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Recent Sudden stratospheric warming … its cause / more specific implications


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#1
richard mann

Posted 31 December 2014 - 02:26 PM

richard mann

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.. First, a bit of history, looked at along together with explanation of the phenomenon. 
 
excepted from a "poster" working to explain a warming event having occurred back in 2003-04, generated and put out online in the form of a PDF, by NOAA's "National Centers for Environmental Prediction"(NCEP).
 
 
Introduction
 
Sudden stratospheric warmings (SSWs) are winter hemisphere phenomena characterized by the rapid increase in temperature in the polar stratosphere and the weakening of the zonal mean zonal flow. In the most dramatic cases, stratospheric temperatures locally can rise by 40°-50°C and the zonal mean zonal flow can reverse in direction in the span of just a few days. The WMO defines a sudden stratospheric warming as when the 10 hPa temperature gradient poleward of 65°N changes sign and becomes positive for at least five consecutive days. This is deemed a “minor” warming. Additionally, a “major” warming is defined as when the 10 hPa zonal wind at 65°N reverses and becomes easterly for more than 5 consecutive days. By this definition the 2003/2004 event was then a “Major” warming. During this event the stratospheric zonal mean temperatures increased by as much as 28°C. The zonal mean zonal wind changed from 65 m/s westerlies into 20 m/s easterlies over the span of two weeks.This wind reversal encompassed the entire stratosphere. This event was not just confined to the stratosphere, but anomalous features extended well into the troposphere impacting the surface for much of January and parts of February, 2004.
 
What causes sudden stratospheric warmings?

Stratospheric warmings involve interactions between the zonal flow of the polar stratosphere and upward propagating planetary waves consisting primarily of zonal wave numbers 1 and 2. Normally, the zonal flow is very strong in the wintertime lower polar stratosphere and vertically propagating waves tend to be deflected equatorward. But if the lower stratosphere is “pre-conditioned” by earlier wave activity, the zonal flow is weakened or moved poleward and vertically propagating waves tend to be deflected poleward. The vertical component of the E-P Flux (Fz) which is proportional to the poleward heat flux maximizes at about 10 hPa at this time. The area above the heat flux maximum (divergence of wave forcing) acts to decelerate the eastward zonal flow. A residual circulation then induces sinking motion below and poleward of this forcing region. The sinking motion causes the temperatures to increase due to adiabatic warming. This reduces the thermal gradient which in tur reduces the zonal wind speed. These large temperature and wind anomalies then propagate downward into the lower stratosphere (Baldwin and Dunkerton, 1999; Limpasuvan et al., 2004). Almost all stratospheric warming events get to this stage. It is still a research topic as to what allows the warming influences to proceed further into the troposphere or be impeded from further downward propagation. Zhou et al. (2002) show that warmings that penetrate into the troposphere are characterized in the onset by a double pulse of the vertical component of the E-P Flux (Fz). The 2003/2004 stratospheric warming event had such a double pulse.
 
.. Here following, accessible, is that PDF generated poster, with a fuller discussion replete with graphics working to cover the event suggested here above.  (With its dimensions, and where considering it more online display, perhaps better downloaded toward the idea of manipulating it.) 
 
ftp://ftp.cpc.ncep.noaa.gov/long/misc/SSW Poster new.pdf


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#2
richard mann

Posted 31 December 2014 - 02:39 PM

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... If and when one occurs, cold air will go somewhere in the mid-latitudes.  The PNW needs the SSW to coincide with favorable tropic and mid-pattern levels to see the snow/cold.  Our window for that may be closing, although there are plenty of other scenarios that could still deliver the goods. The GFS does seem to be coming on board for a SSW.  I don't have access to the ECMWF.


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