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Adjunct Western Gulf of Maine Model

The western Gulf of Maine model grid is 90 x 45 cells and has a typical grid spacing of 2-4 km (Figure 2.5).

The bathymetry was generated in identical fashion to the Massachusetts Bay model and the model was driven by the same wind and heat flux forcing. The elevation boundary conditions were specified as , and tides derived from Lynch and Naimie (1993). There were no subtidal fluctuations specified, so the low-frequency elevation was effectively clamped at zero along the offshore boundary. At the northern boundary, a mean sea surface slope is imposed to drive a coastal current. Along this boundary the low-frequency elevation varied linearly from 0.06 m at the coast to 0.00 m at the open boundary 45 km offshore. This was picked to drive a mean alongshore flow of 5--10 cm s at locations where mean currents were observed by Vermersch et al (1979). The mean flow in the Gulf of Maine model is held constant throughout the year. Recent data collected on the eastern flank of Stellwagen Bank at a location determined to be a good measure of the large-scale circulation in the Gulf of Maine (Bogden et al, 1995) suggests, encouragingly, that there might be little variability in the seasonal mean flow. A comparison of the mean flow observed in spring, summer and fall of 1994 at the Stellwagen Bank mooring with seasonally-averaged mean flow obtained from the model at this location over 1990--1992 shows remarkable agreement (Figure 2.6).

Outside of the buoyant surface layer, the observed mean flow due to large scale circulation in the Gulf of Maine changes very little with season (Table 2.2). The mean flow at 27 m, for example, appears to be about 10 cm s throughout the year, and the mean flow at 50 m is nearly constant at 4 cm s .

The temperature and salinity along the open boundary were specified by monthly climatology determined from the Bedford Institute of Oceanography AFAP database. Model output averaged over the tidal cycle was output along the cells that constituted the open boundary for the Massachusetts Bay model, the converted into low-frequency elevation boundary conditions and temperature and salinity boundary conditions at standard levels (1, 2.5, 5.0, 7.5, 10, 15, 20, 30, 40, 50, 75 and 140 m). The resulting boundary conditions for Massachusetts Bay, as intended, represent more variability and Gulf of Maine discharge-driven events than would have been possible with open boundary conditions based on climatology (Figures 2.7, 2.8).

Inter-annual variability at frequencies less than the synoptic scale weather band (3-10 days) is not dramatic, although certain events stand out, such as the sustained decrease in surface salinities near the coast during the fall of 1990, consistent with the abnormally large runoff during this time period.