Theories of BlockingBrian Hoskins

B. Bolin (1949) in Beggren et al, Tellus 1

What is a block?

Chicago School 1940sRossby, Bolin, Rex

A block for the westerlies with their weather systems

H

L

H

L

Z500

A block for the westerlies with their weather systems

+

-

+

-

PV on θ

A block for the westerlies with their weather systems

W

C

W

C

θ on PV2

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 flowe.g. Modon (McWilliams, 1980, 1981)

•Soliton

•Instability of a basic wavy westerly flowe.g Frederiksen (1982)

W

C

Transient eddy feedbacks

Hoskins, James & White (1983)

Shutts (1983)eddy straining

W

C

PV inversion, e.g. HMR (1985)

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

Swanson 2002Initial value problem

Onset

B = north - south

Pelly & H (2003)Tyrlis & H (2008)θ on PV2

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 systemsDrip feed or big meal?

Importance of diabatic processes?

Erosion of blocking high by radiative damping ~ 1 weekErosion 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)