CHAPTER 7 



REGIONAL VARIATION OF CURRENTS 



Another question associated with the development of 

 anoxia is its apparent confinement to approximately the 

 inner half of the continental shelf off New Jersey. Current 

 meter observations made off Long Island (stations P31 

 and LT4) during the same times as those off New Jersey, 

 and some shorter records from observations near the shelf- 

 break off New Jersey (stations P12 and LT3), were studied 

 to determine whether the interannual current variation 

 observed in the region of anoxia was experienced through- 

 out the entire New York Bight. Anoxic conditions were 

 not observed at either site during 1976. 



Data from LT3 and P12 were available for only 1 month 

 (J.D. 115-145 in April and May) common to both 1975 

 and 1976 (table 7-2). In 1975, the flow at station P12 was 

 toward the southwest as elsewhere on the shelf, and at 

 substantial speed. The 30-day displacement was over 200 

 km. In 1976, at station LT3D, the flow was to the south- 

 west, as normally expected over the shelf, at a time when 

 the flow closer inshore was reversed. The speeds at LT3D 

 were about one sixth those of the previous year, even at 

 a level farther (11 vs. 5 m) from the influence of bottom 

 friction. The total variance at P12 is only about half that 

 at LT3D, but this may be due to closer proximity of the 

 current meter to the bottom on station P12 and to the fact 

 that the subsurface float on this mooring was much farther 

 below the surface. 



The best data for comparing conditions off Long Island 

 with those in the anoxic area off New Jersey during 1975 

 are data from the bottom station. P31 (fig. 7-4). These 

 data are inconsistent with most other data acquired at this 

 time (Beardsley et al. 1976; Hansen 1977) in that the 

 observed flow was weakly to the northwest, generally up- 

 slope across the continental shelf. This measurement also 

 was relatively close to the bottom (5 m) and consequently 

 may have been perturbed by local bathymetry, although 

 no likely cause appears on the best available bathymetric 

 charts. When observations in this area were resumed in 

 October 1975, a relatively strong southwestward flow was 

 observed off both Long Island and New Jersey. In No- 

 vember, off New Jersey a northeastward flow (LT2) was 

 observed while off Long Island (LT4) there was a period 

 of essentially no net flow; which was followed in Decem- 

 ber and January by northwest and northward flows that 

 were not observed off New Jersey. This 3-month midwin- 

 ter perturbation of the normal southwestward flow over 

 the shelf is of the same time and amplitude scales as that 

 observed coincident with the development of anoxia off 

 New Jersey, but anoxic conditions are not expected during 

 the unstratified conditions of winter. From February 

 through July the flow off Long Island was steadily to the 

 southwest at speeds of 55 km/mo (2 cm/s) or more. The 

 monthly means off Long Island do not indicate the slowing 



and reversal of flow that occurred off New Jersey during 

 the summer. The July 1976 flow off Long Island is in fact 

 slightly stronger than in the months immediately preced- 

 ing. 



UPWELLING 



A possible aspect of the circulation that can contribute 

 to development of anoxic conditions is upwelling, or 

 shoreward and upward flow of water from below the pyc- 

 nocline offshore. This water may tend to be somewhat 

 higher in nutrients than typically found in coastal bottom 

 waters. 



It is frequently not possible to discern in local current 

 measurements the occurrence of upwelling, because the 

 alongshore component of flow is much larger than the 

 cross-shelf flow and the local orientation of the shelf 

 bathymetry is insufficiently defined. It is apparent in figure 

 7-2 that the flow is generally parallel to the bathymetry 

 (48°-228° T) much of the time. When the flow crosses the 

 nominal isobath by more than 15° (ratio of components 

 1:4) for significant periods we believe meaningful state- 

 ments can be made about local onshelf flow or upwelling. 

 This crossing angle is about three times greater than our 

 uncertainty in determining the shelf direction. Such situ- 

 ations are identified in figure 7-4. Indication of net up- 

 welling is seen off both Long Island and New Jersey. Off 

 New Jersey it is associated primarily with the flow per- 

 turbation of summer 1976. In the 13 months of observa- 

 tions available (stations 49B and LT2B), upwelling and 

 downwelling occurred five times each in the monthly 

 means; shelf-parallel flow occurred in three monthly 

 means. Off Long Island, upwelling occurred in 8 of the 

 12 monthly means available (stations P31 and LT4B). but 

 occurred most strongly in connection with the midwinter 

 current perturbation. The information on the monthly 

 mean near-surface currents included in table 7-2 and fig- 

 ure 7^, considered jointly with either the currents below 

 the pycnocline or the winds observed at JFK, demonstrate 

 only that the surface currents off Long Island were 

 strongly influenced by local winds. 



LOCAL METEOROLOGICAL EFFECTS 



Recent studies (e.g., Beardsley et al. 1976) show that 

 a large fraction of the kinetic energy in water movements 

 in nearshore coastal waters is correlated with local winds. 

 Because there are indications of anomalous atmospheric 

 conditions during winter and spring 1976 (ch. 3), it is 

 appropriate to review here the winds that influenced cir- 

 culation in the New York Bight during these seasons of 

 1975 and 1976. For this purpose we obtained wind obser- 



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