The Deep Interior Downwelling, the Veronis Effect
and Mesoscale Tracer Transport Parameterizations in an OGCM
ALBAN LAZAR* , GURVAN MADEC AND PASCALE DELECLUSE
Laboratoire d'Océanographie Dynamique et de Climatologie (LODYC),
Unité Mixte de Recherche CNRS-ORSTOM-UPMC,
Universtité Pierre et Marie Curie, Paris, France
Journal of Physical Oceanography
ABSTRACT
Numerous numerical simulations of basin-scale ocean circulation display
a vast interior downwelling and a companion intense western boundary layer
upwelling at mid-latitude below the thermocline. These features, related
to the so-called Veronis effect, are poorly rationalized and depart strongly
from our classical vision of the deep circulation where upwelling is considered
to occur in the interior. Furthermore they significantly alter results
of ocean general circulation models (OGCMs) using horizontal Laplacian
diffusion. Recently, some studies showed that the parameterization for
meso-scale eddy effects formulated by Gent and McWilliams allows integral
quantities like the streamfunction and meridional heat transport to be
free of these undesired effects. In this paper, an idealized OGCM is used
to validate an analytical rationalization of the processes at work and
help understand the physics.
Our results show that the features associated with the Veronis
effect can be related quantitatively to three different width scales that
characterize the baroclinic structure of the deep western boundary current.
In addition, since one of these scales may be smaller than the Munk barotropic
layer, usually considered to determine the minimum resolution and horizontal
viscosity for numerical models, we recommend that it be taken into account.
Regarding the introduction of the new parameterization, diagnostics in
terms of heat balances underline some interesting similarities between
local heat fluxes by eddy-induced velocities and horizontal diffusion at
low and mid-latitudes when a common large diffusivity (here 2000 m2 s-1)
is used. The near quasi-geostrophic character of the flow explains these
results. As a consequence, the response of the Eulerian-mean circulation
is locally similar for runs using either of the two parameterizations.
However, it is shown that the advective nature of the eddy-induced heat
fluxes results in a very different effective circulation, which is the
one felt by tracers