Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 270

9.2.1 Coastal buoyancy currents

As low salinity water from the estuary enters the shelf sea on an open coast, the
Coriolis force tends to deflect it back towards the coast and to establish a flow
parallel to the coast. In the northern (southern) hemisphere, such coastal currents
or river plumes move with the land boundary to the right (left) of the direction of
flow, i.e. in the direction of propagation of a Kelvin wave (see Section 3.6.2 ) . Except
near the equator, where f 0, the influence of the Earth's rotation frequently

dominates, and discharge from an estuary results in a current flowing along the
coastal boundary. This flow transports the buoyancy input away from the estuary
source and distributes it into the ROFI where it is mixed into the water column at a
rate depending on the level of stirring imposed by the tides and wind stress.
A well-defined example of such a density-driven current is the Norwegian Coastal
Current (NCC) which flows northwards along the Norwegian coast with a transport
of up to 3 Sv (3

!

10 6 m 3 s 1 ). This current is mainly driven by a large input of
brackish water which enters from the Baltic Sea via the Skagerrak (the 'estuary' in
this case). The flow of this low salinity water along the coast is clearly evident in
Fig. 9.2 , which shows the salinity distribution of the northern North Sea and a
salinity section normal to the coast (Mork, 1981 ; Rodhe, 1998 ) . The current, which
is 200 metres) of the Norwegian

trench which lies close to the coast. Because the water depth generally exceeds that of
the current, the flow is scarcely influenced by frictional stresses at the seabed so that,
in the absence of wind stress, we would expect the current to be in geostrophic
balance. To a first approximation this seems to be the case, although the dynamics
are complicated by instability of the current. Satellite imagery (Mork, 1981 ) shows
that the flow can be unstable and develops large meanders and eddies with scales of
∼

75 km wide, flows mostly over the deep water ( >

∼

80 km. This baroclinic instability is essentially the same as that which we found can
occur in the frontal jets of TM fronts ( Section 8.4 ) and which has been studied in
laboratory experiments (Griffiths and Linden, 1981 ) . Although the eddies resulting
from the instability induce some horizontal mixing which tends to broaden the
current, the large water depth means that the brackish water is far from the stresses
at the bottom boundary and is only slowly mixed downwards. This limited vertical
mixing, together with some reinforcement of the flow by other freshwater sources
along the coast, allows the flow to remain as a distinct low-salinity feature for
∼ 1000 km along the Norwegian coast.

More usually, the depth near the coast is shallower and the coastal current
interacts with the seabed generating significant frictional stresses which modify the
flow. This is the situation along the Dutch coast in the North Sea, shown in Fig. 9.3a ,
where the estuarine discharge from the Rhine, carrying the large freshwater input
from the river ( 2000 m 3 s 1 ), is deflected to the right as it mixes with the ambient

water of the North Sea and establishes a persistent, density-driven current flowing
northwards along the coast. This Rhine ROFI is located in water of depth 10-20
metres and exhibits a salinity deficit reaching

∼

D S 4 near the coast. It extends

∼

30 km from the coast and, in the along coast direction, more than 200 km into

∼