blocking and regime transitions
DESCRIPTION
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 PresentationTRANSCRIPT
![Page 1: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/1.jpg)
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
![Page 2: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/2.jpg)
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.
![Page 3: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/3.jpg)
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)
![Page 4: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/4.jpg)
NAO+
EA+
Increasing jet latitude
NAO+
EA+
Increasing jet speed
(Woollings et al 2010, QJRMS)
![Page 5: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/5.jpg)
NORTH
CENTRAL
SOUTH
There appear to be preferred transitions between different jet positions.
Wavetrain seen before northward jet shifts.
- MJO? (Cassou, Lin)
![Page 6: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/6.jpg)
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'
![Page 7: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/7.jpg)
Frame et al (in prep)
Jets closer to the equator are more persistent. (Barnes and Hartmann papers)
Forecasts are least skillful for poleward jets.
![Page 8: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/8.jpg)
Many models still have a systematic zonal bias.
(Woollings 2010, Phil Trans)
![Page 9: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/9.jpg)
Stronger equatorward jet bias = more skewed jet latitude distribution (Barnes and Hartmann 2010, GRL).
![Page 10: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/10.jpg)
• Southward jet position = NAO- = Greenland / Atlantic
blocking
• Intraseasonal regimes set the flavour for the season.
![Page 11: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/11.jpg)
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
![Page 12: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/12.jpg)
Is the zonal bias due to a lack of blocking?
![Page 13: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/13.jpg)
Mio Matsueda: http://tparc.mri-jma.go.jp/TIGGE/tigge_map.html
![Page 14: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/14.jpg)
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.)
![Page 15: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/15.jpg)
Or do blocking indices actually measure mean bias rather than problem with model variability…?
(Scaife et al, Jclim, in press)
![Page 16: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/16.jpg)
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…
![Page 18: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/18.jpg)
![Page 19: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/19.jpg)
Look for structure in the space spanned by the two leading EOFs.
![Page 20: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/20.jpg)
Now with colours representing jet speed.
![Page 21: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/21.jpg)
The North Atlantic Oscillation (NAO) essentially describes variations in the
latitude of the North Atlantic eddy-driven jet.
![Page 22: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/22.jpg)
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.
![Page 23: Blocking and regime transitions](https://reader036.vdocuments.net/reader036/viewer/2022062517/56813b38550346895da40d1c/html5/thumbnails/23.jpg)
A Gaussian mixture model identifies three very similar regimes in NAO/EA space.