An alternative mechanistic model of the Antartic polar vortex

Walter Legnani1, Rolando Garcia2, Pablo Jacovkis3, Murry Salby4, and Pablo Canziani5,6 1 Instituto de Cálculo, Universidad de Buenos Aires - Argentina

2National Center for Atmospheric Research - U.S.A. 3Instituto de cálculo/Departamento de Computacion, Universidad de Buenos Aires - Argentina

4UCenter for Atmospheric Theory and Analysis University of Colorado - U.S.A. 5 Programa de Estudios de Procesos Atmosféricos en el Cambio Global –Pontificia Universidad Católica Argentina/CONICET – Argentina

6Departamento de Ciencias de la Atmosfera y los Océanos, Universidad de Buenos Aires/CONICET - Argentina

A quasi-3D global atmospheric model was developed to study the dynamics of the Antartic polar vortex. The atmospheric system is represented by shallow water equations system together with thermodynamics, from a classical point of view. The integration of the equations on the horizontal plane was made using a Hough harmonic base. These conform an orthonormal system which is the eingensystem of the shallow water operator of the linearised system of equations. The non-linearities were retained in the forcing terms of the equations. Thus the model uses the non linear shallow water systems of equations. In the vertical, the model was adjusted at each time step using a spline base to fit the solution between two consecutive model layers, on isentropic surfaces. This mechanistic model is forced from below at the lower boundary (near the tropopause) with the temperature from NCEP reanalysis, while the upper boundary is free. The model was developed to study the dynamical behaviour of the Antartic polar vortex, in particular the interaction with synoptic scale disturbances of tropospheric origin. The model was able to reproduce observations that show that synoptic scale perturbations of tropospheric origin can penetrate sufficiently into the lower stratosphere and can thus significantly contribute to the deformation of the polar vortex.

The models equations

,),,,(),,,(),,,(

tstst

ts FWt),,L(

W

,),(),(),(,

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NM

nmjkjkjk tsts HW

Model Results

Model Validation

A 1 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 1 2

- 1 0

- 8

- 6

- 4

- 2

0

B 1 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 1 2

- 1 0

- 8

- 6

- 4

- 2

0

A 2 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 0

- 1 8

- 1 6

- 1 4

- 1 2

- 1 0

- 8

- 6

- 4

- 2

0

B 2 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 0

- 1 5

- 1 0

- 5

0

A 3 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

B 3 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

A 1 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 1 2

- 1 0

- 8

- 6

- 4

- 2

0

B 1 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 1 0

- 8

- 6

- 4

- 2

0

A 2 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

B 2 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 0

- 1 5

- 1 0

- 5

0

A 3 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 3 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

B 3 )

0 . 2

0 . 4

0 . 6

0 . 8

1

3 0

2 1 0

6 0

2 4 0

9 0

2 7 0

1 2 0

3 0 0

1 5 0

3 3 0

1 8 0 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

Model’s Output for event type II (from 14 to 21km)

Results with all wave Results with wave components

components in the forcing up to wave number 4

Model’s Output for event type I (from 14 to 21km)

Results with all wave Results with wave components

components in the forcing up to wave number 4

Summing upThe model uses a month warm up period (September) to reproduce the dyamical and thermodynamical conditions of the atmosphere.

The validation with datasets was completely successfully.

The outputs shown on the left, show that the presence of the synoptic scale waves, with zonal wave numbers greater than 4 have an important contribution in the shape of the polar vortex for certain conditions, dependent on the mean flow and longer planetary wave structure.

The model presented here is a mechanistic one, because it uses an input as temperature of tropopause to integrate the isentropic shallow water equations with to purpose of reproduce a certain picture of the atmosphere.

For an event type II the presence of synoptic scale waves is important to conform the real structure of the vortex, in the cases studied.

For events type I, the activity of synoptic scale waves do not contribute on the conformation of the main structure of the polar vortex.

Shallow water equations in isentropic coordinates

The coordiante system

Spectral solution (using Hough harmonics) of the system in the horizontal direction and interpolation beta

spline scheme in the vertical direction

NCAR

University of Colorado Boulder