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

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

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

4 Variation/persistence of the jet latitude with time (0-60W, mb) (Woollings et al. 2010, QJ) 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 (ERA40)

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

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 proportions covariances centres Circulation regimes

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

Preferred paths and transition probabilities 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 Preferred paths

Day Sequence of the fourth regime centroid in a mixture with K=4, and delay parameter of 40 days Preferred path towards GB regime (southern jet position) Z500, contour interval 10 m

Transition path (GB) and wave-breaking Blocking index: reversal in on PV2 surface Day Mean state of blocking index (delay space) associated with transition path toward WB B =  north -  south

Example: on PV2 surface for 20 Jan – 28 Feb /1 20/1 29/1 3/28/2 13/2 18/2 23/228/2

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

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