Etudes et évaluation de processus océaniques par des hiérarchies ...

4.7. ON THE BASIC STRUCTURE OF OCEANIC GRAVITY CURRENTS 135
Ocean Dynamics (2009) 59:551–563 555
tel-00545911, version 1 - 13 Dec 2010
of the friction layer attained the boundary of the domain.
All the experiments discussed in this publication
are listed in Table 1, which includes information
concerning the physics of the experiments: temperature
anomaly T, mean geostrophic velocity v G and
Richardson number Ri, as well as numerical values:
horizontal resolution and time of integration.
3 Results
I start with a qualitative description of the dynamics
of the gravity current before quantitatively determining
the key parameters.
3.1 Qualitative description
The gravity current is initially in a geostrophic state as
expressed by Eq. 1. The velocity near the boundary
is rapidly reduced due to friction. This rapid decrease
of the velocity leads to inertial oscillations throughout
the gravity current. Near the ocean floor, in the bottom
Ekman layer, frictional forces reduce the long-slope velocity
so the flow is no longer geostrophically adjusted,
as the Coriolis force no longer balances the hydrostatic
pressure gradient and the water in the Ekman layer
Fig. 1 Structure of the
gravity current after 60 h in
exp. G00 (top) and G15
(bottom), see Table 1 for
details. The inclination of the
floor is exaggerated in the
figure, for pedagogical
reasons only; the real angle is
only 1 ◦ . Please note that the
vertical extension is given in
metres and the horizontal
extension in kilometres. The
temperature anomaly, with
respect to the ambient water,
is shown (given in Kelvin).
Isolines of the y component
of the velocity (positive into
the plane) are given every
0.05 ms −1 in black for
positive values and red for
negative values, zero line is
omitted
flows down the pressure gradient. At the downslope
border of the vein, a friction layer forms, which consists
of gravity current water moving downslope (see Fig. 2).
This feature is observed in laboratory experiments by
Smith (1977) and Lane-Serf and Baines (1998) (and references
therein) and measured by Wirth and Sommeria
(unpublished manuscript).
In Fig. 1, one sees a typical result of our numerical
integrations for two experiments with different density
and temperature differences. In the interior of the gravity
current, the isolines of the y component are vertical
due to geostrophy, which says that this velocity component
is determined by hydrostatic pressure gradient,
that is, the angle of the surface of the gravity current
to the horizontal. In the upslope part of the gravity
current, the y component of the velocity is negative
due to the negative angle of the surface of the gravity
current to the horizontal.
Figure 1 clearly shows that the gravity current consists
of two parts, a “vein” which is the thick part of
the gravity current (thicker than about 20 m) and a
“friction layer”, the thin layer of dense fluid that extents
downstream of the vein. The vein of the gravity current
water detrains through the Ekman layer into the friction
layer, which is responsible of the major part of the
downslope transport of gravity current water. Please