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SUMMARY:Variability of the winter stratosphere: the sudden stratospheric w
 arming of 2009 - Dr Yvonne Hinssen\, Institute for Marine and Atmospheric 
 Research\, Utrecht University
DTSTART:20100615T093000Z
DTEND:20100615T103000Z
UID:TALK24867@talks.cam.ac.uk
CONTACT:Dr Francis Pope
DESCRIPTION:Using the ECMWF ERA-interim reanalysis data for 1989-2008\, it
  is shown that a relation exists between the stratospheric polarcap potent
 ial vorticity (PV) and the 100 hPa eddy heat flux. A difference in PV betw
 een years is found to be linearly related to the flux difference integrate
 d over time\, taking into account a decrease in relaxation timescale with 
 height in the atmosphere. \n\nThis mean PV-flux relation is then used to e
 stimate the polarcap PV for the 2008/2009 winter from the climatological P
 V and the 100 hPa flux difference between 2008/2009 and the climatology.\n
 The ERA-interim polarcap PV for 2008/2009 shows a large and abrupt change 
 in midwinter\, related to the occurrence of the Sudden Stratospheric Warmi
 ng (SSW) in January 2009\, and this is also captured by the PV estimated f
 rom the 100 hPa flux and the PV-flux relation.\nThe results show that the 
 large interannual variability in the state of the stratosphere that is obs
 erved in the northern hemisphere is largely determined by the interannual 
 variations in the 100 hPa heat flux. \n\nPiecewise potential vorticity inv
 ersion is then used to examine the influence of the stratospheric PV on th
 e zonal wind before and after the occurrence of the 2009 SSW. A change in 
 this influence from westerly before the SSW to easterly afterwards is foun
 d\, which results in changes in the stratospheric influence on the troposp
 heric winds up to 5 m s-1 within a few weeks. These changes are found to b
 e a substantial contribution to the wind difference observed in the ERA-in
 terim data\, indicating that the stratosphere influences the tropospheric 
 conditions\, leading to a weakening of the westerlies due to the occurrenc
 e of the January 2009 SSW. The PV changes accompanying the SSW in the lowe
 r stratosphere persist for several months\, indicating that stratospheric 
 information might be valuable for extended range weather forecasts.\n
LOCATION:Todd-Hamied Room\, Department of Chemistry
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