Chemistry Reference
In-Depth Information
characteristic time
L
/
U
for advection over a distance
L
cannot be longer
than the time available with low or moderate winds.
3.1. A note on wind and waves
So far we have assumed the slick material to be collected and compacted
by regions of surface convergence of the currents in the upper ocean. A
wind of velocity
V
causes a surface drift that on the average is uniform and
of the order of magnitude 0.03
V
. Such a surface drift will then tend to
move the surface material relative to the underlying regions of conver-
gence and thus upset the process of compactification of film material.
In the first case study above a wind velocity
V
in the
x
-direction would
remove the stagnation points, and thus the source of accumulated slick
formation if
U
k
< 0.03
V
. For example, with
U
k
= 15 cm s
-1
this would take
a wind of velocity > 5m s
-1
.
The waves caused by small wind velocities are damped by a slick. The
momentum loss of the waves induces a wave stress gradient acting on the
surface film (Foss 2001). This results in a compacting influence on the
upwind side of the slick.
3.2. Surface renewal
At the short end of the scale (of the order of 10 cm) the motion may be due
to convective instability in a shallow surface layer as explained by Gem-
merich and Hasse (1992). They made field observations of the surface mo-
tion by spraying powdered corkwood on the surface, watching how it col-
lected on a time scale of 10 seconds or less, into various structures
depending on the condition. Lines were by far the most common structure
observed. They may, for example, be caused by convective rolls along the
wind drift direction.
Will a slick be able to suppress such a convectively induced surface mo-
tion? Gemmerich et al. measured characteristic lengths between the lines,
and the corresponding surface velocities. One can now use the relation (4)
to estimate the critical film pressure needed to cover such an area. We find
using their data that a film pressure of 2 mN m
-1
would be enough. Thus a
slick easily suppresses the surface motion induced by shallow convection,
causing a boundary layer over which heat must be transported by conduc-
tion rather than convection. The viscous boundary layer is thicker than a
corresponding thermal layer because the kinematic viscosity is much larger
than the thermal diffusivity. This lead to a reduced efficiency of heat
transport through a slick that can be detected through the surface tempera-
ture signature it leaves (Jarvis 1962, MacLeish 1970).
