A Lagrangian-trajectory study of a gradually mixed ... - Kristofer Döös

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B.F. Jönsson et al. / Continental Shelf Research 31 (2011) 1811–1817
0
750
0
4500
σ0 (kg -1 m 3 )
4
350
-350
σ0 (kg -1 m 3 )
4
1500
-1500
8
-750
24°E 26°E 28°E
24°E 26°E 28°E
8
-4500
σ0 (kg -1 m 3 )
0
4
4500
1500
-1500
8
24°E 26°E 28°E
-4500
Fig. 7. Lagrangian overturning stream-functions projected with density ðs 0 Þ as the vertical coordinate in the diagrams. The upper left panel (A) shows the behaviour of the
water debouching into the Gulf of Finland from the river Neva (River Neva Meridional Stream-function). The upper right panel (B) represents the motion of the Baltic water
entering across the Hanko–Hiiumaa transect (Baltic Proper Meridional Stream-function). The lower panel (C) shows a combination of the results from the upper two panels
(Total Meridional Stream-function).
62°N
61°N
60°N
59°N
58°N
18°E 21°E
24°E
27°E 30°E
33°E
Fig. 8. Example of a trajectory originating from the Baltic Proper which enters the southern part of the Gulf of Finland in the surface layer and exits the Gulf in a subsurface
layer farther north. The color scale indicates the subsurface depth of the trajectory in meters. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
A closer examination of the vertical dynamics of the Gulf of
Finland using Lagrangian overturning stream-functions confirmed
the picture of a system dominated by a haline-driven circulation
from the Baltic Proper with a magnitude of about 5000 m 3 s 1 .
When this circulation was projected on the depth, it was also
possible to detect an additional closed shallow circulation cell,
most likely caused by a meridional slope of the sea-surface
height.
The present study has shown the feasibility of using Lagrangian
trajectories for studying complex and/or less well-defined
estuaries. When investigating systems of this type, a balance must
be struck between choosing a simple and robust model approach
as typified by mass-balance models (which, however, require a
number of specific criteria to be satisfied) and more advanced 3-D
numerical models. The latter type of model is somewhat complex
to apply, but does not, e.g. call for the system under investigation
(or parts thereof) to be more-or-less spatially homogeneous, and
furthermore yields a much better understanding of the physical
processes than mass-balance models do. 3-D modeling, however,
generates an abundance of highly resolved data in time and space.
Even if physical processes are described in a satisfactory manner,
it remains a challenge to specify the ‘‘representative state’’ of the
system, a common goal in not least estuarine studies. In situations
such as these, Lagrangian-trajectory methods can serve a useful
purpose, since these techniques are capable of providing a
coherent synthesis of the time-evolution of large data-sets, while
still including any intrinsic variability of the system. It is also
possible to define prognostic scalars characterizing the estuaries,
which facilitates a systematic comparison between different
systems.
Lagrangian approaches have classically been employed for
studies of the dispersion of pollutants, sediments, etc. In contrast,