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

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 hPa -> Low-pass filter (10 day) -> Find maximum -> Remove seasonal cycle.

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)

NAO+ EA+ Increasing jet latitude NAO+ EA+ Increasing jet speed (Woollings et al 2010, QJRMS)

NORTH CENTRAL SOUTH There appear to be preferred transitions between different jet positions. Wavetrain seen before northward jet shifts. - MJO? (Cassou, Lin)

Transient eddies forcing northward jet shifts: Eddies contribute to forcing regime onset They subsequently act to maintain the anomalous jet position

Frame et al (in prep) Jets closer to the equator are more persistent. (Barnes and Hartmann papers) Forecasts are least skillful for poleward jets.

Many models still have a systematic zonal bias. (Woollings 2010, Phil Trans)

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

Southward jet position = NAO- = Greenland / Atlantic blocking Intraseasonal regimes set the flavour for the season.

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

Is the zonal bias due to a lack of blocking?

Mio Matsueda:

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.)

Or do blocking indices actually measure mean bias rather than problem with model variability…? (Scaife et al, Jclim, in press)

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…

Jet stream regimes: Weather or climate?

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

Now with colours representing jet speed.

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

Can we diagnose the latitude of the eddy-driven jet directly? Method: Zonal wind -> Average over 0-60W and hPa -> Low-pass filter (10 day) -> Find maximum -> Remove seasonal cycle.

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