Blocking and regime transitions

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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 Department of Meteorology

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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. - PowerPoint PPT Presentation

Transcript of Blocking and regime transitions

Page 1: Blocking and regime transitions

Blocking and regime transitions

Tim WoollingsWith thanks to: Brian Hoskins, Abdel Hannachi, Christian

Franzke, Joaquim Pinto, Joao Santos, Olivia Martius, Giacomo Masato, Tom Frame, Adam Scaife, Libby Barnes

Department of Meteorology

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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.

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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)

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NAO+

EA+

Increasing jet latitude

NAO+

EA+

Increasing jet speed

(Woollings et al 2010, QJRMS)

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NORTH

CENTRAL

SOUTH

There appear to be preferred transitions between different jet positions.

Wavetrain seen before northward jet shifts.

- MJO? (Cassou, Lin)

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Transient eddies forcing northward jet shifts:

Eddies contribute to forcing regime onset

They subsequently act to maintain the anomalous jet position

ua ( E), E v'2 u'2, u'v'

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Frame et al (in prep)

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

Forecasts are least skillful for poleward jets.

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Many models still have a systematic zonal bias.

(Woollings 2010, Phil Trans)

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

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• Southward jet position = NAO- = Greenland / Atlantic

blocking

• Intraseasonal regimes set the flavour for the season.

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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

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Is the zonal bias due to a lack of blocking?

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

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Matsueda (2009, SOLA)

• 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.)

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

(Scaife et al, Jclim, in press)

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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…

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Jet stream regimes: Weather or

climate?

www.met.reading.ac.uk/~swr01tjw/ [email protected]

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Look for structure in the space spanned by the two leading EOFs.

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Now with colours representing jet speed.

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The North Atlantic Oscillation (NAO) essentially describes variations in the

latitude of the North Atlantic eddy-driven jet.

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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.

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A Gaussian mixture model identifies three very similar regimes in NAO/EA space.