Saturday, October 11, 2008

Global Circulation Going Down with the Markets - 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 (snr) anomaly composites are now updated daily, centered on the date shown. Please see product descriptions. Part-1 of WB (2008), where the GWO is formally introduced, has been accepted for publication in MWR. Please see link in Appendix.

SSTs have continued to warm across the Indian Ocean, with recent daily totals ~30C north of the equator around 60E. Anomalies, significant for these waters, are at least plus 0.5-1C. Large portions of the Arabian Sea and regions of the Bay of Bengal have anomaly magnitudes greater than 1C. The west central and southwest Pacific Ocean are the warmest globally, with totals ~30-31C, but only having anomalies ~0.5-1C. The warm Indian Ocean will support further intensification of tropical convection (discussed below) during the next few weeks. The relatively warm west central Pacific Ocean increases the odds for tropical forcing to propagate east into the warm pool region this coming boreal winter. Links below provide additional SST details. (note the initial projection) (link 18).

The characteristics of the global circulation have become solidly La-Nina. Global relative AAM, updated through 9 October removing the QBO signal, is nearly 2 standard deviations below the R1 data climatology. This is the lowest value since boreal spring. Zonal mean AAM anomalies are ~minus 2 AMUs near 25S and ~minus 1 AMU around 25N. The latter translates to 200mb zonal mean easterly wind flow anomalies of ~10m/s (~5m/s) ~25S (~25N). Relatively weak zonal mean westerly wind flow anomalies are on the poleward flanks, once again suggestive of anomalous midlatitude ridges and poleward shifted zonal mean polar jet streams.

Zonal mean AAM sinks are pronounced ~25S and 35N (keep in mind boreal autumn and austral spring issues) with flux convergence of AAM transport near 45N and 50S (latter shifting poleward). This is an important characteristic of a low AAM base state (GWO phases 2-3), having midlatitude synoptic eddies northeast-southwest tilted (southeast-northwest tilted Southern Hemisphere), favoring anticyclonic wave breaking. For instance, roughly a week ago, after a GWO phases 8-1 transition, a cyclonically wave breaking trough amplified into the western USA. As I type, the current western states mid/upper tropospheric trough/closed low is undergoing anticyclonic wave breaking baroclinic development. This kind of dynamical behavior is best seen in isentropic potential vorticity (IPV) space.

Full disk satellite imagery show tropical convective forcing intensifying across much of the Indian Ocean, centered just north of the equator ~80E. Other enhancement persists from the Americas into equatorial Africa. The meridionally oriented interhemispheric Rossby wave energy dispersions (RWDs) that were across the Western Hemisphere (see phases 8-1 of the MJO 250mb snr psi composite anomaly plots) about a week ago are currently propagating into the Eastern Hemisphere. A response is that upper tropospheric divergence is increasing over the anomalously warm Indian Ocean leading to an enhancement of tropical convection. This chain of events is not unusual when a MJO component re-emerges into the Indian Ocean, and the extratropics are generally involved.

Removing the interannual component, the WH (2004) measure of the MJO (updated through 8 October) only shows a very weak projection in octant 2 of phase space. However, that projection increases to roughly 1.5 standard deviations in the same octant with the base state left in (used for this discussion). The WB (2008) GWO measure also shows approximately a 1.5 sigma projection in octants 2-3 of phase space. Hence, based on these measures, the former empirical/equatorially confined and the latter global/dynamical, the tropics and extratropics are becoming coupled. This situation has been expected to occur. MJO phase 2 of the 250mb snr psi composite anomaly plot best represents the current tropical circulation anomalies as does GWO phase 2 for the extratropics.

Summing up, the issues discussed in my 4 October posting remain and will not be repeated here. Since ~10 September the dynamics explained by the WB (2008) measure of the GWO have been dominating the global circulation. The RWD cited above was a response from the MJO component. Animations of daily mean anomalies of upper tropospheric vector wind currently show zonally oriented chains of midlatitude RWDs while a transition to twin tropical anticyclones (cyclones) occurs across the Indian Ocean (west Pacific Ocean). An important monitoring concern is how much SST cooling will occur in the region of the equatorial Dateline forced by enhanced trades over the next few weeks. Any statement about the outcome of the 2008-09 boreal cold season must consider roles of subseasonal atmospheric variability and global tropical SSTs, such as the above (yes, insert the angry Rottweiler!). I do not think statements like “ENSO-neutral conditions are expected to continue into early 2009” serves weather sensitive users well.

I feel fairly confident where the global atmosphere will go during the next several weeks; however, that is somewhat difficult to illustrate in the format of these discussions (and timing is white noise). Currently there is some interhemispheric meridional symmetry of zonally symmetric zonal mean wind anomalies, typical of a positive phase of the Branstator (2002) circumglobal teleconnection. GWO phases 2-3 of the 250mb snr psi composite anomaly plots illustrate that response. While a slow evolution shown by phases 3-5 of the 250mb snr psi composite anomaly plots for both GWO and MJO is probable during the next 2-4 weeks, faster embedded GWO variations tied to the surface torques are likely. For instance, an extended North Pacific Ocean jet outrunning the Eastern Hemisphere tropical forcing may occur during week-2, possibly collapsing into the western USA trough. Weather outcomes should be understood.

In the longer term, perhaps early November to around Thanksgiving, anomalous moist tropical convective forcing may shift into the west central Pacific Ocean leading to another transition shown by MJO/GWO phases 7-8-1 of the 250mb snr psi composite anomalies (see the corresponding 2m air temperature anomaly plots). At that point, this behavior may be a perturbation upon a La-Nina base state. Weather ramifications during that time may include another stormy period focusing on the western and central USA.

There is no change to my thoughts for boreal winter. The anomalously warm west central Pacific Ocean always concerns me, and that may be the “bail out” from keeping the global atmosphere spiraling “too low (AAM)” in GWO phase space. Stay tuned to see if our nemesis of 2 regions of tropical convective forcing returns and/or robust equatorial west Pacific Ocean westerly wind bursts starts to occur.

Internationally, while tropical forcing including the risk of cyclones winds down weeks 1-2 for the Western Hemisphere (hybrids understood), intense to severe thunderstorm activity is likely to increase/remain anomalously intense across Africa and the equatorial Indian Ocean this upcoming week. During weeks 2-3 locations throughout much of the central and eastern Indian Ocean, Indonesia and Southeast Asia may be strongly impacted with enhanced rainfall. Furthermore, tropical cyclogenesis across the Bay of Bengal may be greater than climatology during the next few weeks. High impact weather due to tropical convection may focus on the TNWP into the west central Pacific Ocean weeks 3-4. 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. For instance, our current base state favors a poleward shifted storm track across the North Atlantic Ocean which may adversely impact much of Europe during the next few weeks.


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 revised and accepted 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 ESRL/PSD GSDM web site 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! I should be able to do a discussion next weekend, ~18-19 October.

Ed Berry

No comments: