FISHERIES OCEANOGRAPHY
Fish. Oceanogr. 8:4, 255±263, 1999
Nonlocal wind-driven fjord±coast advection and its potential
effect on plankton and ®sh recruitment
LARS ASPLIN,1,* ANNE GRO VEA
SALVANES2 AND JON BENT
KRISTOFFERSEN2
1
Institute of Marine Research, PB 1870 Nordnes,
N±5024 Bergen, Norway
2
Department of Fisheries and Marine Biology,
University of Bergen, N±5020 Bergen, Norway
early life stages between neighbouring fjords and thus
enhance genetic exchange.
Key words: advection of ®sh eggs and larvae, ®sh
recruitment, fjord±coast advection, nonlocal forcing,
numerical 3D simulation, wind-generated upwelling
and downwelling
ABSTRACT
INTRODUCTION
The Bergen Ocean Model (BOM), a three-dimensional physical coastal ocean model, was used for a
numerical simulation experiment to investigate shortterm effects of wind-generated coastal upwelling and
downwelling on the dynamics of adjacent large outer
and smaller inner fjords. The effect of the real
alongshore wind regime on advection for an idealized
fjord topography, resembling Masfjorden, western
Norway, is used as an example. This modelling exercise is a supplement to, and its predictions support, the
various hypotheses investigated in ecosystem simulation studies of the Masfjorden. The model predicts
that coastal winds from the north cause upwelling and
transport the upper water layer out from the fjords.
Winds from the south cause downwelling and
transport the upper water layer into the fjords. The
transport is rapid and »50% of the upper water layer
may be replaced within 1±2 days. Implications of
these physical processes for the dispersal and retention
of planktonic organisms and the early life stages of ®sh
are discussed. If strong southerly winds occur
frequently, this will transport planktonic organisms
into the fjord and may increase the carrying capacity
for planktivorous ®sh. In contrast, frequent strong
northerly winds may reduce the abundance of planktonic organisms, including the early life stages of
marine ®sh, and thus possibly reduce recruitment to
fjord ®sh populations. Frequent shifts between southerly and northerly winds would cause an exchange of
The western coast of Norway is rugged, with numerous fjords, which are typically long and deep, with
widths varying from several kilometres in the outer
parts to some hundreds of metres in inner regions.
Many ®sh species spawn in the fjords, with the early
pelagic life stages inhabiting the upper part of the
water column (Sundby, 1991; Ellertsen et al., 1992).
The major proportion of mesozooplankton also occurs
in the upper 30 m of the water column (Aksnes et al.,
1989).
Outside the fjords, the coastal waters are generally
strati®ed, with the baroclinic Norwegian Coastal
Current (NCC) as an important dynamic feature
(Sñtre and Mork, 1981; Sñtre et al., 1988). There is
normally a signi®cant exchange of water between the
fjords and the NCC (Svendsen, 1980; Aksnes et al.,
1989; Aure et al., 1996). Many physical processes are
responsible for this water exchange (e.g. wind, tides,
freshwater run-off; review: Farmer and Freeland,
1983). Of special interest for the present work is the
importance of nonlocal forcing (such as coastal wind)
of fjord water mass dynamics as described by Klinck
et al. (1981) and Proehl and Rattray (1984). The topography of fjord areas is a complicated but important
factor in the determination of water exchange, and its
complexity is probably a major reason why this subject
has received relatively little attention.
In the Northern Hemisphere, the net wind-driven
transport of water is to the right of the wind direction
(Ekman transport; Gill, 1982). Along the western
coast of Norway, northerly alongshore wind thus
produces coastal upwelling, and southerly alongshore
wind produces coastal downwelling. Associated with
the upwelling and downwelling are coastal currents.
The widths of the coastal currents are comparable to
*Correspondence. e-mail: [email protected]
Received 6 April 1998
Revised version accepted 30 September 1998
Ó 1999 Blackwell Science Ltd.
