Geology Reference
In-Depth Information
Fig. 14.3
Dynamics of internal waves, showing the velocity
relationships of water above and below the internal wave, and
how that is impacted when the internal wave touches ground
along a slope. (
a
) Water below the internal wave boundary
moves opposite to the propagation direction of the wave as a
whole, whereas water above the boundary moves in the same
direction as the wave (Modifi ed after LaFond
1962
) . (
b
) If more
than one wave is present in the ocean at different depths, which
can occur in well-stratifi ed water with signifi cant seafl oor topog-
raphy (e.g. Robertson
2005
), current directions along the seafl oor
can become quite complicated
primarily generated by the interaction of surface tides
with seafl oor topography (Cacchione et al.
1988
;
Huthnance
1989
), although in quite a complicated
fashion (Garrett and Kunze
2007
; Sherwin et al.
2002
) .
Once generated, internal tides may behave differently
than surface tides, as the internal tidal waves can radi-
ate out from the generation location in different direc-
tions (Garrett
2003
), and in fact a number of separate
internal tides may occur at differing depths, sometimes
slightly out of phase with each other (Fig.
14.3
)
(Mulder et al.
2009
; Robertson
2005
; Shepard et al.
1979
). One implication of this is that the dominant
fl ow direction at the seafl oor can differ depending on
the depth of the seafl oor, and which internal tidal wave
is involved. Above the density boundary of an internal
tidal wave, the fl ow will typically be up-slope, whereas
below this boundary, fl ow is down-slope (Fig.
14.3
). If
a series of internal tidal boundaries are present
(Robertson
2005
) , current fl ow related to the same tidal
cycle can be reversed at different locations, creating a
confusion of sediment transport directions along and
down the continental slope (Fig.
14.3b
). Current meter
evidence for this exists from a number of localities,
where the current direction during the same time period
at the seafl oor is reversed at different localities and
depths (Mulder et al.
2009
; Shepard et al.
1979
; Xu
et al.
2008
). The time-velocity curve of internal tidal
currents is apparently quite variable; some currents
start abruptly and strongly and then taper off, whereas
others start slowly, grow in strength, and end rather
abruptly, and still others appear to have a more normal
velocity distribution, with a gradual increase in veloc-
ity, a peak, and a gradual decrease in velocity (Fig.
14.4
)
(Nash et al.
2006
; Shepard and Emery
1973
; Shepard
and Marshall
1973
; Shepard et al.
1979
; Xu et al.
2008
). Why different time-velocity curves exist at the
same locations during succeeding cycles is unclear at
present, but may have to do with harmonic convergence
