How Topographic Smoothing Contributes to Differences Between
the Eddy Flows Simulated by Sigma- and Z-level Models
Thierry Penduff, Bernard Barnier,
Marie-Aurelie Kerbiriou, and Jacques Verron
The characteristics of the mesoscale turbulence simulated at a
resolution of 1/3 deg by a sigma-coordinate model (SPEM) and a
geopotential-coordinate model (OPA) of the South Atlantic
differ significantly. These two types of models differ with respect to
not only their numerical formulation, but also their topography
(smoothed in SPEM, as in every sigma-coordinate application).
In this paper, we examine how these topographic differences result
in eddy flows which are different in the two models.
When the topography of the Agulhas region is smoothed locally in
OPA, as is done routinely in SPEM, the production mechanism
of the Agulhas rings, their characteristics and their subsequent
drift in the Subtropical Gyre, are found to converge toward those
in SPEM.
Furthermore, the vertical distribution of eddy kinetic energy (EKE)
everywhere in the basin interior becomes similar in SPEM and OPA
and, according to some current meter data, seems to become more
realistic when mesoscale topographic roughness is removed from the
OPA bathymetry (as in SPEM). As expected from previous process
studies,this treatment also makes the sensitivity of the Agulhas
rings to the Walvis Ridge become similar in SPEM and OPA. These
findings demonstrate that many properties of the eddies produced
by sigma- and geopotential-coordinate models are, to a significant
extent, due to the use of different topographies, and are not intrinsic
to the use of different vertical coordinates. Other dynamical differences,
such as the separation of western boundary currents from the shelf or
the interaction of the flow with the Zapiola Ridge, are attributed to
intrinsic differences between both models. This study shows that
topographic smoothing, which is of major importance in defining
sigma-coordinate model configurations, should also be considered
seriously when defining z-level configurations.