Saturday, August 23, 2008

Ressurecting the Atmosphere from the Noise

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 anomaly composites are now updated daily, centered on the date shown (see product descriptions). Stay tuned as our work slowly moves forward. Again, this effort is a work in progress with extremely limited resources.

Global tropical SSTs continue to exhibit spatial patterns that cannot be summarized by relatively trivial indices such as Nino 3.4, IOD, PDO (which also considers the extratropical North Pacific Ocean), etc. Anomalous warmth (~1-3C) broadly exists in locations such as the equatorial Indian Ocean, west central and northwest Pacific Ocean, equatorial east Pacific and finally the areas of the Caribbean into the Atlantic Ocean. Cool anomalies of similar magnitude are present around Indonesia and the equatorial Dateline. The warmest SSTs globally (totals ~30C) having the largest areal extent are still across the Tropical Northwest Pacific Ocean (TNWP).

It is not the point of these discussions to offer SST attribution (see links below). However, the above summary is given to emphasize that atmospheric responses to global tropical oceans constantly impact locations such as the USA. Statements such as “ENSO neutral” are extremely misleading, and not scientifically complete. In fact, weather-climate linkage signals involving the global tropical SSTs may be rising out of the noise as I type. (note the initial projection) (link 18)

I want to follow-up from the 15 August posting. The weather-climate situation remains tremendously complicated and difficult to write about in a focused manner. However, for the first time in at least the past 1-2 months, tropical convective forcing has been slowly intensifying farther east across the anomalously warm equatorial Indian Ocean during the last week. Per full disk satellite imagery this region of enhanced tropical rainfall is located ~60-100E, having 3-day OLRA ~minus 30-50W/m**2. Other pockets of enhancement remain across the TNWP and the Americas.

Regardless of all the “wandering” details, there is evidence that a subseasonal event is developing, including a MJO component. WH (2004) plots updated through 21 August show a significant projection of ~1.5 sigma in octant 2 of phase space. Leaving the interannual component in, the projection is roughly 2 standard deviations in octant 2, not trivial. The phase 2 MJO streamfunction (psi) composite anomaly per GSDM web link strongly represents the global tropical wind circulation. Repeating from a week ago, there has been a decided shift toward Indian Ocean tropical forcing in WH (2004) phase space. As pointed out by Weickmann of the WB (2008) GWO, the MJO projection, a component to the GWO, is increasing as the interannual portion slowly weakens.

To fully appreciate the recent behaviors to the WB (2008) GWO, in addition to carefully understanding some of the dynamical principles discussed in our submitted paper, rigorous daily monitoring has been critical. Loosely, faster time scale processes due to midlatitude mountains and baroclinic eddy Rossby wave energy dispersions (RWDs) have contributed to ~10-20 day orbits in GWO phase space (recall 15 August posting). These circuits have been from octants 4-5 to 8-1, with a “drift” toward low AAM.

Taking out the westerly QBO phase of the stratosphere, updated through 21 August global relative AAM has dipped to ~1.5 standard deviations below the R1 data climatology. Not only has anomalous zonal mean easterly wind flow (~5-10m/s at 200mb) returned to the tropical and subtropical atmospheres, there is evidence of poleward propagation. Furthermore, anomalous zonal mean westerly wind flow on the poleward flanks of these easterlies has brought back our familiar pattern strong midlatitude ridges, including the central North and South Pacific Oceans.

So, lets try to put all the above together. It is too early and uncertain for me to say subseasonal tropical-extratropical coupling involving the MJO and GWO is starting. However, even though the time scale is unclear, my suspicion is yes (per above), and stay tuned. A few of the serious subseasonal monitoring issues include: 1) if we are starting some rejuvenation of our low AAM base state similar to the boreal 2007-08 cold season, 2) if we are seeing the first of several coupled subseasonal events that may eventually shift into the anomalously warm west central Pacific Ocean, perhaps initiating a warm event (El-Nino), and 3) the weather-climate situation remains in a sea of noise. Whatever happens, there will be upcoming high impact weather globally, and stating “ENSO neutral” does not “cut it”.

Until I get impressed, my card will be to stick with the notion of a weakly low AAM base state having superimposed rapid GWO variations. Unlike my previous discussion, however, I do want to take into consideration the possibility of a subseasonal event for week 1-3 predictions, which includes an east-northeast shift of the Indian Ocean tropical convective forcing.

For weeks 1-3, rapid global circulation variations given by phases 4-5 and 8-1 of the 250mb composite anomaly psi from the WB (2008) GWO are most probable. These will be superimposed upon a slower evolving base state shown by phase 2 and eventually phases 3-4 of the WH (2004) MJO anomalous psi composites, constructively and destructively interfering. The probable USA temperature outcomes can be seen from the corresponding temperature composite anomaly plots, as well as the tropical convection from the composite OLRA plots. Obviously this is a very low confidence outlook.

Numerical model solutions for the USA have been inconsistent and varied for the next 1-2 weeks. Not only will model performance suffer going into boreal autumn, the weather-climate situation discussed above will also contribute (another item on list of work for WB to show). Broadly, the barrage of troughs digging into the Rockies and Plains then shifting east has started. Individual synoptic details beyond ~day 3 in this type of circulation regime are white noise, and attempts to predict them (for the public) is a waste of resources as well as scientifically indefensible. Skill is derived from statements of probability.

Should an eastward shifting subseasonal event including the MJO component occur, a more persistent and deeper western USA trough shown by MJO psi composite anomaly phase 4 may occur late week-2 into week-3. In addition to intense/severe precipitation including thunderstorms for the central USA, early season heavy snow may occur for portions of the northern Rockies during the ~second-third weeks of September. Reiterating from my previous discussion, western and central USA troughs with subtropical westerly wind flow may be the most likely outcome going into boreal winter (having my reasons).

Intense to severe tropical thunderstorm activity is probable continue across the equatorial Indian Ocean week-1 shifting east and north into the Indian and Asian monsoon systems while also impacting portions Indonesia by week-2. It is not out of the realm of possibilities for this intense thunderstorm activity to impact the TNWP by week-3. Regardless, the west central and northwest Pacific Ocean will be a wild card for numerous reasons until further notice, including the risk of tropical cyclones. The tropical Americas and North Atlantic Ocean appear to remain anomalously active week-1 and likely into week-2.

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 (there has been a recent increase).


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. In addition, the first of a two-part paper has been submitted to MWR where WB formally introduce the GWO. A pdf version can be downloaded from the following link:

Overlapping seasonally varying subseasonal composites for variables such as surface temperature, precipitation, geopotential height and streamfunction anomalies are planned on being posted on the web site mentioned above and presented in part-2 of our paper. 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!

Given shift work and travel, updates are extremely difficult. I hope to do another discussion next weekend, ~30-31 August.

Ed Berry

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