How topographic smoothing contributes to differences between the eddy flows simulated

by sigma- and z-level models.

T. Penduff Center for Ocean-Atmospheric

Prediction studies,

Tallahassee, FL, USA

B. Barnier, Laboratoire des Ecoulements

M.-A. Kerbiriou Geophysiques et Industriels

J. Verron Grenoble, France.

DYNAMO: Sigma-coordinate, Z-level, and isopycnic

ocean models. 1/3o resolution. North Atlantic

THIS STUDY:Sigma-coordinate, Z-level ocean models

1/3o resolution. South Atlantic

Why do the eddy flows simulated by SPEM and OPA differ?

What is the impact of their different topographies?

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South Atlantic configurations

Z-level OPA8.1

Sigma-coord SPEM5.1

• ROUGH bathymetry

+ 3 smoothed bathymetries

• SMOOTHED bathymetry

Same horizontal resolution 1/3o

Same open boundary conditions

Same climatological forcing (ECMWF)

Similar domains

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ROUGH SPEM

Beranger, 2000

Buffer zone

Simple sea-ice model

Eddy field in ROUGH and SPEM

Mean Surface EKE

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ROUGH SPEM

Snapshots of T(300 m)

ROUGH SPEM

EKE(z)

Intrinsic or topographically-induced differences???

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Absolute

Relative to 350 m

Subtropical

ACC

Subtropical

ACC

SPEM

ROUGH

Global topographic smoothing in OPA

ROUGH GS

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ROUGH GS

Snapshots of T(300 m)

ROUGH GS

SPEM

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ROUGH GS

Surface EKE

SPEM

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EKE(z)

Significant topographically-induced differences.Near-coast processes? Sea-floor roughness?

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Absolute

Relative to 350 m

Subtropical

ACC

Subtropical

ACC

SPEM

ROUGH

GS

Local topographic smoothing in OPA

ROUGH LS

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ROUGH LS

Snapshots of T(300 m)

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GSSPEM

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Surface EKE

ROUGH LS

GS

Impact of mesoscaletopographic roughness in OPA

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• Reduces eddy-topography interaction• Increases lifetime of Agulhas Rings

GS GSR

GS GSR

The removal of seafloor roughness in SPEM topography explains:• most of OPA/SPEM differences south of 40oS• part of north

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GS GSR

Impact of Seafloor roughness in OPAAbsolute

Relative to 350 m

SPEM

ROUGH

GS

GSR

Subtropical

ACC

Subtropical

ACC

Conclusions

• Different eddy flows in SIGMA and LEVEL

topographies

• Local topographic smoothing in the Agulhas region makes

LEVEL Rings behave like those in SIGMA

• Removing mesoscale roughness makes LEVEL EKE(z)

similar to that in SIGMA

more realistic

• Seemingly intrinsic differences between models in

WBC system

Confluence region

Some smoothing may improve LEVEL solutions in the absence

of adequate parameterization of current-topography interaction

Perspectives

• Eddy-resolving models, other domains ?

• Along-slope currents in sigma- and z-level systems

• Parameterization of topographic effects

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