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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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slide1
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

slide2
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
slide3
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.
slide4
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

slide5
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

slide6
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

slide7
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

slide8
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

slide9
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

slide10
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

slide11
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

slide12
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

slide13
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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