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Wind Driven Circulation
To examine the influence of the wind
on the sub-tidal circulation and mixing in the NB/MHB,
we repeated the model run with tidal forcing and river
discharge with the addition of a wind field constructed
from data interpolated from all available meteorological
measurement stations in the region. During March 15-April
14, 2001, the wind direction varied over a time scale
of 1-5 days. The wind speed was about 5-10 m/s.
The interaction of wind driven and river
discharge buoyancy induced circulation results in a complex
time-dependent near surface sub-tidal current field. In
the case with tidal forcing plus river discharges, the
model predicts two strong currents flowing out from MHB
to NB through a deep channel, with a strong anticyclonic
eddy east of Hog Island and a weak cyclonic eddy around
the upper left coast of Aquidneck Island. This pattern
remain unchanged under the southeasterly wind condition,
even though the intensity of the southward current and
eddies slightly weaken (Fig. 1a). The northeasterly wind
increases the southwestward current, which produces more
spatially uniform outflow to NB from the NB/MHB channel
(Fig. 1b). The anti-cyclonic eddy field in the north convex
coastal region in the western MHB is also intensified
in this case. When the wind turns northwesterly, the near-surface
current weakens remarkably. This water splits into two
sections in the southern region of MHB: the first turns
clockwise and flows through the MNB/NB channel to NB;
the other returns northward to join the southward flow
into the SR (Fig. 1c). The southwesterly wind causes the
wind-induced water transport against the buoyancy flow.
As a result, the outflow into the SR reduces significantly
(Fig. 1d).
The vertical profile of the sub-tidal
currents on the cross-section of the NB/MHB channel varies
with the wind. The profiles under southeasterly and northwesterly
winds are very similar to that predicted in the case with
the river discharge (Fig. 2a, c). Northeasterly and southwesterly
winds tend to intensify the outflow near the surface and
thus return flow in the deep region (Fig. 2b,d).
The wind seems to be a key process giving rise to the
short-term variation of the sub-tidal circulation in NB/MHB.
Since the near-surface, sub-tidal current varies almost
instantaneously with a wind direction which changes over
hourly to daily time scale, it is difficult to interpret
the current data measured on the ship-mounted ADCP without
the assistance of numerical models.
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