255
256
L. Asplin et al.
the internal radius of deformation (i.e. the ratio of the
internal wave speed and the Coriolis parameter; Gill,
1982). Along the west coast of Norway, the coastal
currents associated with the upwelling and downwelling typically extend »5 km offshore.
The coastal upwelling or downwelling will be
reduced if the coastline is broken by a fjord mouth,
and a coastal trapped, internal Kelvin wave bore will
be generated. Such a Kelvin wave bore can propagate
into and inside fjords having a width comparable to
the internal radius of deformation. The propagation of
the bore also sets up a temporary current ®eld, which
generates exchange of water between the coast and the
fjord, but the extent of this exchange has hitherto
been largely unknown.
Field studies in fjords generally report the highest
biomass of mesozooplankton, including Calanus ®nmarchicus, in the upper water layers (Aksnes et al.,
1989). Simulation modelling of the ecosystem
dynamics of Masfjorden (Fig. 1) and other west
Norwegian fjords has shown that a highly ¯uctuating
transport of C. ®nmarchicus into the fjords has a pronounced effect on the carrying capacity for ®sh at
higher trophic levels (Salvanes et al., 1992, 1995).
Moreover, ®eld investigations in Masfjorden reveal
large interannual variations in cod recruitment as
Figure 1. Map of the western coast of Norway, showing the Fensfjorden and Masfjorden.
Ó 1999 Blackwell Science Ltd., Fish. Oceanogr., 8:4, 255±263.
Nonlocal wind-driven fjord±coast advection
one-year-olds (Salvanes and Ulltang, 1992; Nordeide
et al., 1994), partly caused by density-independent
factors at the egg and larval stage (Ellertsen et al.,
1992), but modi®ed by density-dependent predation
and cannibalism after settlement (Ellertsen et al.,
1992; Salvanes et al., 1994; Salvanes et al., 1995;
Salvanes and BalinÄo, 1998; Giske and Salvanes,
1999). Because the early life stages of marine ®sh
generally inhabit the upper water layers (Sundby,
1983; Ellertsen et al., 1992), they are affected by
advection of these layers (Bailey, 1981). Hence, for
local ®sh populations, wind-generated advection has the
potential to transport eggs and larvae to or from suitable
habitats, and similarly to transport zooplankton.
The importance of advection for ®sh recruitment
relative to other factors such as starvation and predation is not well known. There are indications that the
in¯uence of advection can be signi®cant (Taggart and
Leggett, 1987; Pepin et al., 1995; Svendsen et al.,
1995), but it is not clear at what time or spatial scales
advection operates. In the present paper, we demonstrate that the ¯uctuations in water transport caused
by nonlocal wind conditions operate at short time
scales.
The modelling exercise presented is based on
existing knowledge and is a natural extension of, and
supplement to, the previous ecosystem simulation
modelling developed for an idealized food web of
Masfjorden (Salvanes et al., 1992, 1995). The ecosystem simulations of Salvanes et al. (1992, 1995) and
the ®eld observations reported in Aksnes et al. (1989)
provided the framework for the present modelling
study by indicating connections between zooplankton
advection and ®sh carrying capacity. However, the
ecosystem modelling simpli®ed advection to a bordering forcing function, which changed monthly and
which did not have any depth solution. Furthermore,
the physics of fjord/coast water dynamics were not
incorporated in the ecosystem model, because the
focus was on the dynamics of the biological processes
and species interactions within the ecosystem. In the
present study, we use a three-dimensional numerical
simulation experiment to show that the short-term
effects of nonlocal wind-generated coastal upwelling
and downwelling are important for water exchange
between coast and fjords. The wind regimes used for
the simulation experiments are frequently observed on
the west coast of Norway as well as at Shetland or
elsewhere in the northern North Sea. The fjord
topography is idealized, keeping fjord sizes and distances from the coast resembling the real topography
of Masfjorden and Fensfjorden, through which
Masfjorden is connected to coastal waters of western
Ó 1999 Blackwell Science Ltd., Fish. Oceanogr., 8:4, 255±263.
