plots.

Phases 8-1 of the 250mb snr composite psi anomaly plots best describe the global extratropics. During ~mid October there was a large positive global AAM tendency forced by eddy poleward transports and surface torques. A response was to add weak zonal mean westerly wind flow anomalies to the subtropical atmospheres and decouple the WB (2008) measure of the GWO from the WH (2004) MJO measure. Tropical forcing then intensified across the Western Hemisphere, and recently left severe tropical cyclone Paloma in the wake.

Subseasonal dynamical processes explained by the WB (2008) measure of the GWO are not only currently working to decrease global relative AAM, but also push the global atmosphere back toward the La-Nina attractor. For instance, updated through 6 November the computed global AAM tendency was ~minus 20 Hadleys having zonal mean contributions from negative East Asian, North American and Andes mountain torques. The strong positive Coriolis torque and negative earth AAM tendency, working along with the above, have forced the GWO to orbit back to octants 8-1 of phase space. The latter terms, part of the earth-atmosphere AAM budget, suggests an intensifying Hadley cell leading to mass removal from the tropics.

Regardless of how confusing the above may appear to the reader, my point is to give some evidence that the tropics and extratropics may be getting back in sync shifted toward La-Nina. I think it is probable to for both the WB (2008) GWO and WH (2004) MJO to orbit in similar octants of phase space during the next few weeks. That is, initially octant 3 then perhaps 4-5. The corresponding snr composite anomaly plots for both the GWO and MJO should represent North American weather roughly weeks 2-4.

Sources of uncertainty include the likelihood of more western Pacific Ocean and Western Hemisphere tropical forcing, the seasonal cycle, and the inherent red noise of the GWO. In fact, some models of the WH (2004) MJO (La-Nina removed) suggest a “loop” into the Western Hemisphere weeks 1-2. In any case, I think our quasi-stationary global circulation component per above may dominate going into DJFM, resulting in at least some weather similarities to boreal winter 2007-08 including the USA.

It is possible that the “8-1 evolution” of the global circulation discussed in past postings may be occurring as I type. Regionally for North America, the Pacific Ocean jet is extended leading to a split flow storm track. As tropical-extratropical coupling occurs, this jet should retract leading to a spatial pattern similar to phase 3 of the WB (2008) GWO 250mb snr composite psi and 2m air temperature anomaly plots by ~weeks 2-3. That is suggestive of troughs focused on the Plains, after initially impacting the western states. On average, this situation shifts west heading into boreal winter. Whatever the details, there may be weather concerns for much of the country during Thanksgiving travel. Additionally, there is no change to the “December cold scenario” discussed in the last few postings.

Internationally, intense to severe thunderstorm activity is likely to remain focused on Indonesia (~120-140E) weeks 1-2 while locations including Southeast Asia and portions of the TNWP also have weather impacts. The on-going trade surge should limit tropical cyclone activity; however, hybrids could affect locations such as the Philippines. While currently convectively suppressed and would be expected to remain so, flare-ups particularly across the South Indian Ocean are possible “anytime”, including a “rogue” tropical cyclone for locations such as Madagascar. No more than the typical diurnal thunderstorm activity appears probable for regions such as Brasil. Rainfall may also be quite intense at times for equatorial Africa (shifting south).

Broadly, going into boreal winter enhanced rainfall appears probable to have a quasi stationary component ~120E while shifting slowly south. The warm west central and southwest Pacific Ocean will remain the “wild card”, as there will likely be severe convective flare-ups including cyclones in those regions. The role of the MJO is unclear, and there will be tropical impacts from at least rapid ~10-20 day GWO variations that can only be captured from daily monitoring.

Please see the latest official tropical cyclone forecasts for all basins. I trust the expertise of the appropriate meteorological centers to alert the public of additional weather hazards worldwide.


Links to CPC and PSD ENSO discussions:

The following is a link to information about the stratosphere and other nice monitoring tools:

The following is a link to NCEP model verifications (surf around for lots more) .

The following is a link discussing recent global weather and related events

These are probabilistic statements. We hope that an opportunity will arise for us (soon) to allow our dedicated web page effort to mature, expediting objectively and accountability. This web page effort will hopefully include an objective predictive scheme for the GWO with hindcasts.

The WB (2007) paper on the GSDM has been published in the February issue of MWR. The first of a two-part paper, where WB formally introduce the GWO (WB (2008)), has been accepted for publication MWR. A pdf of the in press version can be downloaded from the following link:

In addition to the subseasonal snr composite anomaly plots, we hope near real-time discussions with “weather maps” will become a routine part of the ESRL/PSD GSDM web site sometime soon. Part-2 of our paper will discuss the latter. We want to emphasize notions such as global-zonal mean-regional scale linkages as well as forcing-response-feedback (with subsequent interactions) relationships. An important purpose is to provide a dynamical weather-climate linkage framework to evaluate the numerical models in a sophisticated manner as part of a subseasonal (and any time scale) forecast process, in addition to a climate service for all users. Relying on the numerical models alone is a cookbook! I plan on posting a discussion the weekend of 14-16 November.

Ed Berry

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