Abstract:
Concern with the impact of human activities on the coastal region of the
world's oceans has elicited interest in the so-called "coastal boundary
layer"-that band of water adjacent to the coast where ocean currents adjust
to the presence of a boundary. Within this zone, roughly 10 km wide, several
physical processes appear to be important. One of these, the tides, is of
particular interest because their deterministic nature allows unusually
thorough analysis from short time series, and because they tend to obscure the
other processes.
The Coastal Boundary Layer Transect (COBOLT) experiment was
conducted within 12 km of the south shore of Long Island, New York to
elucidate the characteristics of the coastal boundary layer in the Middle
Atlantic Bight. Analysis of data from this experiment shows that 35% of the
kinetic energy of currents averaged over the 30 m depth are due to the
semidiurnal and diurnal tides.
The tidal ellipses, show considerable vertical structure. Near-surface
tidal ellipses rotate in the clockwise direction for semidiurnal and diurnal
tides, while near-bottom ellipses rotate in the counterclockwise direction for
the semidiurnal tide. The angle between the major axis of the ellipse and the
local coastline decreases downward for semidiurnal and increases downward
for diurnal tides. The major axis of the tidal ellipse formed from the depth
averaged semidiurnal currents is not parallel to the local shoreline
but is oriented at an angle of -15 degrees. This orientation "tilt" is a
consequence of the onshore flux of energy which is computed to be about 800
watts/m.
A constant eddy viscosity model with a slippery bottom boundary
condition reproduces the main features observed in the vertical structure of
both semidiurnal and diurnal tidal ellipses. Another model employing long,
rotational, gravity waves (Sverdrup waves) and an absorbing coastline explains
the ellipse orientations and onshore energy flux as a consequence of energy
dissipation in shallow water. Finally, an analytical model with realistic
topography suggests that tidal dissipation may occur very close (2-3 km) to the
shore.
Internal tidal oscillations primarily occur at diurnal frequencies in the
COBOLT data. Analysis suggests that this energy may be Doppler-shifted to
higher frequencies by the mean currents of the coastal region. These motions
are trapped to the shore and are almost exclusively first baroclinic mode
internal waves.