257
Norway. Finally, the consequences of the results for
retention and dispersal of planktonic organisms and
early life stages of ®sh are discussed.
MATERIALS AND METHODS
Model design
The numerical model used for the simulations of
fjord and coastal water exchange is a threedimensional, primitive equation, time-dependent,
r-coordinate, ocean circulation model (the Bergen
Ocean Model; BOM) developed by Berntsen et al.
(1996). The prognostic variables of the model are
three components of velocity, potential temperature, salinity, surface elevation and two variables
representing turbulent length scale and turbulent
kinetic energy. The BOM has an embedded turbulence closure submodel (Mellor and Yamada,
1982). The governing equations are the equations
for conservation of mass, momentum, temperature
and salinity, along with the hydrostatic equation in
the vertical and an equation of state relating
salinity and potential temperature to potential
density. The equations are solved by ®nite
difference techniques on a staggered Arakawa
C-grid. The time differencing is implicit.
Two numerical experiments were performed to
simulate the hydrodynamic response to winds along
the coast in a strati®ed, coupled coastal ocean and
fjord system. The ®rst experiment simulated wind from
the south, the open ocean being to the left of the
southern wind component and the coast on the right.
The combination of this wind direction and coastal
topography generated coastal downwelling. The other
experiment, with the wind in the opposite direction,
generated coastal upwelling. The numerical experiments were idealized so as to focus on the physical
processes and to keep the Kelvin wave problem
tractable, although still having realistic characteristics
of Norwegian fjord and coastal areas.
The model domain consisted of a rectangular
coastal ocean and a fjord system (Fig. 2). The main
fjord topography (outer fjord) was idealized to 7 km
wide and 15 km long, and the inner fjord arm was
1500 m wide and 12 km long, thereby resembling the
topography of Masfjorden and Fensfjorden (Fig. 1).
The bottom depth throughout the modelled area (i.e.
both the coastal ocean and the fjords) was set constant
at 100 m. The initial conditions were of no water
velocities, no surface elevations and a constant
temperature ®eld of 10°C. The salinity ®eld was horizontally homogeneous, but with linear strati®cation in
258
L. Asplin et al.
Figure 2. Top view of the idealized model domain used for
the numerical simulations. A and B refer to sections
displayed in Fig. 5.
Wind stress on the coastal ocean was the only
forcing of the model (i.e. no local wind was speci®ed
for the fjords). The wind direction was along the coast
only. The total simulation period was 72 h. The simulated wind pattern was chosen, based on observations
in the spring frequently registered at the Norwegian
Meteorological Institute. The maximum wind speed
was 16 m s±1, and the absolute wind speed values were
the same for the two simulations. Wind speed was
increased linearly from 0 to 16 m s±1 over the ®rst
12 h, then held constant at 16 m s±1 for the next 36 h
before it was reduced linearly to 10 m s±1 through the
last 24 h of the simulations.
The model grid had a horizontal resolution of
500 m and was thus able to resolve adequately the
internal radius of deformation (»4 km) based on a
typical internal wave speed of »0.5 m s±1. Vertically,
35 grid nodes were used, with the grid size (expressed
in z-coordinates, assuming no surface elevation)
increasing from 0.25 m in the upper few metres to 5 m
below 10 m depth. This ®ne vertical grid resolution
was needed to simulate adequately the wind-driven
¯ow and internal wave activity (Asplin, 1999). The
time step of the simulations was 75 s, obeying the
time-step constraints of the numerical model grid.
RESULTS
the upper 50 m, increasing from a salinity of 33 at the
surface to 35 at 50 m depth.
Coastal downwelling and advection into the fjords
Coastal downwelling generated by the southerly wind
caused a substantial up-fjord advection of upper-layer
(0±45 m depth) water masses (Table 1; Fig. 3). During
the ®rst 2 d of the simulation, the internal Kelvin
wave bore created by the reduced downwelling in the
fjord mouth area propagated into and out of the outer
fjord (with the land on its right). The Kelvin wave was
Table 1. Relative distribution (%) of the tracer water masses after 72 h of the simulations of coastal downwelling (southerly
wind) and upwelling (northerly wind). The initial distributions of the water masses are shown by the 100% values in parentheses, e.g. under downwelling conditions, MASS 1 was initially 100% in the outer fjord, between 0 and 45 m depth. See text
for explanation.
