Saturday, April 19, 2008

Circumglobal Teleconnection

The spatial distribution of global SSTs still includes weakening but persistent below normal waters along the equatorial Pacific Ocean from ~160E-120W. Magnitudes are as low as roughly minus 1.5C extending to 150m deep per latest 5-day averaged TAO buoy data. Shallow but very warm anomalies continue across the far eastern Pacific Ocean to South America, while significant positive anomalies in excess of 5C are present around 200m deep/160E along the equator.


Needless to say careful detailed rigorous daily monitoring is a must as part of any forecast process to gain some understanding on the future of El-Viejo. There are a couple of outlier statistical and dynamical models suggesting El-Nino SSTs by boreal winter 2008-09. Whatever the case, confidence in any objective predictive scheme must be very low right now, and remember that SSTs from other basins including the tropical Indian and Atlantic Oceans are also important. As discussed below, the current global circulation base state is strongly La-Nina. Links below are to additional SST information.


http://iri.columbia.edu/climate/ENSO/currentinfo/technical.html


http://www.pmel.noaa.gov/tao/jsdisplay/


http://www.cdc.noaa.gov/forecast1/IndoPacific.frcst.html (note the initial projection)


http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/index.primjo.html (link 18).


In the spirit of brevity, full disk satellite imagery and other diagnostic monitoring tools indicate that the tropical convective forcing across the central and eastern equatorial Indian Ocean has been strongly organizing during the last several days. The greatest concentration is ~0/80E having 3-day averaged OLRA in excess of minus 70 W/m**2. Sporadic convection continues across other portions of the tropics that we should all be familiar with. SSTs have cooled too roughly minus 1-2C in the region of where the severe Indian Ocean thunderstorm activity is, with warming across the equatorial west central Pacific Ocean. My point is I suspect, with forcing from the dynamics captured by the GWO, MJO #5 for this boreal cold season (time of year, and all other arguments understood) may be evolving. WH (2004) phase space plots for the MJO through 18 April have a near zero projection. There is already a westerly wind burst (WWB) across the equatorial Indian Ocean and a WWB across the west central Pacific Ocean is a possibility during the next few weeks.


Global relative AAM, updated through 17 April per ESRL/PSD, is roughly 2.5 sigma below the R1 data climatology. Not only is the global circulation in our familiar low AAM base state, there is evidence of (again) merdional symmetry of zonally symmetric zonal mean wind flow anomalies (vertically and zonally integrated for the latter). That is, weak equatorial zonal mean anomalous westerlies flanked by zonal mean easterly wind flow anomalies across the subtropical atmospheres. The greatest subtropical zonal mean easterly wind flow anomalies persist across the Northern Hemisphere, having magnitudes ~10-15m/s at 200mb. Westerly wind flow anomalies are present across the mid and higher latitudes; again, particularly the North Hemisphere. That is why anomalous ridges are present across the midlatitudes (as seen in the earth component of the angular momentum budget) leading to anomalous poleward shifted storm tracks, in the zonal mean.


Various animations of several wind fields show that the zonal mean equatorial westerly wind flow anomalies are largely coming from the upper troposphere of the Western Hemisphere Pacific Ocean. The latter is part of the expected baroclinic response to the Eastern Hemisphere tropical convective forcing. Anomalous twin upper tropospheric tropical/subtropical anticyclones are developing in the region of the Indian Ocean/Indonesian divergent outflow.


The WB (2007, 08) GWO, which is a much better quantitative measure of the global circulation than an equatorially confined empirical MJO index in our current situation, has orbited to a roughly 2 sigma phase 4 projection. The global tendency of relative AAM is ~plus 25 Hadleys which not only has had a contribution from the tropical forcing, but also a recent spike in the global frictional torque of ~plus 20 Hadleys. This strong positive frictional torque has been forced by anomalous surface easterlies from the strong midlatitude ridges discussed above. The point to all this is we have another example of tropical-extratropical coupling, observed so often this past cold season. The WB (2007, 08) GWO can tell us a lot about the dynamics of the non-linear complicated forcing-response-feedback relationships involved.


The punch line to this posting is because of all the scientific issues discussed above (and those I left out), zonally oriented Rossby wave energy dispersions (RWDs) continue to be favored in our current base state (due to trapping of baroclinic energy in the jets). This has been the case since at least November 2007, with a few exceptions such as ~1 December 2007 and earlier this month. As discussed in my last posting, most numerical model predictions failed badly on the latter (see link below as an example).


http://www.cpc.ncep.noaa.gov/schemm/z500ac_wk2_na.html .


