14 



3.0 OCEANOGRAPHIC ANALYSES 



3.1 Water Column Characteristics 



Water mass characteristics above 200 m depth in the Gulf of Maine are controlled 

 mainly by seasonal changes in the surface heat transfer processes, river runoff, and influx of 

 relatively cold waters from the northeast, originating on the Scotian shelf (Hopkins and 

 Garfield 1977). During winter, storms induce considerable vertical mixing and near- 

 elimination of the density stratification in the water column, thereby allowing the relatively 

 fresh, near-surface waters to mix with the more saline deep water within the Gulf of Maine. 

 The resulting cold water mass, which is formed only in winter, has been named Gulf of 

 Maine Intermediate Water (MIW) by Hopkins and Garfield (1977). In support of this 

 hypothesized winter process, Brown and Beardsley (1978) have demonstrated that cold 

 offshore winds during winter are effective at cooling the near-surface waters and causing 

 intermittent overturning of the upper water column, which, in turn, leads to formation of 

 MIW. 



With the onset of spring along the western Gulf of Maine, river runoff contributes a 

 significant volume of much fresher (and somewhat warmer) water, resulting in significant 

 density stratification in the upper 20 m of the water column. This stratification continues to 

 intensify during summer and early fall due to continual warming of the surface waters, 

 although near-surface salinities remain relatively constant. 



As described in Section 2.0, vertical profiles of temperature, salinity, and density 

 (sigma-t) were acquired at a single location in the Portland Disposal Site (PDS) during three 

 events associated with deployment, servicing, and recovery of the moored instrumentation: 

 February 27, April 22, and May 14, 1996, respectively. Vertical profiles of all three 

 variables over the depth range from 3 to 58 m (2 m above the seafloor) during February are 

 presented in Figure 3- la. The February profiles of salinity and density indicate an 

 isohaline/isopycnal layer above 6 m depth, and gradually increasing salinity and density over 

 the depth range from 9 m to the seafloor. Salinities ranged from roughly 31.0 to 32.4 ppt 

 over the full extent of the water column. Water temperature in February exhibited more 

 vertical structure than salinity: temperatures were relatively constant (near 3°C) over the 

 upper 20 m of the water column, but beneath that level, temperatures increased 

 monotonically to a maximum of roughly 4.8°C near the bottom. Overall, Figure 3-la 

 illustrates a water column that is very weakly stratified, as is typical for the coastal region of 

 the Gulf of Maine during later winter (Brown and Beardsley 1978). 



Water property characteristics during late April (Figure 3- lb) illustrate that relatively 

 fresh/warm waters had been introduced to the near-surface layer as a result of river 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



