Marine Ecosystem Dynamics Modeling Laboratory

Wind Driven Circulation

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.

Posted on January 16, 2014