Blocking and Rossby Wave-breaking

Brian Hoskins

Vangelis Tyrlis, Tim Woollings

Jo Pelly, Paul Berrisford, Mike Blackburn

Department of Meteorology

Symposium in Honour of Maurice Blackmon

A typical blocking dipole: 20 November 1993 12 UTCA typical blocking dipole: 20 November 1993 12 UTC

on 2 PVUon 2 PVU

Geopotential on 250 hPaGeopotential on 250 hPa

Blocking

B = B = northnorth - - southsouth

Situations giving a reversal of the meridional contrast in θ on PV2

C

W

W

Anticyclonic cut-off

C

CW

Cyclonic cut-off

C

C

W

W

Dipole

Anticyclonic wave-breaking

Cyclonic wave-breaking

W

W

C

C

(B positive)

Annual averaged synoptic time-scale 300hPa EKE

What central latitude for calculation of B?

Blocking of eastward motion of mid-latitude weather systems

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

230°E

270°E

Longitudinal profiles of statistics

mean

Standard deviation

skewness

Annual mean frequency of blocking

Local instantaneous blocking

Sector blocking episodes

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

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

Anticyclonic or Cyclonic Rossby Wave-breaking?

yGroup velocity

Phase speedHigh PV on θ

Low θ on PV2

Low PV on θ High θ on PV2

+

+

_

Now consider the 2-D distribution of the wave-breaking index for NH in winter.

European blocking

‘High latitude blocking’

Tim Woollings (JAS, in press)

We refer to these as “Greenland Blocking Episodes”

An example of a NW Atlantic wave-breaking event

MSLP Surface temperature

NAO- months

NAO- months with Greenland blocking days removed

Contribution of Greenland blocking to NAO- pattern

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.

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

Blocking and Rossby Wave-breaking

Symposium in Honour of Maurice Blackmon

Annual cycle of the frequency of Blocking (LIB)

time

longitude0 270

Variability of mean winter blocking (SBE)

longitude

Signature of Greenland blocking episodes