CHAPTER 7 



vations at JFK and the two environmental buoys (EB34 

 and EB41). These are shown in PVD format for both 

 velocity components and stress (figure 7-5). Time series 

 of stress were computed according to Wu (1969). The 

 stress data for both 1975 and 1976 clearly show the tran- 

 sition in early May (J.D. 120-125) from the generally 

 northwesterly winds characteristic of winter to the more 

 southwesterly winds of summer (southerly at JFK in 1975). 

 In late winter and early spring 1976, the winds at JFK 

 were slightly weaker and more westerly than during the 

 previous year. Data obtained from EB34 and EB41 show 

 that the winds were more westerly in the southern reaches 

 of the Bight. 



A relatively normal transition to summer conditions 

 occurred about the first of May 1976 at JFK and EB34, 

 but about 2 weeks earlier at EB41. A period of slightly 

 greater than normal wind speed of unusual persistence 

 from the south and southwest occurred during much of 

 June. This period of persistent winds evidently caused a 

 large amount of floatable material to wash ashore on Long 

 Island (Swanson et al. 1977). Although these winds were 

 clearly much stronger and steadier than those of June 

 1975, they were not dramatically different from those of 

 July 1975. In July 1976 the winds again weakened and 

 were more southwesterly. The major difference in low- 

 frequency wind behavior between 1975 and 1976 seems 

 to be that in 1976 there was a greater tendency for winds 

 to have a westerly component in late winter and early 

 spring and a long period during June 1976 when the winds 

 were persistently from the southwest, in opposition to the 

 normal flow of water. 



Early in the morning of August 10, 1976, hurricane 

 Belle passed through the New York Bight (figure 7-6). 

 Though unrelated to the genesis of anoxia in the Bight, 

 this event is of interest in connection with possible ad- 

 vective renewal of oxygen-depleted waters or increased 

 vertical mixing (ch. 2), either of which may be expected 

 to alleviate low D.O. conditions. Figure 7-6 shows the 

 currents observed over the few days spanning the passage 

 of the hurricane at stations LT2 and LT4. Prior to the 

 hurricane, the normal semidaily tidal currents are most 

 noticeable, especially at station LT2. These rotary tidal 

 currents have speeds of 15 to 20 cm/s and produce no net 

 displacement over a complete tidal cycle. The hurricane's 

 time of arrival and speed of passage were such that its 

 effects appear to be almost in phase with the tidal currents, 

 approximately doubling their speed for about one cycle 

 (one-half day). The sequence of enhanced flows is offshelf 

 (SE), downshelf (SW), onshelf (NW), and upshelf (NE). 

 Unlike the tidal currents, which are almost in phase at 

 these two stations, the storm-induced disturbance at sta- 

 tion LT2 occurred before that at LT4 by about 4 hours, 

 about equal to the time it took for the hurricane to pass 

 through the Bight. Following this is a period of weaker. 



but significant, residual flow upshelf at both stations, per- 

 haps caused by the southerly winds following the hurri- 

 cane. This upshelf flow is a continuation of the anomalous 

 trend observed earlier at LT2 and may therefore have 

 contributed to continuation rather than alleviation of an- 

 oxic conditions. 



The current meters also sample and record temperature. 

 Long temperature series from stations 49 and LT2 are 

 used in chapter 5 so are not discussed extensively here. 

 Some data, however, are of interest in connection with 

 vertical mixing of D.O. and other constituents in the water 

 column. At the time of the hurricane, because tempera- 

 tures and dissolved oxygen are correlated (ch. 2), tem- 

 perature can serve as an indicator for vertical transfer of 

 D.O. Figure 7-7 shows temperature observations made 

 concurrently with the current observations in figure 7-6. 



Before the passage of the hurricane, the temperatures 

 at station LT2A, 13 m below the surface but above the 

 thermocline, were above the maximum range of the sensor 

 and therefore were not plotted in figure 7-7. During the 

 storm the temperature dropped abruptly into the working 

 temperature range of the thermistor or sensor. The tem- 

 perature continued to drop more slowly during the sub- 

 sequent 2 days. At meter LT2B, the temperature in- 

 creased about 8° C, fluctuated over several degrees in 

 about 4 hours, then after about half a day fell to less than 

 2° C above its initial value. Our interpretation is that the 

 large transient temperature increase primarily reflects 

 downwelling and offshelf flow followed by upwelling and 

 onshelf flow of water described earlier at this level. The 

 relatively small residual temperature change (2° C) is 

 probably indicative of vertical mixing that can be expected 

 to provide only a modest resupply of oxygen to the region 

 below the pycnocline. 



More complete information about the vertical structure 

 of temperature changes is available from station LT4. Four 

 hours after the temperature increase at LT2B, a temper- 

 ature rise of about 4° C occurred at 21 m depth at LT4A 

 and persisted for several days. Near-surface temperatures 

 (LT4S) rapidly decreased more than 8° C at about the 

 same time as the final rapid temperature drop at LT2B. 

 Subsequently, the temperatures at the 3- and 21-m depths 

 were only slightly different and increased at about the 

 same rate during the following days. This combination of 

 responses suggests vertical mixing to a depth of more than 

 20 m and possibly upwelling and offshore flow of surface 

 water. It is not clear why the changes observed at the 13- 

 m depth at station LT2 were so much less than those 

 observed in the upper 20 m at LT4. These differences 

 probably cannot be explained by local mixing processes 

 alone. More to the point regarding mixing across the pyc- 

 nocline, aside from a transient 3° C increase for only about 

 4 hours at 8 m above the bottom and a 1° C increase 

 spanning about 1'/: days at both 8 m and 1 m above the 



159 



