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Sensitivity of the position and variability of the eddy-driven jet and storm-track to different SST profiles in an aquaplanet general circulation model (Arpège). Clio Michel and Gwendal Rivière Météo-France/CNRS, CNRM-GAME, Toulouse, France. EGU Vienna - April 25, 2012. - PowerPoint PPT Presentation
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Sensitivity of the position and Sensitivity of the position and variability of the eddy-driven jet and variability of the eddy-driven jet and
storm-track to different SST profiles in storm-track to different SST profiles in an aquaplanet general circulation an aquaplanet general circulation
model (Arpège)model (Arpège)
Clio Michel and Gwendal Rivière
Météo-France/CNRS, CNRM-GAME, Toulouse, France
EGU Vienna - April 25, 2012
Intensity of the subtropical jet (SJ).
•Eddy-driven jet (EDJ) and storm-track position and intensity controlled by:
Anchoring effect of EDJ and storm track by the SST front.
(Nakamura et al. 2004, Brayshaw et al. 2008, Ogawa et al. 2011)
(Lee and Kim 2003, Son and Lee 2005)
•EDJ climatology intensively studied but much less the EDJ variability (linked to teleconnections such as AO, NAO).
A strong SJ leads to a single jet, a weak SJ leads to two jets.
Literature and problematicLiterature and problematic
Close but on the poleward side of the SST front.
ProblematicProblematic
Goal: Study of the eddy-driven jet variability using an aquaplanet AGCM
Approach: •Aquaplanet simulations using AGCM Arpège-Climat (T127L31) with prescribed axisymmetric SST at the lower boundary and equinoctial conditions.
•Systematic analysis of the storm-track and eddy-driven features (latitudinal position, intensity) relative to the SST gradient features (latitudinal position, intensity and width).
SST profilesSST profiles
Idealized piecewise linear distributions to only modify midlatitudes.
3 latitudes, 2 widths, 2 SST gradient intensities 12 experiments
ERA40 SST Atl-
DJF
40°
20°
The two jet regimesThe two jet regimes
One single jet Two well-separated jets
SST gradient at=30° SST gradient at=50°
Mass streamfunction (red and blue contours), zonal wind (shading)
y
p
v
Higher the gradient to the south, stronger the storm-track. Higher the gradient intensity (intensity and width),
stronger the strom-track.
Storm-track intensityStorm-track intensity
(Brayshaw et al. 2008, Graff and LaCasce 2012)
wide weak front
wide strong front
narrow weak front
narrow strong front
Baroclinicity (Meridional gradient of Baroclinicity (Meridional gradient of ))
Baroclinicity linked to SJ
Baroclinicity linked to SST
gradient
When the SST gradient is more equatorward, the two zones of baroclinic waves growth merged, the two types of baroclinicity add and the storm-track is stronger.
SST gradient at=30° SST gradient at=50°
Eddy-driven jet latitude Eddy-driven jet latitude (Umax at 850 hPa)(Umax at 850 hPa)
EDJ located on the poleward side of the SST gradient because of the predominance of AWB.
More poleward the gradient, more poleward the EDJ relative to the SST front.
One-to-one lineweak gradientstrong gradient
Wave-breaking asymmetriesWave-breaking asymmetries
anticyclonic WB
cyclonic WB
AWB = CWB AWB >> CWB
U U
SST gradient at=30° SST gradient at=50°
SST SST
WB event: reversal of the absolute vorticity gradient.
Interpretation in terms of refractive index Interpretation in terms of refractive index asymmetriesasymmetries
222
2
22
22 sin
cos4cos
am
NHf
cauyq
n
n2<0: evanescent waves. n2>0: waves propagation.
AWB
AWB
CWB
(Rivière 2009)Proposed interpretation:
SST gradient more poleward WB asymmetries (much more AWB) EDJ much more poleward relative to the SST gradient.
Eddy-driven jet variabilityEddy-driven jet variability
SST gradient at=30° SST gradient at=60°
node
U max
Only latitudinal shifting Mixed pulsing and latitudinal shifting
U regressed on PC1 (black contours) and U (shading)EOF1: first EOF of the vertically and zonally averaged zonal wind
node
U max
PC1>1PC1<-1U
PC1>1PC1<-1
U
ERA40 reanalysis (JJA – Southern ERA40 reanalysis (JJA – Southern hemisphere)hemisphere)
Pacific 55°S 120°E-210°E
Indian 45°S 30°E-120°E
Meridional SST gradient
South Pacific SST gradient poleward the South Indian SST gradient
ERA40 reanalysis (JJA - SH)ERA40 reanalysis (JJA - SH)
Pacific ocean 120°E-210°E
nodes
U regressed on PC1 (black contours) and U (shading)SST gradient at=55°S
Indian ocean 30°E-120°ESST gradient at=45°S
PC1>1
PC1>1 PC1<-1PC1<-1
UU
U max
U max
EOF1: first EOF of the vertically and zonally averaged zonal wind
Latitudinal shifting Mainly pulsing
ConclusionsConclusions
•We find two regimes of EDJ variability: latitudinal fluctuations (middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans. (similar to barotropic simulations of Barnes and Hartmann 2011)
ConclusionsConclusions
•We find two regimes of EDJ variability: latitudinal fluctuations (middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans. (similar to barotropic simulations of Barnes and Hartmann 2011)
•Intensification of the storm-track for SST gradient closer to the subtropical jet by addition of the two sources of baroclinicity.
•The position of the eddy-driven jet relatively to the SST gradient can be explained by wave-breaking asymmetries.(consistent with more idealized simulations of Rivière 2009)
ConclusionsConclusions
•We find two regimes of EDJ variability: latitudinal fluctuations (middle and low latitudes) and pulsing (highlatitudes), similar to the difference between the observed SH Pacific and Indian oceans.
Stronger subtropical jet. (by increasing SST in the subtropics) Zonally asymmetric SST forcing.
(similar to barotropic simulations of Barnes and Hartmann 2011)
•Intensification of the storm-track for SST gradient closer to the subtropical jet by addition of the two sources of baroclinicity.
•The position of the eddy-driven jet relatively to the SST gradient can be explained by wave-breaking asymmetries.(consistent with more idealized simulations of Rivière 2009)
•Perspectives:
End of talkThank you for your attention
SST profilesSST profiles
•Idealized piecewise linear distributions to only modify midlatitudes 12 experiments:
Solid lines L=20°
Dashed lines L=10°
Blue lines weak SST gradient=-8.54 10-6 K/mBlack lines strong SST gradient=-10.79 10-6 K/m
3 latitudes: 30°,40°,50°
Rossby wave breakingsRossby wave breakings
RWB = large-scale and irreversible overturning of the PV gradient over an isentropic surface.
Two types (Thorncroft et al., 1993):
anticyclonic (SW-NE) cyclonic (SE-NW)
• RWB detection algorithm (based on geometrical considerations)
Refractive index asymetries Refractive index asymetries (Rivière 2009)
222
2
22
22 sin
cos4cos
am
NHf
cauyq
n
n2 increases with
NW-SE orientation, poleward
propagation, u’v’<0 CWB
ū
φ
φ0
n2 decreases with
φ0
φ
ū
perturbation
SW-NE orientation, equatorward
propagation, u’v’>0 AWB
AWB
AWB
CWB
Eddy-driven jet variabilityEddy-driven jet variability
latitudinal shift
pulsing
Results partially similar to those found with a simple barotropic model. (Barnes and Hartmann 2011)
One-to-one line