1. Theories of Blocking Brian Hoskins B. Bolin (1949) in Beggren et al, Tellus 1

2. What is a block? Chicago School 1940s Rossby, Bolin, Rex A block for the westerlies with their weather systems H L H L Z 500

3. A block for the westerlies with their weather systems + - + - PV on θ

4. A block for the westerlies with their weather systems W C W C θ on PV2

5. Some Theories Hydraulic jump, Rossby State with amplified Rossby wave ridge e.g Charney & Devore (1979) Mulitiple equilibria in the presence of topography Self-stabilisation of a vortex dipole in a westerly flow e.g. Modon (McWilliams, 1980, 1981) Soliton Instability of a basic wavy westerly flow e.g Frederiksen (1982) W C

6. Transient eddy feedbacks Hoskins, James & White (1983) Shutts (1983) eddy straining W C PV inversion, e.g. HMR (1985)

7. Group velocity considerations Wave activity leakage, e.g. On uniform westerly flow, U, barotropic stationary waves exist & have eastward group velocity 2U Modons constructed by McWilliams on equivalent barotropic westerly flow for which stationary waves do not exist. Swanson (2001,2) considered an equivalent barotropic atmosphere with 2 regions of uniform PV separated by a discontinuity: Possibility of R waves with zero group velocity & phase speed

8. Swanson 2002 Initial value problem

9. Onset B =  north -  south Pelly & H (2003) Tyrlis & H (2008) θ on PV2

10. Central latitude: Line of latitude (Tibaldi & Monteni, 1990), mean jet or storm-track latitude (x) (Pelly & H, 2003) or 2-D e.g. Berrisford et al (2007) for SH, Woollings et al (2008) for NH Reinforcement By interaction with subsequent weather systems Drip feed or big meal?

11. Importance of diabatic processes? Erosion of blocking high by radiative damping ~ 1 week Erosion of cut-off low by latent heat release ~ days Anchoring by underlying surface? Importance of ambient planetary waves & forcing of them e.g. Rockies but also anomalous SSTs, sea ice,.... How non-linear is the behaviour of blocking? persistence Masato et al (2009) frequency of occurrence Scaife et al (2010)