Thursday, August 07, 2008

The Day the Atmosphere "Stood Still" ???

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 90-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.

The warmest SSTs globally are centered on the tropical northwest Pacific Ocean ~10-20N/150E having totals ~29-30C. The anomalous warm and cool horseshoes of SSTs remain across the Pacific Ocean basin, along with the positive anomalies across the equatorial east Pacific. Broadly similar to the last 2 boreal summers, a positive-negative-positive anomaly spatial SST pattern is present from the Indian Ocean to the west central Pacific Ocean. SSTs across much of the tropical Atlantic Ocean basin are well above climatology.

The general consensus in regard to the interannual component is “ENSO neutral”. Regardless of “labeling”, constant evolutionary dynamical processes involving all components of the weather-climate system continue. These include complex subseasonal processes, and will contribute to additional high impact weather globally. I do remain concerned that a renewed tilt toward a low AAM base state (a “weakened rendition” of the 2007-08 La-Nina) is the most probable outcome for at least the next 3-6 months (more said below). See the following links for SST details. (note the initial projection) (link 18)

Eastern Hemisphere enhanced tropical convective forcing has shifted well north, extending from approximately northern India/Pakistan (where flooding deaths have occurred) through southeast China into the northwest Pacific Ocean. The core of this activity is at ~15-20N/120E, with recent 3-day averaged OLRA less than minus 90 W/m**2 per BMRC. Tied to the Southern Hemisphere extratropics, flare-ups continue over the anomalously warm west central equatorial Indian Ocean. There has also been an increase of tropical convection along Pacific Ocean ITCZ as well as the Americas during the last few days.

WH (2004) MJO plots updated through 6 August, while showing a slightly less than 1 sigma projection in octant 2 of phase space without the base state left in, present a somewhat greater than 1 standard deviation signal with the interannual component. As was suggested by many MJO predictions in WH (2004) phase space at least a week ago, a westward shift of this tropical signal has occurred. I think much of the signal has come from the winds (tied to the processes explained by WB (2008) GWO).

In any case, not only must users carefully understand the derivation of the empirical WH (2004) technique and related predictive tools, but much more importantly the dynamics responsible for the recent behavior of this tropical forcing. The bottom line is I think coupling between the tropics and extratropics is still present meaning the now familiar low AAM quasi-stationary component to the global weather-climate situation may be (again) starting to dominate. Going into the boreal cold season, subseasonal processes may change this.

The enhancement across the Western Hemisphere can be attributed to faster processes such as convectively coupled Kelvin waves (per coherent modes Hovmollers) and those linked with the WB (2008) GWO. I do have more confidence relative to my 1 August posting this Western Hemisphere signal of enhanced tropical forcing will not be as robust as ~late May and early July.

Through 5 August global relative AAM (including the westerly phase of the stratospheric QBO) was slightly below the R1 data climatology with its computed tendency ~minus 10 Hadleys forcing the WB (2008) GWO to roughly octant 8-1 of phase space. Since approximately 20 July a relatively fast orbit has been occurring. One contributor has been the global mountain torque, with the positive East Asian component (~10 Hadleys) out of phase with the global signal. The latter is very complicated (but explainable), and has been a characteristic feature of our low AAM base state for the past several months. Regardless, I think the current Western Hemisphere tropical convective signal is partly a response to this fast WB (2008) GWO. The trough that most models predict to impact the USA during roughly days 4-7 is also a response to the fast WB (2008) GWO including the positive East Asian mountain torque.

Zonal mean westerly wind flow anomalies (~5m/s at 200mb) have shifted off the equator to roughly 15N, while 5-10m/s anomalous zonal mean easterly wind flow anomalies persist in the subtropical atmospheres. In fact, anomalous midlatitude ridges are again present across the global midlatitudes, having a zonally oriented character across the Northern Hemisphere. The latter is characteristic of a Branstator (2002) circumglobal teleconnection (and is also connected to the recent negative NAO phase – red noise!!!), as observed frequently during the boreal 2007-08 cold season.

Specifically, everything considered (this discussion is already too long!!!), WB (2008) GWO and WH (2004) MJO phase 3 streamfunction (psi) and 2m air temperature anomaly composites best describe the current global weather-climate situation. Animations of upper tropospheric daily mean vector wind anomalies show twin anticyclones becoming better defined in the region of Africa/Indian Ocean with downstream twin cyclones. This supports the notion of a low AAM quasi-stationary regime, as well as the composites I am “pointing to”.

Weeks 1-3 across the USA should see synoptic events featuring troughs digging into the middle of the country, then moving to the East Coast. Ridging is most probable in the region of the west coast. I think the 2m air temperature anomaly pattern given by WB (2008) GWO phase 3 may be the most representative, on average. However, there will be orbits in phase space, and keep in mind other GWO/MJO composites such as phases 2 and 4. Phases 5-7 are the least likely. Similar to the last few days, intense rainfall and MCS activity (including severe local storms) will continue to periodically impact much of the central and eastern portions of the country. I think the latest official week-2 forecast is too dry for much of the middle USA, especially the Upper Mississippi Valley and particularly around Iowa. The increasing subtropical westerly wind flow mentioned above supports wetness from the Desert Southwest into the Plains.

Should this low AAM quasi stationary component (key word) of the global circulation regime persist well into the boreal 2008-09 cold season, outcomes of USA temperature and precipitation anomalies having some similarity to 2007-08 are again possible. The barrage of western USA troughs may resume. One difference may be for wetness to expand farther south into the Plains given what may be increased subtropical westerly wind flow (I have my reasons).

Intense to severe tropical thunderstorm activity should increase across the equatorial Indian Ocean during the next 1-3 weeks while expanding northeast into the monsoon and frontal band systems. Given the very warm SSTs, the tropical northwest Pacific Ocean is likely to remain periodically active, including an elevated risk of tropical cyclones. In fact, both the Indian and west Pacific Oceans may be anomalously convectively active going into boreal autumn. Stay tuned to see if our "old nemesis of the new world atmosphere" returns. The central and east Pacific Oceans will have a tropical cyclone risk at least week 1, while the Atlantic basin may get into the act weeks 2-3 (Saharan dust understood).

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. 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, ~15-17 August.

Ed Berry

No comments: