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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 B = north - south on 2 PVU Blocking Geopotential on 250 hPa Situations giving a reversal of the meridional contrast in θ on PV2 (B positive) C C W W W Anticyclonic cut-off C Cyclonic cut-off W C W C Anticyclonic wave-breaking C W Dipole W C 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 230°E 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 Sector blocking Local instantaneous 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? 1. Spherical domain gives bias towards equatorward propagation & anticyclonic wave-breaking 2. Ambient shears: anticyclonic (cyclonic) breaking on poleward (equatorward) side of jet 3. Sense of latitudinal displacement y High PV on θ Low θ on PV2 Phase speed + Low PV on θ High θ on PV2 + _ Group velocity 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 An example of a NW Atlantic wave-breaking event We refer to these as “Greenland Blocking Episodes” Contribution of Greenland blocking to NAO- pattern MSLP NAO- months NAO- months with Greenland blocking days removed Surface temperature 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 wavebreaking/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. 1. European blocking 2. Rossby wave-train from the Pacific 3. 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 0 270 longitude Variability of mean winter blocking (SBE) longitude Signature of Greenland blocking episodes