Inner fjord (Masfjorden)
Outer fjord (Fensfjorden)
Coastal ocean
0±45 m
Simulated Conditions
Water Mass
0±45 m
45±100 m
0±45 m
45±100 m
Downwelling
MASS 1a
MASS 2b
13
28 (100)
3
42
31 (100)
6
46
24
2
0
5
0
Upwelling
MASS 1a
MASS 2b
1
30 (100)
0
0
42 (100)
51
0
0
55
19
2
0
a
b
45±100 m
Representing an idealized Fensfjorden.
Representing an idealized Masfjorden.
Ó 1999 Blackwell Science Ltd., Fish. Oceanogr., 8:4, 255±263.
Nonlocal wind-driven fjord±coast advection
259
Figure 3. Simulated dynamic transfer and retention of
tracer water masses between the large outer fjord (resembling
Fensfjorden) and a small inner fjord (resembling Masfjorden)
for the downwelling and upwelling simulation experiments.
(a) The initial horizontal distribution of the tracer water
masses; vertically, the tracer water masses extend from the
surface to 45 m depth. (b and c) Changes in the distribution
of the tracer water masses above 45 m depth in (b) Fensfjorden and (c) Masfjorden through the simulated period.
DW refers to downwelling and coastal wind from the south.
UPW refers to upwelling and coastal wind from the north.
below this depth range (out of the fjords). Large horizontal variations were found in both the ¯ow ®eld and
the hydrography, with in¯ow along the left fjord side
(looking out from the fjord) and out¯ow along the
right fjord side. About 20 h after initiation of wind
forcing, the circulation connected with the Kelvin
wave bore had started to push water into the upper
40±50 m of the water column of the inner fjord
(1.5 km wide). Downwelling of upper-layer water took
place inside the inner fjord, with out¯ow of water into
the outer fjord in the lower layer (below 40±50 m
depth). Seventy-two hours after the initiation of the
wind, most of the water mass originally present in the
upper layer of the outer fjord (MASS 1 referring to the
idealized Fensfjorden, Table 1) was retained in the
fjord system, with 31% still in the upper layer whereas
46% had been transported to the lower layer of the
outer fjord and 13% to the upper layer of the inner
fjord. Of the water mass originally in the inner fjord
(MASS 2 referring to the idealized Masfjorden,
Table 1), 28% was still retained in the upper layer
after 72 h, 42% had been transported down to the
out¯owing lower layer within the inner fjord, and 24%
to the lower layer of the large outer fjord. The drift of
the passive particles in the surface layer for the
downwelling experiment also showed a tendency for a
movement into the fjord system (Fig. 4a).
approximately two-layered, with ¯ow in the direction
of propagation in the upper 40±50 m of the water
column (into the fjords) and in the opposite direction
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Coastal upwelling and advection out of the fjords
Coastal upwelling, generated by a northerly wind,
¯ushed the upper water layer out of both the large
outer and the small inner fjord (Table 1; Fig. 3).
During the ®rst 2 d of the simulation, the internal
Kelvin wave bore, created by the reduced upwelling in
the fjord mouth area, propagated into and out of the
outer fjord in the same direction as the bore created by
the coastal downwelling (land on its right). The simulated upper-layer (0±20 m depth) ¯ow was in the
opposite direction to the wave propagation, and the
lower-layer ¯ow was in the same direction as the wave
260
L. Asplin et al.
Figure 4. Advection of passive particles (identi®ed by
number) in the surface layer (upper 1 m) for (a) the downwelling experiment (southerly coastal wind) and (b) the
upwelling experiment (northerly coastal wind). The open
triangles refer to start positions of the particles (at 0 h) and
the open squares to end positions (after 72 h).
