1. 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

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

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

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

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

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

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

17. 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.

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

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

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

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

22. Variability of mean winter blocking (SBE)
longitude

23. Signature of Greenland blocking episodes