The tropospheric jet response to prescribed zonal forcing in an idealized atmospheric model
Gang Chen1,
Pablo Zurita-Gotor2
in Atmospheres, Oceans and Climate, Massachusetts Institute of Technology
2Universidad
4. Responses to the zonal torque near the surface
δU(φ0=30,
σ0=0.85)
Zonal
wind
5. Responses to the zonal torque in the lower stratosphere
0.01
0.02
0.02
0.01
-3
0.1
0.2
10
20
30
40 50 60
Latitude(deg)
70
80
10
20
30
40 50 60
Latitude(deg)
70
80
0.01
0.02
0.02
0.05
0.05
0.2
0.1
0.2
30
40 50 60
Latitude(deg)
70
80
90
• The tropospheric winds are characterized by a midlatitude jet, and
the wind anomalies vacillate about the mean jet latitude.
• The extratropical stratospheric winds are weak in the
climatological mean and display relatively little variability.
•This configuration minimizes stratospheric intrinsic variability and
its influence on the troposphere in the control simulation. While the
results are mostly analogous to the dynamics in the Southern
Hemisphere summertime, the forced response in this model is often
similar to that in models with a stratospheric polar jet.
Eddy momentum flux convergence
spectra in upper troposphere
80
latitude(deg)
70
60
50
40
30
20
10
-10 -5
A key feature is that the
absorption of upper
tropospheric eddies in the
subtropics is confined in a
critical layer of 10-20 degrees in
latitude poleward of their linear
Time mean U
critical latitudes, and therefore
the movement of the
critical latitudes may be useful to
momentum
divergence
understand the latitudinal
wave sink
movement of eddy momentum
5 10 15 20 25 30 35 40
fluxes (e.g. Chen et al. 2007).
angular phase speed (m/s)
momentum
convergence
wave source
0
20
30
40 50 60
Latitude(deg)
70
80
90
1
0
-3
30
40 50 60
Latitude(deg)
0
0.01
0.02
0.02
0.05
0.05
0.2
20
70
80
90
70
80
90
changes in the full
δdF(φmodel
=70, σ =10-1.25)
0.01
0.1
10
0
0.1
0.2
1
30
40 50 60
Latitude(deg)
70
80
90
1
0
0.5
1
10
20
30
40 50 60
Latitude(deg)
70
80
90
1
0
0.5
10
20
30
40 50 60
Latitude(deg)
70
80
90
1
0
10
20
30
40 50 60
Latitude(deg)
This downward penetration of zonal winds displays a poleward slope for the
subtropical torque, an equatorward slope for the subpolar torque, and less tilting for
the midlatitude torques.
6
20
latitude(deg)
90
-3
10
80
10
20
Changes in eddy momentum flux convergence spectra in the full model
3
70
10
0.5
80
80
70
70
60
60
latitude(deg)
Sigma level
Sigma level
1
0
tropopause
0.2
1
0
10
-1.25
EP
flux
divergence
δdF(φ
=30,
σ0=10
)
0
Sigma Level
Sigma level
Sigma level
0.1
1
0.1
0.5
3
1
0
90
Sigma Level
0.01
0.5
0.5
40 50 60
Latitude(deg)
0.2
3
1
0
90
Torque
Torque
δUzm(φ0wind
=30, σ0=0.85)
δUzm(φ0=50,
σ0=0.85)
Zonal
changes in the zonally symmetric
model
3
30
0.1
3
1
0
1
0.05
tropopause
20
0.05
3
0.02
10
0.2
3
0.5
-3
0.5
30
1
0
-3
3
0.02
0.5
0.1
0.5
-3
U mode U AM
Annular
AM
0.01
0.2
0.05
Sigma level
0.2
0.05
-3
0.1
Sigma level
0.05
3
Zonal windU climatology
0.01
-10
0.02
-3
3. The control simulation
0.1
0.01
0.02
3
GFDL spectral atmospheric dynamical core
Held-Suarez Physics (1994)
Zonally symmetric radiative forcing and surface friction
No topography, therefore no stationary waves
T42, enhanced stratospheric resolution with a sponge layer at the top
0.05
-1.25
Torque
δU(φ =30, σZonal
=10-1.25wind
)
Torque
changes in the full δU(φ
model
=70, σ0=10
)
0
0
0
δU(φ0=50, σ0=0.85)
changes in the full model
0.01
2. The idealized dry model
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•
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Complutense de Madrid. Madrid, Spain
3
We study the response of tropospheric jets in an idealized
atmospheric model to a prescribed zonal torque in a systematic
manner. We first examine the process that gives rise to the
tropospheric jet shift, when a zonal torque is applied in the
troposphere, as in Ring and Plumb (2007), and then explore the
stratospheric influence on the tropospheric jet with a zonal torque in
the stratosphere, as in Song and Robinson (2004). The insights
obtained from these idealized experiments can help to understand
the shifts of tropospheric jets and surface westerlies during climate
change.
Sigma level
1. Introduction
Sigma level
1Program
50
40
30
40
30
20
20
Time mean U
10
-10
The appearance of anomalous EP flux convergence in the stratosphere corresponds
to the increased EP flux divergence near the tropopause between the latitudes 45o
and 55o, and the direction of anomalous wave propagation coincides with the slope of
the zonal wind change.
50
-5
0
5 10 15 20 25 30
angular phase speed (m/s)
35
-10
Despite the differences in the stratospheric changes, the tropospheric jet shifts
poleward in both cases, and projects strongly onto the tropospheric internal mode.
Time mean U
10
40
-5
0
5 10 15 20 25 30
angular phase speed (m/s)
35
40
Interpretations:
These jet movements can be explained by changes in the critical latitudes
of tropospheric eddies (e.g. Chen et al. 2007). For a westerly torque near
the surface, the response can be thought of a two-step adjustment:
1) A zonally symmetric balance between the torque and surface friction.
The zonally symmetric response in the zonal wind is nearly barotropic
above the forcing.
2) The subsequent modifications on the eddies and the eddy-driven
circulations.
While the increased zonal winds in the subtropics allow the midlatitude
eddies to propagate further into the tropics and result in the
equatorward shift in the critical latitudes, the increased winds in the
midlatitudes accelerate the eastward eddy phase speeds and lead to the
poleward shift in the critical latitudes.
6. Summary
The tropospheric jet shifts equatorward for the westerly zonal torque on the
equatorward flank of the jet in the troposphere. In contrast, the tropospheric jet
moves poleward for the torque on the poleward flank of the jet in the troposphere,
and for the torque in the extratropical stratosphere.
These jet movements can be explained by changes in the critical latitudes
of the tropospheric eddies, due to the eastward acceleration of the tropospheric
torque or the downward influence of the stratospheric torque.
References
Chen, G., I.M. Held, and W.A. Robinson, 2007: Sensitivity of the Latitude of the Surface Westerlies to Surface Friction.
J. Atmos. Sci., 64, 2899–2915.
Ring, M.J., and R.A. Plumb, 2007: Forced Annular Mode Patterns in a Simple Atmospheric General Circulation Model.
J. Atmos. Sci., 64, 3611–3626.
Song, Y. and W. A. Robinson, 2004: Dynamical mechanisms for stratospheric inuences on the troposphere. J. Atmos. Sci.,
61, 1711-1725.