propagation. Upwelling of lower-layer water took
place inside the inner fjord, and in¯ow of water from
the outer fjord was in the lower layer. During coastal
upwelling, a rapid ¯ush-out of the upper layer was seen
(Table 1). After 72 h, 57% of the water mass originally in the outer fjord (MASS 1) had been advected
to the open ocean. Also, a signi®cant proportion of the
water mass originally in the inner fjord (MASS 2) had
been advected out and only 30% of MASS 2 was
retained within the inner fjord; 51% had been transported to the outer fjord and as much as 19% reached
the coastal ocean. The illustration of the passively
drifting particles in the surface layer for the upwelling
experiment also showed a tendency towards outwards
drift from the fjords to the coastal ocean (Fig. 4b).
Water exchange in the inner fjord (Masfjorden)
Figure 5 illustrates vertical sections of current vectors
and isohalines along the small inner fjord 30 h after
the initiation of the coastal wind (which represents
the time at which maximum advective ¯ow occurs).
The maximum current speed in the inner fjord is about
0.2±0.3 m s±1 and con®ned to the upper 20 m of the
water column. The results from the downwelling
simulation show an in¯ow of water in the upper 40±
50 m of the water column and an out¯ow below that
depth range (Fig. 5a), whereas the results from the
upwelling simulation show the opposite (Fig. 5b).
Note that the upper layer was thinner (20±30 m) for
the upwelling simulation than for the downwelling
simulation (40±50 m).
Figure 6 shows that the simulated coastal downwelling generated a net positive volume ¯ux to the
upper layer of the inner fjord and that the simulated
coastal upwelling caused a negative volume ¯ux
re¯ecting a ¯ushing-out of the upper layer of the inner
fjord.
DISCUSSION
Previous explanations of the observations of large
volume exchange between coast and fjord systems
were incomplete because only selected aspects of the
fjord dynamics were considered (Aksnes et al., 1989;
Kaartvedt and Svendsen, 1995; Wassman et al., 1996),
or the solutions were restricted to narrow fjords
(nonrotating dynamics) and for time-independent
dynamics (Stigebrandt, 1990; Aure et al., 1996). The
present study focuses on the effects of real, but nonlocal wind-generated fjord±coast advection on the
dynamics of an idealized topography of an actual fjord±
coast system (Masfjord±Fensfjord±coast) and represents an alternative and more complete approach.
Ó 1999 Blackwell Science Ltd., Fish. Oceanogr., 8:4, 255±263.
Nonlocal wind-driven fjord±coast advection
Figure 5. Vertical sections along the middle of the inner
fjord (transect A±B; Fig. 2) of current vectors and isohalines
after 30 h for (a) the downwelling simulation and (b) the
upwelling simulation.
The predictions from the simulations of the present
study support the hypothesis of Aksnes et al. (1989)
and Salvanes et al. (1992) that regional wind conditions are of importance in the dynamics of fjords
having a greater width than the appropriate dynamic
length scale (internal radius of deformation). In the
Northern Hemisphere, the internal radius of deformation decreases towards the north (Gill, 1982),
which means that narrow fjords at northern latitudes
may have rotational dynamics whereas fjords of the
same dimensions further to the south may not. Wind
regimes that are common along the western Norwegian coast, as well as in areas of other North Atlantic
fjord systems, are shown to generate advection of
water into fjords located far from the coast and thus
from the origin of the physical driving processes. Our
modelling approach demonstrates that nonlocal
Ó 1999 Blackwell Science Ltd., Fish. Oceanogr., 8:4, 255±263.
