| The mean current typically flows southerly through Massachusetts Bay and turns offshore into the Gulf of Maine (fig. 5). During much of the year, this weak counterclockwise circulation persists in Massachusetts and Cape Cod Bays, principally driven by the southeastward coastal current in the Gulf of Maine. The current proceeds southwesterly into the bay south of Cape Ann, southward along the western shore, and easterly out of the bay north of Race Point, typically at a strength of about 5 cm/s (0.1 knot). This flow pattern may reverse in the fall, especially near the western shore, because preferential cooling of the shallow water creates denser water near the shore. Fluctuations of the current caused by wind and density variations alter this simple flow pattern on any day (figs. 5 and 6). In most of Massachusetts Bay, the flow-through flushing time for the surface waters ranges from 20 to 45 days. In western Massachusetts Bay near the new outfall site, mixing and transport of water and material into the regional mean flow pattern are accomplished by a variety of processes, including the action of tides, winds, and river inflow. The distance that particles travel in a day is typically less than 10 km. The future outfall is located in a region generally to the west of the basinwide residual flow pattern. |
Figure 5. Observed mean flow (small blue arrows) and the variability (shown as a green ellipse centered around the tip of the mean flow arrows) for near-surface currents (4‚8 m below sea surface) measured between December 1989 and September 1991. Typically, the daily-averaged current originates at the station symbol (red squares) and flows toward any location within the ellipse. In general, the fluctuations are larger than the mean. The bold gray arrows indicate the overall direction of the residual drift. The mean wind during this period was from the west at about 0.1 dyne/cm2 as measured at the Boston buoy. (See Geyer and others, 1992.) |
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Figure 6. Paths of a few of the drifters released in May 1990 illustrating the variability of the surface currents in Massachusetts Bay. Drifters (surface floats attached to an underwater drogue) were released (initial locations at the red dots) and tracked via satellite until they either left the bay or grounded ashore. For drifters set out during various times of the year as part of a field experiment carried out in 1990‚91, traveltimes from western Mass- achusetts Bay to Cape Cod Bay ranged from 4 to 7 days and from western Massachusetts Bay to Race Point ranged from 7 to 50 days. (Geyer and others, 1992.) |
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