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Department of Meteorology. Blocking and regime transitions. Tim Woollings With thanks to: Brian Hoskins, Abdel Hannachi, Christian Franzke, Joaquim Pinto, Joao Santos, Olivia Martius, Giacomo Masato, Tom Frame, Adam Scaife, Libby Barnes.

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slide1

Department of Meteorology

Blocking and regime transitions

Tim Woollings

With thanks to: Brian Hoskins, Abdel Hannachi, Christian Franzke, Joaquim Pinto, Joao Santos, Olivia Martius, Giacomo Masato, Tom Frame, Adam Scaife, Libby Barnes

slide2

Use low-level wind to identify the North Atlantic eddy-driven jet stream (Woollings et al 2010, QJRMS)

Zonal wind

-> Average over 0-60W and 925-700hPa

-> Low-pass filter (10 day)

-> Find maximum

-> Remove seasonal cycle.

slide3

This analysis suggests three preferred locations of the jet.

Z500 anomaly patterns resemble NAO and EA patterns.

LATITUDE ANOMALY (DEG)

(Woollings et al 2010, QJRMS)

slide4

EA+

EA+

NAO+

NAO+

Increasing jet latitude

Increasing jet speed

(Woollings et al 2010, QJRMS)

slide5

There appear to be preferred transitions between different jet positions.

NORTH

CENTRAL

SOUTH

Wavetrain seen before northward jet shifts.

- MJO? (Cassou, Lin)

slide6

Transient eddies forcing northward jet shifts:

Eddies contribute to forcing regime onset

They subsequently act to maintain the anomalous jet position

slide7

Jets closer to the equator are more persistent. (Barnes and Hartmann papers)

Forecasts are least skillful for poleward jets.

Frame et al (in prep)

slide9

Stronger equatorward jet bias = more skewed jet latitude distribution (Barnes and Hartmann 2010, GRL).

slide10

Southward jet position

  • = NAO-
  • = Greenland / Atlantic blocking
  • Intraseasonal regimes set the flavour for the season.
slide11

Empirical Mode Decomposition:

  • 97% of the jet latitude variance is in the intra-annual range.
  • The shoulder in the autocorrelation function does not reflect enhanced predictability (as in Keeley et al 2009).
  • ~50% of the interannual variance in winter is climate noise (~70% in summer).

ACF

30 days

slide13

Mio Matsueda: http://tparc.mri-jma.go.jp/TIGGE/tigge_map.html

slide14

Model representation of blocking has improved but still some tendency to underestimate frequency.

  • Persistence of blocking linked to subsequent wave-breaking, which is missed in some case studies. (Masato, Reading Uni.)

Matsueda (2009, SOLA)

slide15

Or do blocking indices actually measure mean bias rather than problem with model variability…?

(Scaife et al, Jclim, in press)

slide16

Conclusions

  • There are three preferred positions of the Atlantic eddy-driven jet stream.
  • Dynamical features, especially precursors, provide useful benchmarks for testing model skill – eg wave-trains, eddy forcing, preferred transitions.
  • Certain regimes are particularly persistent, which can lead to high model skill.
  • Many models are still too zonal and underestimate blocking – but this may really be a mean-state problem.
  • Still much debate on intrinsic time-scales of circulation patterns…
slide17

Jet stream regimes: Weather or climate?

www.met.reading.ac.uk/~swr01tjw/ t.j.woollings@reading.ac.uk

slide21

The North Atlantic Oscillation (NAO) essentially describes variations in the latitude of the North Atlantic eddy-driven jet.

slide22

Can we diagnose the latitude of the eddy-driven jet directly?

Method: Zonal wind -> Average over 0-60W and 925-700hPa -> Low-pass filter (10 day) -> Find maximum

-> Remove seasonal cycle.