Saturday, July 12, 2008

Another Weather-Climate Crossroad

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

Following the lead started last week, please keep in mind the ESRL/PSD GSDM web link, below, while reading this discussion.

Below are the links to various sources of global SST information. Overall, any interannual component of variability involving the SSTs is still unclear. There are some aspects similar to a year ago appearing such as the loose spatial pattern of anomalous warm-cool-warm of equatorial/tropical SSTs from the Indian Ocean into the west central Pacific Ocean. There has been little change since at least a week ago. (note the initial projection) (link 18)

Full disk satellite imagery and other tools used to help understand evolutionary processes still indicate multiple regions of enhanced tropical convective forcing. Similar to a week ago, they include the equatorial Indian Ocean into the Eastern Hemisphere monsoon systems and the eastern Pacific Ocean ITCZ into the Americas. Some important differences, however, are increased suppression over the west central Pacific Ocean and a significant increase in coverage and intensity along the equator over the Indian Ocean. In fact, latest 3-day averaged OLRA for the latter are ~minus 70-90 W/m**2. This increase of Indian Ocean forcing was expected a week ago, linked to the dynamics of complicated Rossby wave energy dispersions (RWDs) arcing through the extratropics, particularly the Southern Hemisphere (more said below).

As I have been also expecting to happen for at least the past 2 weeks, global relative AAM is decreasing. In fact, ~7 July the calculated relative AAM tendency was ~minus 30 Hadleys. This was one of the largest negative magnitudes since December 2007. In contrast to the computed tendency (GWO2), the latter takes into consideration the zonal mean AAM transport signal. Updated through 10 July relative AAM was roughly 1 AMU below the R1 data climatology with the calculated tendency of ~minus 10 Hadleys.

While not feasible to discuss the details here, from daily monitoring and working to have a better understanding of the dynamics involved, I feel fairly comfortable offering attribution to this recent AAM behavior. Much of this removal of global westerly wind flow has been through frictional dissipation of poleward and downward propagation of zonal mean westerly wind flow anomalies into the midlatitude storm track regions for at least the past several weeks. That has been especially true of the Southern Hemisphere, given austral winter, having zonal mean negative frictional torque anomalies of more than 2 Hadleys ~50-60S contributing to a global frictional torque of ~minus 20 Hadleys around 1 July. Interesting question to ask is if this behavior is a loose mirror image to what occurred last February for boreal winter? The point is this type of understanding helps to anticipate subseasonal behaviors including the MJO, as part of the GWO. All numerical models will do generally poorly with these types of processes even after a few days into their prediction cycles.

Updated through 10 July, both the WH (2004) and WB (2008) measures of the MJO and GWO respectively had similar locations in phase space. Both were in approximately octant 1 having projections greater than 1 sigma. I think there is good coupling between the tropics and extratropics, including (again) arguably meridional symmetry of zonal mean zonal wind anomalies. As bands of zonal mean westerly wind flow anomalies shift poleward through the extratropical atmospheres, easterly wind anomalies are increasing in the tropical and subtropical atmospheres. This all goes “hand in hand” with the MJO dynamical signal of tropical convective forcing starting to return to the Eastern Hemisphere, particularly the Indian Ocean.

Referring to the GWO and MJO composites for ~250mb streamfunction (psi) and 2m air temperature, the ongoing progressive synoptic trough currently in the central USA and the model predictions of another for the USA Pacific Northwest are consistent with roughly phases 1-2. Most week-2 ensembles were predicting the current trough to dig into the western USA, which did not occur (another issue).

While unclear about the time scale, I do think the spatial patterns depicted by especially GWO phases 3-4 is where the global circulation is going. My suspicion for the latter is ~weeks 3-4 (by early August). Until then, the patterns illustrated by MJO phases 2-3 may be the best offering of a prediction roughly weeks 1-2. The bottom line is I still have my concerns for some rejuvenation of the low AAM base state going into the upcoming boreal cold season. Obviously exact weather ramifications are unclear. Statements like, “ENSO neutral with lingering La-Nina circulation”, while perhaps currently true, tell you nothing about the critically important global subseasonal dynamics (as measured by the GWO).

The probable USA temperature patterns per above can be seen from the 2m air temperature composite signals. Again, remember these are composites and individual cases can be much stronger. Locations periodically focusing on the Northern Plains into the Ohio Valley are the most probable storm track for MCS activity. That includes more heavy rainfall for locations such as Iowa and the Upper Mississippi Valley.

Intense to severe tropical thunderstorm activity is probable to increase from the equatorial Indian Ocean into the monsoon systems and frontal bands of India and Asia during the next few weeks. The west and northwest Pacific Ocean will remain a “wild card”, including the possibility of tropical cyclone spin-ups threatening the Philippines and Southeast Asia. The East Pacific Ocean tropical cyclone hazard should wane by week-2.

Anomalous upper tropospheric twin tropical anticyclones are well defined across the Western Hemisphere, while they slowly propagate through Africa into the Indian Ocean. This situation is favorable for additional strong African easterly waves. 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 introduces the GWO. A pdf version can be downloaded from the following link

Overlapping seasonally varying subseasonal composites for numerous variables including 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. An update is unlikely next weekend, 19-20 July. I hope to do another discussion ~23 July.

Ed Berry

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