OCEANOGRAPHY 797
Water, eventually reaching the South Atlantic and flowing
eastward around Antarctica.
The travel time for the North Atlantic Deep Water to
reach the Antarctic continent has been estimated as being
on the order of 1000 years. Of course, we do not expect this
water to remain unchanged during such a journey. Mixing
processes alter the density of the water by mixing it with
the less dense water overlying it. It is this gradual mixing,
in fact, that creates the relatively warm, saline deep water
which arrives at the Weddel Sea to be mixed with the cool,
saline shelf water and form Antarctic Bottom Water.
Current speeds of the North Atlantic Deep Water and the
Antarctic Bottom Water vary greatly. Maximum speeds as
high as 20 cm/sec have been measured near the ocean bottom
beneath the Gulf Stream (Pierce and Joyce, 1988). However,
as indicated by the estimated travel time of the North Atlantic
Deep Water, the average deep water current velocity is much
lower, most likely on the order of 1 cm/sec.Coastal Ocean CirculationWe have already discussed the generation of surface (wind-
driven) and bottom (density-driven) currents, identifying the
dominant, or best known, of each. Our examples, however,
were confined to the “interior”, or deep ocean regions, far
removed from the shallow water areas that border most of
the continents. As mentioned earlier, the coastal oceans are
typically characterized by a shallow continental shelf, gently
sloping seaward to a continental slope, which drops rela-
tively abruptly to the deep ocean. Most seismically inactive
areas, such as the eastern coast of the United States, possess
a relatively wide (100–200 km) continental shelf, whereas
seismically active areas, such as the Pacific coast of the
United States, typically possess a very narrow shelf-slope
region.
Coastal ocean regions differ from the deep ocean both in
forcing and response. As one would expect, many of these
differences arise from the relatively shallow water depths
encountered along a continental shelf. Clearly, the surface
wind stress is responsible for much of the coastal ocean cir-
culation, especially the short term, highly variable compo-
nents of the flow. However, the response of the water column
differs from that of the deep ocean for the simple reason that
the water depth is often considerably less than the “depth of
penetration” of the wind stress, the Ekman Depth alluded to
earlier. Since the wind-induced motion does not therefore
decrease to zero before the ocean bottom is reached, the flow
experiences a resistance due to skin friction and form drag
over roughness elements on the bottom. For ocean circulation
modelers, the estimation-parameterization of this bottom flow
resistance is a formidable task. A knowledge is required, not
only of the roughness characteristics of the bottom, but also
the variation of these characteristics in both time and space
as the sediments are influenced by near-bottom motions and
bottom-dwelling marine organisms. For an excellent treat-
ment of the complexities of the flow within the continental
shelf “bottom boundary layer”, the reader is referred to Grant
and Madsen (1986).Of course, the proximity of coastal ocean regions to
land also influences the water circulation. In the vicinity of
freshwater inflows, the nearshore circulation is altered by
the presence of density-driven motions. Typically, the fresh
water moves on top of the saline ocean water, eventually
becoming mixed in the vertical and horizontal directions,
primarily through wind-induced turbulent mixing. Before
this mixing is complete, however, the horizontal and vertical
density gradients will induce water motions, often quite dif-
ferent from those anticipated from the local (i.e., wind) forc-
ing. As an example, the mean flow along the Middle Atlantic
Bight (the continental shelf region running from Cape Cod
south to Cape Hatteras) is toward the southwest, opposed
to the mean eastward wind stress experienced in the area.
Although still a topic of active research, the explanation for
this flow appears to lie in the presence of a mean along shelf
pressure gradient, quite possibly originating from freshwater
input north of the region (Chapman et al., 1986). The land
boundary itself is also responsible for phenomena unique to
the coastal ocean. Topographic variations, both in the hori-
zontal plane and along the ocean bottom can induce sec-
ondary motions by virtue of the variation in flow resistance
along the boundaries. These motions, as in the case of the
density-driven flows mentioned previously, can often run
counter to the local, primary forcing.
One of the most significant differences between the two
types of coastal boundaries mentioned earlier (narrowshelf
and wide-shelf) is the influence of deep ocean water motions
on the nearshore circulation. As one might expect, narrow
shelf areas are more prone to deep ocean forcing. For this
reason, fluctuations in the location and magnitude of deep
ocean surface currents (e.g., the California Current along
the Pacific coast of the United States) can greatly influence
the nearshore water motion, as can wind-driven upwelling
events. Regions possessing a wide continental shelf, however,
are characterized by quite different flow regimes. Recent evi-
dence (Chapman and Brink, 1987; Chapman et al., 1986)
indicates that shallow continental shelves remain effectively
isolated from deep ocean forcing. The circulation on these
shelves is dominated by wind-driven motions, with tidal forc-
ing and freshwater inflows (hence, density-driven currents)
important, especially in nearshore areas.
We should here note that in regions of energetic deep
water motions, the shelf and slope are occasionally influ-
enced by these motions. As a case in point, continental shelf
motions along the east coast of the United States can be
affected by the northward flowing Gulf Stream. This influ-
ence takes the form of circulation cells, or eddies (com-
monly referred to as Gulf Stream Rings) shed from the Gulf
Stream and driven toward the shelf region. As these short-
lived eddies (typically 1–3 weeks, Lee and Atkinson (1983))
approach the shallow shelf-slope boundary, their forward
motion is impeded by the bottom topography, thereby limit-
ing their influence on inner shelf motions. However, the rota-
tional motion of the eddies themselves can entrain extremely
large volumes of shelf-slope water, removing this water out
to the deep ocean and replacing it with Gulf Stream water
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