CHAPTER 7 



Table 7-1. — New York Bight 1975-76 current-meter and wind-obsenation stations and characteristics 



Station 



Water depth 



Distance from 

 bottom 



Distance from 

 surface 



Location' 

 Latitude Longitude 



Years 



New Jersey shelf 



Pll 



49S 



49B 



LT2S 



LT2B 

 New Jersey shelf break 



PI2 



LT3D 

 Long Island shelf 



P3I 



LT4S 



LT4B 



LT4C 

 Meteorological stations 



JFK 



EB34 



EB4I 



Station locations are shown on figure 7-1. 



number of data points. Ortman ( 1978) described the meth- 

 ods and editing criteria used. Raw weather data were used 

 with sampling intervals of 1 hour for the EB stations and 

 3 hours for JFK. Water motion statistics were computed 

 for appropriate time scales, and progressive vector dia- 

 grams (PVDs) were plotted for these same time scales. 

 For the statistics and the plots of velocity components, the 

 coordinate system was rotated 48° clockwise so that the 

 north component transformed into the upshelf (toward 

 Cape Cod) component and the east component trans- 

 formed into the offshelf (toward Bermuda) component. 

 This rotation provides an approximation to the orientation 

 of the bathymetry off both Long Island and New Jersey. 

 Small-scale topographic variability prohibits conclusive 

 statements about cross-shelf flow where the crossing angle 

 is small. 



Because the PVD presentation is extensively used in 

 the arguments that follow, it is useful to review what a 

 PVD portrays. For a series of velocity measurements, a 

 PVD is a virtual displacement diagram derived by inte- 

 grating the velocity in time to yield a representation of 

 water movement past the current meter. It is virtual dis- 

 placement in the sense that a drogue at that depth would 

 move along this displacement diagram (PVD) only if all 

 water adjacent to the point of measurement (the current 

 meter mooring) were moving in exactly the same way. 

 The flow field would then be spatially homogeneous. This 

 is not the case, because water particles distant from the 

 current meter may have moved into a different flow field, 

 which, if current meter measurements were available, 

 would produce a different displacement diagram. The 



PVD does, however, yield valuable information about the 

 cumulative displacement of water particles past the point 

 of measurement. A PVD is a useful tool for visualization 

 of the average and slowly varying components of flow. 



TIME SCALES 



The basic question addressed here is whether weakened 

 or otherwise altered circulation during 1976 may have con- 

 tributed to development of anoxic conditions by reducing 

 advection of D.O. into the region or by concentrating 

 oxygen-demanding material. This question does not have 

 a simple answer within the confines of the data. As will 

 be shown, the net movement over several weeks was in- 

 deed less during spring and summer 1976 than during 

 spring 1975. Over shorter periods, however, the measured 

 currents usually were more energetic. This is revealed in 

 the larger variance statistics in 1976 for corresponding 

 months in the 2 years. Spectrum analyses show the in- 

 creased variance to be distributed over a broad band of 

 time scales, covering periods from at least 0.5 to 6 days. 

 The measured tidal currents were greater in 1976 than in 

 1975, which is more likely to be due to uncertainty of 

 measurement than true secular variation. Although there 

 are undoubtedly measurement uncertainties in both years, 

 the general trends are believed to be real. Increased var- 

 iance at all frequencies, including tidal, can result from 

 high-frequency mooring motion, usually induced by sur- 

 face waves. The moorings (49 and LT2) used off New 

 Jersey during 1975 and 1976 were almost identical, and 



151 



