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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. Background.

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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


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


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.


  • 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


Jet positions and circulation regimes associated with jet stream.

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


Circulation regimes associated with jet stream.

  • 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


Scatter plot within EOFs associated with jet stream.

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


Preferred paths and transition probabilities associated with jet stream.

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


Preferred path towards GB regime (southern jet position) associated with jet stream.

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


B =  associated with jet stream.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


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


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


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


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