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Statistical Comparison of Currents at the Mooring Locations

Reproducing the day to day variations of ocean ``weather'' at specific locations may be required to warrant the use of the model to address some issues, but for other issues it may only be necessary to correctly reproduce the statistics or ``climate'' of the currents over a few weeks or months. To assess the capability of the model to represent the climate, the mean current and low-frequency ellipses from the data and model were compared at the moored locations for each season. The seasons are defined by distinct dynamical regimes instead of the conventional dates: Winter (November 1 to March 1) represents a wind-dominated, well-mixed regime; Spring (March 1 to June 1) represents a runoff-influenced, transitional stratification regime; Summer (June 1 to September 1) represent an upwelling, strongly stratified regime; and Fall (Sepember 1 to November 1) represents a transitional overturning regime.

During the winter, as expected from the relatively high correlations at the moorings, the mean and low-frequency ellipses compare quite well (Figure 3.17).

The ellipses are shown centered around the tip of the mean flow, and the arrows and ellipses are scaled to correspond to the distance that particles traveling at the observed velocity would move in 1 day. This means that about two-thirds of the time (the percentage of time particles move less than one standard deviation), particles released at the moored locations would be found somewhere inside the ellipse after 1 day. Modeled displacements are very close to predicted displacements with the exception of Race Point, where the observed mean flow is close to northward while the modeled flow is close to eastward. Analysis indicates that this discrepancy is most likely due to a shift in the modeled pattern of tide-induced residual currents rather than a major error in representing flow out of Massachusetts Bay. There is strong (10 cm s ) northward mean flow in the model just 2 km to the southwest, the result of a small (5 km) tide-induced residual eddy off the tip of the Cape.

During the spring the surface current variability increases due to intrusions of low salinity water from Gulf of Maine rivers and weaker frictional resistance to wind driving since the surface layer becomes somewhat decoupled from the lower layer. The model reflects the increased intensity of surface currents and the general pattern of the current ``climate,'' doing particularly well at the future outfall site (Figure 3.18).

The modeled surface currents are slightly stronger than the observations near Stellwagen Bank and slightly weaker than the observations at Scituate and Manomet, however. At Race Point there is a discrepancy between the modeled and observed mean flows very similar to winter, consistent with the interpretation that this is due to slight spatial error in representation of the tidal residual eddy off the tip of the Cape. As with the surface currents, the lower layer currents are also stronger than observed near Stellwagen Bank, but are close to the observed currents at the other locations (except Scituate, where the modeled currents are extremely weak).

In the summer, strong stratification results in much stronger currents in the upper layer than in the bottom layer. The model represents this behavior quite well, and again is a close match to the current ``climate'' at the future outfall site (Figure 3.19).

The model currents are again stronger than observed near Stellwagen Bank, and Cape Cod Bay surface currents are also stronger than observed.

In the fall the surface mean flow reverses and flows northward at Scituate and Manomet in both the observations and the model while the mean flow near Stellwagen Bank still flows to the south (Figure 3.20).

The ellipses are generally quite similar except at Race Point, where there appears to be some flow into Massachusetts Bay that is not apparent in the data.

The degree to which the model compares with data can be used to identify appropriate uses of the modeled hydrodynamics. For example, since the current ``climate'' is well represented in western Massachusetts Bay throughout the years, it is appropriate to use the model to study the near-continuous release of effluent in this region by the existing and future outfalls. Likewise, since individual events are well-represented in western Massachusetts Bay during unstratified conditions, the model is well suited to studying the transport of suspended material from this region during individual winter storms. On the other hand, predicting the detailed evolution of a low-salinity plume from a spring runoff event is not possible, at least partially due to inadequate information about the large scale forcing from the Gulf of Maine.