7 4
This choice manifests a problem which, however, for another
choice equally may exist, that is that the flux in and
out of a region may change considerably with slight changes
on the boundaries. In how far should water just passing
through a corner of a region be taken into account? This
problem and the closely related problem of short-time
variations in the flux is in fact a problem of the length-
scale and time-scale of advective versus "diffusive"
transport.
In section 19 Davies gives estimates, based upon numerical
model calculations, of turn-over times obtained from in-
tegrated transports £fIhrough boundaries and from in- and
outflow for the individual grid-points of the model used,
The results differ considerably. For the Norwegian Channel,
where the inflow and outflow from the Norwegian Sea are
close together and pass, for a large part, through the
length of the Norwegian Channel down to the Skagerrak,
these inflows and outflows should be considered separately.
For the other sea areas there appears to be no firm reason
not to take fluxes integrated over the boundaries,
The turn-over times may be estimated form current obser-
vations, from model computations and from the analysis of
the spreading of tracers (Cs 137). With respect to the
latter method the following can be stated,
Artificial and natural tracers, i. e. drift bottles and
cards, fish-eggs and larvae (see for the latter categories
also section 6 by Adams and Harding) have been used to
estimate the Lagrangian flow in the North Sea and assess
its circulation. Recently, artificial tracers with distinet
properties have been used, as i. e. Rhodamine B dye, radio—
nuclides etc. Where iIhe dye experiments were mainly set for
investigations of diffusion on a rather small scale, the
radio-active isotopes, 1. e., Cs 137, discharged from plants
near Cherbourg (Channel) and Windscale (Irish Sea) were
monitored, and these data have been used to gain insight into
the large-scale circulation of the North Sea (Kautsky 10973, 1976}
