Abdel Hannachi

1. Department of Meteorology North Atlantic eddy-driven jet and circulation patterns: Structures, preferred paths and transitions Abdel Hannachi Tim Woollings & Brian Hoskins 1. Background 2. Jet positions and circulation patterns 3. Preferred paths and transitions 4. Summary

2. Background • Linearity + random noise normality of the PDF • Observed large scale flow is non-normal Hannachi et al (2009) J. Clim. Rennert & Wallace, J. Clim, 2009 Non-Gaussian grid points (1%) of NH winter SLP NH winter MSLP skewness • Origin: (i)Nonlinearity/regime behaviour; • (ii) Multiplicative noise; (iii) Cross-frequency coupling

3. Woollings et al (2009) J. Clim. • The North Atlantic region is special: split of the jet. • The NAO essentially describes variations in the latitude of the North Atlantic eddy-driven jet.

4. Much of extratropical weather/climate variability is associated with jet stream. • Link between jet stream and circulation patterns (regimes). • Importance for climate change effect on large scale flow Variation/persistence of the jet latitude with time (0-60W, 925-700mb) (ERA40) (Woollings et al. 2010, QJ) 4

5. Jet positions and circulation regimes Jet preferred locations PDF of North Atlantic jet latitude (three modes) ~+EA Z500 anomaly pattern associated with the preferred jet locations ~-EA -NAO Greenland blocking 5

6. Circulation regimes • The leading two/three EOFs of ERA40 Z500 are used • Skewness is interpreted via the mixture model of the PDF • K: is estimated using arguments from order statistics • Other parameters are estimated using the EM algorithm number of components centres covariances proportions

7. Scatter plot within EOFs 1 and 2 and the three component mixture model (colours refer to latitude anomalies) • Only three components are found • Very good agreement between the regime centres and jet composites and between covariances and jet latitudes 7

8. Preferred paths and transition probabilities Preferred paths • Growth/decay and preferred paths in state space are studied via the mixture model using the extended data (delay space). Embedding dimension • A regime centroid becomes a trunk of a trajectory (length M) We get the most persistent regimes We start getting preferred paths Between regimes in state space

9. Preferred path towards GB regime (southern jet position) Day 1 5 10 Sequence of the fourth regime centroid in a mixture with K=4, and delay parameter of 40 days 15 20 25 35 40 30 Z500, contour interval 10 m

10. B = north - south Transition path (GB) and wave-breaking Blocking index: reversal in on PV2 surface Day 1 5 10 15 20 25 30 35 40 Mean state of blocking index (delay space) associated with transition path toward WB

11. Example: on PV2 surface for 20 Jan – 28 Feb 1986 20/1 24/1 29/1 3/2 8/2 13/2 18/2 23/2 28/2

12. Transition probability Transition probability: Computed using the mixture model: Multivariate normal Probability model: no need to classify each datum 52% 51% C C 32% 32% 30% 38% 28% S N S N 36% 55% 51% 42% 38% Two EOFs Three EOFs

13. Summary • Non-normality of large scale flow • Importance of jet stream in much of extratropical weather/climate variability • Latitudinal position of the NA jet: trimodal (N, S, C) • Very good agreement with regimes from mixture model: N/-EA; C/+EA; S/-NAO/GB • Mixture model with the delay space: preferred paths. Path to S: wave breaking • Transition: S C N • N S: through baroclinic instability