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Blocking and Rossby Wave-breaking

Department of Meteorology. Symposium in Honour of Maurice Blackmon. Blocking and Rossby Wave-breaking. Brian Hoskins Vangelis Tyrlis, Tim Woollings Jo Pelly, Paul Berrisford, Mike Blackburn. B =  north -  south.  on 2 PVU. Blocking. Geopotential on 250 hPa.

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Blocking and Rossby Wave-breaking

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  1. Department of Meteorology Symposium in Honour of Maurice Blackmon Blocking and Rossby Wave-breaking Brian Hoskins Vangelis Tyrlis, Tim Woollings Jo Pelly, Paul Berrisford, Mike Blackburn

  2. B = north - south  on 2 PVU Blocking Geopotential on 250 hPa A typical blocking dipole: 20 November 1993 12 UTC

  3. Situations giving a reversal of the meridional contrast in θ on PV2 (B positive) C C W C W W Anticyclonic cut-off Cyclonic cut-off W C C W Anticyclonic wave-breaking C W W C Dipole Cyclonic wave-breaking

  4. What central latitude for calculation of B? Blocking of eastward motion of mid-latitude weather systems Annual averaged synoptic time-scale 300hPa EKE Evangelos Tyrlis (JAS in press)based on ERA-40 data

  5. Aspects of the frequency distribution of B in DJF 230°E Frequency distributions at 3 longitudes 20°E 270°E Standard deviation Longitudinal profiles of statistics skewness mean

  6. Annual mean frequency of blocking Sector blocking episodes Local instantaneous blocking Sector blocking

  7. Composites of θ on PV2 for NH winter SBE days in representative sectors

  8. Evolution of winter SBEs at 20ºE: composites of θ on PV2

  9. Composites of θ on PV2 for NH summer SBE days in representative sectors

  10. Anticyclonic or Cyclonic Rossby Wave-breaking? • Spherical domain gives bias towards equatorward propagation & anticyclonic wave-breaking • Ambient shears: anticyclonic (cyclonic) breaking on poleward (equatorward) side of jet • Sense of latitudinal displacement

  11. Phase speed Group velocity y _ High PV on θ Low θ on PV2 + Low PV on θ High θ on PV2 +

  12. ‘High latitude blocking’ European blocking Now consider the 2-D distribution of the wave-breaking index for NH in winter. Tim Woollings (JAS, in press)

  13. An example of a NW Atlantic wave-breaking event We refer to these as “Greenland Blocking Episodes”

  14. Contribution of Greenland blocking to NAO- pattern Surface temperature MSLP NAO- months NAO- months with Greenland blocking days removed

  15. How much NAO variability could be attributed to variations in the occurrence of wave-breaking?

  16. The Hypothesis NAO- is a description of periods when NW Atlantic wave-breaking/Greenland blocking is frequent. NAO+ is just a description of periods when it is infrequent (cf Benedict et al, 2004). + feedbacks… Low-frequency variations in the ocean and the stratosphere could modulate the occurrence of Greenland blocking, and so have an NAO signature.

  17. Dynamical precursors could help to identify this modulation. • European blocking • Rossby wave-train from the Pacific • A shift in the stratospheric jet

  18. High latitude Blocking in the NW Atlantic tends to lead that in the N Pacific by a few days: an example

  19. Symposium in Honour of Maurice Blackmon Blocking and Rossby Wave-breaking

  20. Annual cycle of the frequency of Blocking (LIB) time 0 270 longitude

  21. Variability of mean winter blocking (SBE) longitude

  22. Signature of Greenland blocking episodes

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