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

B.F. Jönsson et al. / Continental Shelf Research 31 (2011) 1811–1817 1815
behaviour of specific water masses by following sets of trajectories
from one or more initial sections (in the present case the
mouth of the river Neva and the Hanko–Hiiumaa transect). A brief
general outline of the procedure is given in the appendix.
Applying this formalism to the previously described velocity
fields from the RCO-model, the river Neva inflow to the Gulf of
Finland is first considered. In Fig. 6(A), the resulting, laterally
averaged, Lagrangian overturning stream-function along the Gulf
is shown as a function of the depth. The river discharge is seen to
progress towards the Baltic Proper predominantly in the form of a
surface flow, although note the progressively deepening streamlines
indicating an entrainment of Neva water into the depths of
the Gulf.
In Fig. 6(B), the analogous set of results for the Baltic inflow to
the Gulf across the Hanko–Hiiumaa transect is shown. The
character of these streamlines is seen to differ markedly from
that associated with the Neva discharge. In Fig. 6(B), the dominating
overturning cell is constituted by the incoming saline
deep water from the Baltic Proper, which upwells in the Gulf as it
mixes with the river Neva water and returns westwards as much
fresher water in the surface layers. This is the contribution from
the Gulf of Finland to what has been termed the Baltic haline
conveyor belt (Döös et al., 2004).
It is also possible to add these two separately determined
stream-functions to one which corresponds to the standard
Eulerian stream-function, cf. Fig. 6(C). From this diagram it is
recognised that the fluxes associated with the ‘‘Baltic Proper
circulation cell’’ are an order of magnitude larger than the
transport from the river Neva and thus dominate the vertical
circulation in the Gulf of Finland.
Instead of projecting the stream-function against depth, this
can be done versus density. This intuitively less straightforward
visualisation has the advantage of revealing more of the physical
processes underlying the circulation. Fig. 7(A)–(C) thus show the
Lagrangian overturning stream-function against density. (A projection
versus salinity proved to manifest pronounced similarities,
underlining the well-known fact that the stratification and
circulation of the Baltic are mainly determined by the salinity,
not the temperature.)
A useful feature associated with these projections against
density/salinity is that they facilitate a qualitative as well as
quantitative understanding of how the salinity of water originating
from the Baltic Proper gradually is decreased as it circulates in
the Gulf of Finland. This deep overturning cell is associated with a
flux of around 5000 m 3 s 1 , hereby exceeding the value of
4000 m 3 s 1 deduced from the depth- projected stream-functions.
This discrepancy between the results from the different
projections indicates that the transversal slope of the isopycnals/
isohalines only plays a subordinate role for the vertical circulation
in the Gulf of Finland.
In the depth-projected results there are, however, indications
of a shallow circulation cell which is absent in the corresponding
salinity- and density-projected results. This feature is most likely
due to a horizontal circulation on the transversally inclined
isopycnals, similar to what holds true for the Southern-Ocean
Deacon Cell (Döös and Webb, 1994; Döös, 1994), but which
disappears when lateral averaging is carried out over isopycnals.
(A north–south slope of the shallow part of the horizontal
circulation in the Gulf of Finland can be discerned from individual
trajectories, an example of which is given in Fig. 8.)
5. Summary and discussion
The trajectory results reported here shed some new light on
the estuarine mixing dynamics characterizing the Gulf of Finland.
It has been shown that the main part of this estuary is dominated
by water from the Baltic Proper and that the most pronounced
mixing with Neva water takes place over a rather small area in
the inner parts of the Gulf, the location of this zone tending to
fluctuate somewhat. In contrast to previously held views (Meier
and Kauker, 2003), these variations do not appear to be directly
linked to either the freshwater influx from the river Neva or the
long-term variations of the wind forcing.
Depth (m)
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Depth (m)
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24°E 26°E 28°E
24°E 26°E 28°E
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Fig. 6. Lagrangian overturning stream-functions projected with depth 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. The bottom panel (Baltic Proper Meridional Stream-function) (C) shows a combination of the results from the upper two
panels, and corresponds to the standard Eulerian stream-function (Total Meridional Stream-function).