261
Figure 6. The calculated volume ¯ux (m3 s±1) in the upper
50 m of the water column through the mouth of the inner
fjord (Masfjorden). The solid line represents the results of
the downwelling experiment (southerly coastal wind) and
the dashed line represents the results of the upwelling
experiment (northerly coastal wind).
northerly regional winds will ¯ush out the upper water
layer of fjords, and that nonlocal southerly regional
winds will cause a net transport into the fjords. These
events take place in a remarkably short period (about
1 day) and the absolute value of the volume ¯ux is 1±2
orders of magnitude greater than typical volume ¯uxes
associated with classical estuarine circulation (Stigebrandt, 1990). The validity of the simulation is supported by the good correspondence between the
simulated depth range and direction of the advective
layers with the vertical distribution of residual current
across the sill of Masfjorden, as reported in Aksnes
et al. (1989).
The numerical results clearly demonstrate that
studies on local fjord water mass dynamics should take
into account the in¯uence of remote coastal winds.
Furthermore, in numerical simulations of particular
fjord systems, a better representation of the fjord
topography, wind ®elds and details of the initial density
distribution will improve the realism of the solutions.
As to the bottom topography, details of this will have
only a minor in¯uence on the simulated dynamics as
long as the bottom depth is greater than »100 m.
Because mesozooplankton and most ®sh eggs and
larvae are generally concentrated at depths shallower
than 30 m (Sundby, 1983; Aksnes et al., 1989;
Ellertsen et al., 1992), it is likely that nonlocal winddriven fjord±coast advection of the surface water layers
will have biological implications for these organisms.
This will depend on the wind regime, for example.
262
L. Asplin et al.
· Frequent strong southerly winds will transport
passive particles into fjords and generate advective
supply to the carrying capacity of fjords.
Frequent strong northerly winds will ¯ush the surface layers out from the fjords. This implies a
reduced carrying capacity for planktivorous organisms and reduced retention of the early life stages of
marine ®sh.
Frequent shifts between southerly and northerly
winds will cause an exchange of plankton between
neighbouring fjords via the coastal water and will
thus counteract genetic isolation.
Various strategies may have evolved to cope with
such advective regimes, for example vertical migration (Eiane et al., 1998), selection of spawning
grounds at locations with increased water residence
time (Ellertsen et al., 1992; Brander, 1994), spawning demersal eggs, and `bet-hedging' or `spreadingthe-risk' strategies by spawning repeatedly over a
prolonged period (Den Boer, 1968; Slatkin, 1974).
However, the eggs and larvae of many species are
concentrated in the upper advective layer, where
they are vulnerable to advection. Nevertheless, for
many species the depth of spawning and the buoyancy of eggs may have evolved to reduce dispersal of
the youngest stages. For example, the spawning period for coastal cod (Gadus morhua) in the fjords of
western Norway (February to April) coincides with
frequent winds from the south, and consequently
frequent downwelling. The eggs will ascend in water
of salinity greater than 29 with a vertical speed
(1 mm s±1) that exceeds the typical vertical speed of
downwelling water (0.1 mm s±1; Sundby, 1983,
1991); hence cod eggs are expected to be retained
in fjords during downwelling. The spawning of other
species may be similarly adapted to upwelling situations that predominate in the summer. For example, the mesopelagic ®sh Maurolicus muelleri has a
bet-hedging strategy with repeated spawnings during
the summer, and, usually, a deep egg distribution
(Lopes, 1979; John and Kloppmann, 1989, 1993)
that may prevent advection out of a fjord during
upwelling episodes.
Our study suggests that the sequence of coastal
wind episodes (including wind direction, strength and
duration) could in¯uence the advective transport and
potentially contribute to variability in ®sh recruitment, abundance of planktonic organisms and settlement areas. Pelagic early life stages of ®sh would be
concentrated in inner fjords when coastal wind is from
south, whereas frequent winds from the north would
¯ush out the fjords.
ACKNOWLEDGEMENTS
The authors received funding from The Research
Council of Norway, including the Programme for
Supercomputing (through a grant of computing time).
Thank you, Jarle Berntsen, for providing the Bergen
Ocean Model. We also thank Jarl Giske and Svein
Sundby for discussions and valuable comments to the
paper, two anonymous referees for comments on an
earlier version, and Steve Coombs for his valuable
suggestions which improved the text considerably.
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