Chemistry Reference
In-Depth Information
On surface renewal and sea slicks
Kristian B.Dysthe
Department of Mathematics, University of Bergen, Norway
Abstract.
Interaction between a current and surface-active material is
considered. Some simple cases where the substrate motion is steady and
2D is analysed using standard boundary layer theory. Questions like: how
is the transversal dimension of a slick related to the film pressure and the
substrate convergence? and how strong substrate motion does it take to
break up a surface film?, are addressed. It is pointed out that the answers
depend on whether the film can be considered stagnant, or develops self-
organised motion. It is further pointed out how small scale thermal convec-
tion at the ocean surface is easily suppressed by a slick.
1. Introduction
The surface motion of the sea takes place on a variety of scales from mm
capillary waves to mesoscale eddies. At low wind conditions it seems that
there are some scales that dominate with regard to slick-formation. They
give rise to long filamentary structures observed on optical- as well as
SAR-images of the sea surface, where they have a transverse dimension of
the order of 100 m and longitudinal coherence for several kilometres
(Scully-Powers 1986).
Also readily observed on satellite images are the ship-wakes, which may
extend over large distances (wake-lengths of approximately 70 km have
been reported). Again, the reason why they are visible seems to be con-
nected to the ability of the ship wake to trap surface-active material and to
produce a coherent film (Peltzer et al. 1992).
At the other end of the relevant scales is the surface motion caused by
shallow convection. Gemmerich and Hasse (1992) demonstrated its role in
renewal of the sea surface at low and moderate winds (less than 6 m s
-1
say). Their study indicated a time scale for the renewal of the sea surface
under these conditions of the order of 10 seconds.
The present paper considers some simple case studies with surface film
and substrate motion in search for some criteria for construction and de-
