Saturday, August 30, 2008

Season of the Witch -- Part Deux???

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. There have been on-going map room issues causing missing data and unrepresentative plots. Again, this effort is a work in progress with extremely limited resources.

No large changes to the global pattern of SSTs have occurred since my 23 August discussion. The warmest waters (including aerial extent) continue across the Tropical Northwest Pacific Ocean (TNWP) having totals approaching 31C (~1-2C positive anomalies) east of the Philippines.

An equatorial trade wind surge during July initiated an upwelling oceanic Kelvin wave leading to the negative equatorial subsurface anomalies greater than 3C at roughly 150m depth/150W per TAO buoy data. Another strong trade wind surge (~5m/s 15N-S) is occurring west of the Dateline. Whether or not another upwelling Kelvin wave is generated is unclear. As discussed below, there is some evidence of our low AAM “La-Nina” base state returning. (note the initial projection) (link 18).

Full disk satellite imagery and other tools indicate a strengthening signal of tropical convective forcing centered on the Western Hemisphere and Africa into the Indian Ocean. With the all important interannual component left in, since about 6 August there has been ~2 sigma MJO projection in octant 2 of WH (2004) phase space. Phase 2 of the MJO 250mb psi composite anomaly represents the current tropical circulation anomalies, including a favorable environment for Atlantic Ocean tropical cyclogenesis. Some eastward propagation of this MJO signal has occurred during the last week or so. In fact, tropical convection has been moving east along the equator across Indonesia at ~7m/s. However, having my reasons, I am suspicious the latter is only a transient component within a generally quasi-stationary situation of (extremely complicated) tropical forcing.

Global relative AAM (updated through 28 August) is a good 1 standard deviation below the R1 data climatology, taking out the westerly phase of stratospheric QBO (not to minimize any importance of the latter). After the respectable clustered (mostly East Asia and Antarctica) positive global mountain torque ~11 August, extratropical Rossby wave trains interacting with tropical forcing and mountains have worked to shift zonal mean easterly wind flow anomalies poleward. That has been particularly true for the northern subtropical atmosphere. Hence a zonal mean AAM sink of ~6 Hadleys has appeared ~40-45N, the largest for the Northern Hemisphere extratropics in at least a couple of months.

Synoptically a Northern Hemisphere response has been a “Branstator (2002) like” circumglobal teleconnection of anomalous midlatitude ridges during the last several days. This was the pattern observed, on average, during the boreal 2007-08 cold season, and is typical of La-Nina (defined globally, not just from Nino 3.4!!!).

While the WB (2008) GWO has been orbiting in phase space from 8-1 to 4-5 with approximately a 10-20 day periodicity, there has been a subtle drift toward octants 8-1-2-3 over the last 90 days. That is consistent with the WH (2004) signal of the MJO shifting ~2 standard deviations in phase space toward the western Indian Ocean during this same period. The latter suggests a resumption of tropical-extratropical coupling favoring a return to a La-Nina global circulation.

The bottom line is I think we have an important weather-climate signal to offer at least reasonable week 1-3 predictions. Whether or not La-Nina returns in some form is unclear, especially since there are endless scenarios I can offer to the contrary. However, this is an example of the serious rigorous daily subseasonal monitoring issues that exist, utilizing the WB (2007) GSDM and WB (2008) GWO frameworks.

Having a bit more confidence than a week ago, rapid global circulation variations given by phases 8-1-2-3 of the 250mb composite anomaly psi from the WB (2008) GWO are most probable weeks 1-3. These will be superimposed upon a slower evolving base state shown by phase 2 and “eventually” phase 3 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.

All numerical models have captured an extratropical response consisting of Rossby wave energy dispersing (RWD) from Indian Ocean tropical forcing. Consistent with phase 1 of the WB (2008) GWO 250mb psi composite anomaly plot, western USA troughs are probable week-1. As shown by the GWO phase 2-3 composites, ridging may return to the west coast while troughs amplify across the central states ~weeks 2-3. I would be very careful with the predicted synoptic details of any ensemble numerical model after especially day 5 in this weather-climate regime.

Obviously extreme weather impacts for the USA week-1 are centered on the landfall of severe tropical cyclone Gustav and possibly tropical cyclone Hanna (and others?) going into weeks 2-3. Please see all official statements from the Tropical Prediction Center. I think the North Tropical Atlantic Ocean will remain favorable for additional cyclone development through at least week-2 (above climatology). Substantial precipitation is likely from the Rockies into the Plains and Mississippi Valley week-1, shifting eastward weeks 2-3. I think the notion of some model forecasts to drift the remains of Gustav as far north as Iowa late next week are not unreasonable.

More generally, I think it is much too early in the seasonal cycle to “lock in” to a Branstator (2002) circumglobal pattern of anomalous midlatitude ridges (including the central North Pacific Ocean). However, should it become clear that a rendition of a low AAM base state may persist through boreal winter 2008-09, anomalous western and central USA troughs may be probable starting around November-December. These may have an Arctic cold air source to work with, along with more subtropical westerly wind flow, latter compared to last winter.

Intense to severe tropical thunderstorm activity is probable to continue from the central and eastern equatorial Indian Ocean into Indonesia and the monsoon systems week-1. Unlike the past 2 subseasonal events, this area of rainfall is probable to stay farther south, consistent with the seasonal cycle. How far east the enhanced tropical convection coherently moves is unclear. However, a transient component impacting the west central and TNWP is a possibility anytime until further notice, including at least hybrid tropical cyclones. There may be a reduced risk for significant East Pacific Ocean tropical cyclone development at least week-1, while the tropical Americas have generally climatology for rainfall. Finally equatorial Africa is likely to remain wet with intense easterly wave activity at least weeks 1-2.

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 (high impact weather events are continuing).


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 (WB (2008)). 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, ~6-7 September.

Ed Berry

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