36 



physical processes that contribute to the observed variability in speed and direction and, 

 consequently, it is sometimes difficult to resolve the exact contribution of a tidal constituent 

 versus energy from other processes having equal or similar periods. This is especially true if 

 the tidal amplitude is small as is the case for the Portland measurement site. 



The tidal analysis of the two deployment periods revealed that near-bottom tidal 

 currents at this location are very weak and predominantly driven by the M 2 semi-diurnal 

 constituent. The amplitude of the near-bottom M 2 tidal current constituent for both the first 

 and third deployments in spring of 1996 was approximately 3 cms" 1 compared to amplitudes 

 less than 0.5 cms" 1 for all other semi-diurnal and diurnal constituents. This is consistent with 

 past reports of weak (3-7 cms" 1 ) tidal currents 1.5 m above the bottom at PDS (NUSC 1979) 

 and at mid-depth in the water column at a site northeast of PDS (Vermersch, Beardsley, and 

 Brown 1979). 



The results of the tidal harmonic analysis also can be used to construct tidal current 

 ellipses which represent the speed and direction of the tidal current vector during one period 

 of a specific tidal constituent. Figure 3-7 presents a composite of tidal current ellipses for the 

 M 2 constituent derived from the first and third deployment periods. Note that the ellipse 

 describes the path taken by the end of the tidal vector with its origin at the center of the axes; 

 the speed of the tidal current (in cms" 1 ) is proportional to the scale presented on the axes. As 

 illustrated in this figure, the M 2 tidal current vectors rotate around the predicted ellipse in a 

 counterclockwise direction at a period of 12.42 hr. The major axis of the M 2 tidal current 

 for the first deployment is oriented east-west, whereas, the axis is shifted by 23° for the third 

 deployment. Considering the small amplitude of the tidal currents and the much shorter 

 duration (and presumably less statistical confidence in the tidal analysis) of the third 

 deployment, the results of the tidal analyses of the two deployment periods are reasonably 

 consistent. Ellipses of the other tidal constituents are not shown as their amplitudes are too 

 small to be significant and coherent. 



The observed consistency in tidal currents between the first and third deployments 

 contributes significantly to our confidence in the quality of the moored current records. 

 Although the mean flow directions differ greatly among the deployments, the mean speed 

 and the tidal current characteristics for each deployment are consistent. For the remainder of 

 this report, the analyses focus on the near-bottom current variability due to processes other 

 than the tides. 



As a first step in the investigation of non-tidal processes, one can vectorally subtract 

 the tidal current components from the observed current records to assess the "residual" 

 currents due to other processes. For example, Figure 3-8 presents an analysis of the residual 

 current amplitude in the north-south (V) and east-west (U) current vector components. The 

 Oceanographic Measurements at the Portland Disposal Site during Spring of 1996 



