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

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

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

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

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

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

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

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

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

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

+ + _ 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 + _

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

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”

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

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

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.

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

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

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

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

Variability of mean winter blocking (SBE) longitude

Signature of Greenland blocking episodes