33_ 



Decomposition of the current data into north-south (V) and east- west (U) components 

 (third and fourth tiers from the top of Figure 3-5, respectively) further illustrates the 

 directional shift among the three deployment periods. Currents had a southward (-V) 

 component during most of the first and second deployments, whereas current vectors 

 consistently had a northward (+V) component during the third deployment. The east- west 

 current component demonstrated a gradual shift from mostly eastward (+U) currents to 

 purely westward (-U) currents over the 3 -month measurement period. The decrease in the 

 amplitude of the temporal variability in currents from February to May is evident in both 

 current vector components. 



To further investigate this shift in near-bottom flow direction that corresponded with 

 servicing of the instrumentation, Figure 3-6 presents time series of near-bottom pressure, 

 water temperature, and current vectors for the entire deployment period. The shift from 

 southeastward flow to south westward flow, then to northwestward flow is striking. Notice, 

 however, that at the beginning of the measurement program, near-bottom currents shifted 

 abruptly from eastward to southward within a period of a few days, demonstrating that it is 

 possible for low-frequency (longer than tidal period) currents at this location to change 

 directions on time scales of a day or less. 



Another interesting point that is evident from Figure 3-6 is that water temperatures 

 generally decreased during the first three weeks of the program while the flow was offshore 

 (southward). 



The CTD profile data from February 27, 1996 (see Figure 3- la) illustrated that water 

 temperatures in the near-surface layers were colder than those near the bottom. 

 Consequently, if the near-bottom flow was directed offshore (generally downslope toward the 

 south), it is possible that the relatively cold waters residing at shallow depth inshore of the 

 mooring site could have moved downslope, causing the gradual reduction in water 

 temperatures at the mooring site. 



During the latter half of the third deployment period, near-bottom water temperatures 

 began to rise during a three-week period of persistent northwestward (onshore) flow. 

 Comparison of the near-bottom water temperatures determined from the CTD profiles on 

 April 22 and May 14 (see Figure 3-2) indicates that temperatures were 1.3°C warmer in May 

 and that a 30-m thick layer of roughly 4.5 °C water had displaced the relatively colder water 

 that resided at the mooring site in late April. In the absence of CTD profile data over a 

 broad area at different times during the mooring instrument program, we cannot be sure of 

 the origin of each water type, but it appears possible that the near-bottom warming in early 

 May was, in fact associated with northward (onshore) advection of relatively warmer water 

 from the central Gulf of Maine. 



Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



