Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1977
Description:
A 37 day long field program was carried out in March 1974 on the
New England continental shelf break to study the current and hydrographic
structure and variability on the shelf and in the shelf/slope
front. A second experiment was conducted in the shelf break region for
one week in January 1975 to study frontal exchange processes.
The mean currents during the March 1974 experiment all had a westward
alongshore component, increasing in magnitude progressing offshore
from ~5 cm/sec to a maximum at the nearshore edge of the shelf/slope
front of between 10 and 20 cm/ sec, and decreasing in magnitude with
depth. The current structure was such that the velocity vector rotated
clockwise with depth in the shelf waters inside the front. The mean
alongshore transport of shelf water was on the order of 0.4 Sverdrups
through a cross-shelf transect south of Block Island. About 30% of the
transport occurred in the wedge-shaped region offshore of the 100 m
isobath and inshore of the front. Comparison of the observed mean currents
with those predicted by the steady frictional boundary layer model
of Csanady (1976) indicates that the model captures most of the essential
features of the shelf circulation.
The low frequency currents contain approximately 30% of the total
current variance. An empirical orthogonal modal analysis indicates
that for low frequency alongshore motions the whole shelf together with
the water above the front moves as a unit and that the on- offshore
currents are characterized by opposing flows at surface and bottom. The
alongshore wind stress component is the dominant forcing term for these
low frequency motions and for the subsurface pressure field as well.
For motion with periods longer than 33 hours, the time derivative term
in the cross-shelf momentum balance is comparable with the Coriolis
term while the advective terms are 2 to 10 times smaller, on the average.
The semi-diurnal tide is barotropic over the shelf with current
magnitudes that increase almost by a factor of two between the shelf
break and the inshore mooring 70 km shoreward. At the shelf break one-dimensional
continuity gives the correct relation between the surface
tide and the semi-diurnal currents. The semi-diurnal tide is clockwise
polarized. The diurnal tide is baroclinic, increasing somewhat toward
the bottom, is less clockwise polarized than the semi-diurnal, and has
tidal ellipses aligned with the isobaths. The diurnal tidal energy
decreases toward shore.
Inertial energy in the frontal zone is equal to the semi-diurnal
tidal energy near the surface. The inertial energy decreases with
depth and is an order of magnitude smaller further on the shelf. The
inertial oscillations are shown to be highly correlated with the wind
stress record, arising and decaying on a time scale of 3 to 4 days.
The inertial oscillations are shown to be preferentially forced by wind
stress events that have a large amount of clockwise energy at near
inertial periods.
The frontal zone is shown to be in near geostrophic balance with
an anticipated vertical shear across the front of the order of 5 to 8
cm/sec. Thus, there is a wedge-shaped region of velocity deficit that
is confined directly under the front and above ~200 m. Outside of this
region the velocity is alongshore to the west. Low frequency motion
of the front is shown to exist on time scales from 3 to 10 days although
the complete nature of the motions is not known. An oscillation of the
front about its mid-depth position at periods of 3 1/2 to 4 days was
caused initially by an eastward wind stress event forcing the front offshore
near surface and onshore along the bottom. This was accompanied
by large temperature oscillations near the bottom at midshelf and current
oscillations confined to those current meters near the front.
The internal wave band is most energetic in the center of the
front, is about half as energetic above the front where it is subject
to variations associated with the wind stress, and is smaller and
nearly constant below the front. The internal wave energy decreases
shoreward reflecting the decreasing stratification shoreward of the
wintertime hydrography. Linear internal wave theory seems to break
down in the conditions of the frontal zone.
A stability analysis of the front to small perturbations is
carried out by extending the model of Margules frontal stability of
Orlanski (1968) to include the steep bottom topography of the shelf
break region. The study covers the parameter range pertinent to the
New England continental shelf break region and indicates that the front
is indeed unstable; however, the associated growth rates are so slow
that baroclinic instability does not seem to be a viable explanation
for the observed frontal motions. Application of the theory to the
nearly flat topography of the shelf itself shows that the front would
be at least 20 times more unstable there suggesting that the front
would migrate offshore to the shelf break region until a stable equilibrium
was established between frictional dissipation and the instabilities.
Description:
Funds for 'the field program and the data analysis of the New
England Shelf Dynamics Experiment have been provided by the National
Science Foundation through grants GA-4l075 and DES 74-03001.
Keywords:
Ocean currents
;
Continental shelf
;
Fronts
;
Ocean circulation
;
Dallas (Ship) Cruise
;
A.E. Verrill (Ship) Cruise
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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