|Description||Data from the COBOLT experiment, which investigated the first 12 km
off Long Island's south shore, are analyzed and discussed. Moored current
meter records indicate that the nearshore flow field is strongly
polarized in the alongshore direction and its fluctuations are well correlated
with local meteorological forcing. Complex empirical orthogonal
function analysis suggests that subtidal velocity fluctuations are barotropic
in nature and are strongly influenced by bottom friction.
Wind-related inertial currents were observed within the coastal boundary
layer (CBL) under favorable meteorological and hydrographical conditions.
The magnitude of these oscillations increases with distance from
shore, and they display a very clear 180° phase difference between surface
and bottom layers. Nearshore inertial oscillations of both velocity
and salinity records appear to lead those further seaward, suggesting
local generation and subsequent radiation away from the coast.
The response of the coastal zone to impulsive wind forcing is discussed
using simple slab and two-layer models, and the behavior of the
nearshore current field examined. The major features of the observed
inertial motions are in good qualitative agreement with model predictions.
It is found that, in a homogeneous domain, the coastal boundary
condition effectively prohibits inertial currents over the entire coastal
zone. In the presence of stratification the offshore extent of this prohibition
is greatly reduced and significant inertial currents may occur
within one or two internal deformation radii of the coast. The "coastal
effect", in the form of surface and interfacial waves which propagate
away from the coast, modifies the "pure" inertial response as it would
exist far from shore. The kinematics of this process is such that a 180°
phase difference between currents in the two layers is characteristic of
the entire coastal zone even before the internal wave has had time to
traverse the CBL. It is also suggested that, for positions seaward of
several internal deformation radii, interference between the surface and
internal components of the coastal response will cause maximum inertial
amplitudes to occur for t > x/c2, where c2 is the phase speed of the
The hydrographic structure of the CBL is observed to undergo frequent
homogenization. These events are related to both advective and
mixing processes. Horizontal and vertical exchange coefficients are
estimated from the data, and subsequently used in a diffusive model
which accurately reproduces the observed mean density distribution in
the nearshore zone.
Dynamic balance calculations are performed which indicate that the
subtidal cross-shore momentum balance is very nearly geostrophic. The
calculations also suggest that the longshore balance may be reasonably
represented by a steady, linear equation of motion which includes surface
and bottom stresses.
Evidence is presented which shows that variations in the longshore
wind-stress component are primarily responsible for the energetic fluctuations
in the sea surface slope along Long Island. Depth-averaged
velocities characteristically show net offshore transport in the study
area, and often display dramatic longshore current reversals with distance
from shore. These observations are interpreted in terms of a
steady circulation model which includes realistic nearshore topography.
Model results suggest that longshore current reversals within the CBL
may be limited to the eastern end of Long Island, and that this unusual
flow pattern is a consequence of flow convergence related to the presence
of Long Island Sound.||en_US