The zonally oriented chains of circulation anomalies responding to the RWDs not only favor a central Pacific Ocean ridge, but can also impact the weather globally. Focusing on just the Northern Hemisphere, what I am referring to is not a regional scale teleconnection pattern like the PNA, but a pattern connecting widely separated points all across globe. This pattern has been documented by Branstator (2002), and he has termed it as a “circumglobal teleconnection”. We have observed this type of pattern frequently for years, and this is contained in the legacy WB (2007) Stages 2 and 4 of the GSDM. More recently, WB (2007, 08) have captured this circumglobal teleconnection pattern in composites done on phases 3 (La-Nina like) and 7 (El-Nino like) of the GWO. A two-part paper is in preparation.


The circumglobal teleconnection pattern is currently present, and has been for roughly the last week. Where the atmosphere goes during the next few weeks will depend a lot on whether or not the Eastern Hemisphere tropical forcing comes out into the west central Pacific Ocean. While uncertainty is huge, my feeling is an eastward shift is probable and my outlooks for the USA below reflect that. That eastward shift of tropical convective forcing may also be in sync (at times) with the phases of the GWO, which is also probable to circuit into phase 5 before collapsing. Please recall my discussion from last week about the USA being impacted by the tropical forcing regardless of whether or not we have a “WH (2004) MJO”.


A relatively repeatable pattern of troughs coming into the western USA then the Plains is likely to continue. While timing is white noise, I am thinking the tropical convective forcing will come out into the west central Pacific Ocean ~weeks 2-3. If that is the case, baroclinic energy may become dispersive allowing for great circle RWD routes like that seen earlier this month. An eastward shift of wave trains across the USA would then be expected (with retrogression afterward), as well as a strengthening subtropical jet. In any case, the broken record USA outlooks continue. The storm track will be very active but anomalously northward shifted. Meridional amplification would allow the storm track to temporarily shift south against the seasonal cycle. Weather ramifications are “obvious”.


I do want to make “special mention” of anomalous cold Arctic air that has been building up in western Canada for the last several days. That is a response to blocking developing in the region of Alaska (one of the matters I did not discuss). Initial impacts from this airmass are already being felt across the extreme northern Rockies. Depending on the timing of subseasonal events discussed above, in addition to what may be a vicious severe local storms outbreak for portions of the Plains and the Ohio Valley, a late season significant winter storm (with intense thundersnow) is possible for locations such as the Upper Mississippi Valley and Northern Plains. Having my reasons tied to somewhat faster time scales explained by the GWO (friction-mountain torque index cycle), this extreme weather situation may occur ~week 2. Those faster time scales may also bring the tropical convection into the west Pacific sooner. Stay tuned.


My concerns of prolonged dryness remain for the central and southern High Plains. The faster time scales scenario discussed above would be more favorable for precipitation in these areas, as would any eastward shift of tropical forcing into the west central Pacific Ocean.


Locations from the Indian Ocean into at least western Indonesia are likely to get hammered with intense to severe thunderstorm activity week-1, shifting eastward weeks 2-3. The Philippines and portions of Southeast Asia are probable to be impacted particularly weeks 2-3. Tropical cyclone concerns also remain, and I suspect given the WWB (per above) the Bay of Bengal could be impacted as early as week-1. Climatologically, the Bay of Bengal has one peak period of tropical cyclones during May. The west central into the southwest Pacific Ocean remains a “wild card” until further notice, particularly given the issues discussed above. I continue to leave it to the expertise of the appropriate weather centers internationally to alert the public of additional weather hazards worldwide.


Appendix


An experimental quasi-phase space plot of the GSDM utilizing time series of normalized global relative AAM time tendency (Y-axis) and normalized global relative AAM anomaly (X-axis) can be found at


http://www.cdc.noaa.gov/map/clim/gsdm.shtml


We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the legacy GSDM is to extend current thinking beyond the MJO, the GWO quantifies variations used to derive the original GSDM in a manner that is “user friendly” analogous to the WH (2004) “convention”. In addition, the GWO plot does not have the ENSO signal removed.


Please see the revised description of the GSDM per above link. Also, I encourage the readers to study the annotated MJO and GWO phase space plots to help relate the global variations explained by those techniques to “weather”.


Links to CPC and PSD ENSO discussions:


http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml


http://www.cdc.noaa.gov/people/klaus.wolter/MEI/


The following is a link to information about the stratosphere:


http://ds.data.jma.go.jp/tcc/tcc/products/clisys/index.html


These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability). We hope that an opportunity will arise for us (soon) to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. In addition, a two-part paper is in preparation by WB that will formally introduce the GWO along with subseasonal composites. Given shift work and upcoming travel, updates remain extremely difficult. I will try to post another discussion next weekend, 26-27 April.


Ed Berry

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