CHAPTER! 



depth. The pycnocline is poorly developed in spring, at 

 which time it is characterized by a weak gradient (more 

 than 0.05 or 0. 10 ct, units/m). As the season progresses the 

 pycnocline becomes better developed and is characterized 

 by stronger gradients (as much as 0.10 a, units/m), and 

 there is greater variability of physicochemical conditions 

 above and below the pycnocline. 



SEASONAL PROGRESSION IN 1976 



The 1976 seasonal development of the pycnocline in 

 New York Bight was controlled by two weather/climate- 

 related events. Winter was severe enough to destroy the 

 previous seasonal pycnocline in the shelf water through 

 free and forced convection. The early onset of spring run- 

 off from the Hudson-Raritan estuary established a hal- 

 ocline in the Bight Ape.x. The dominant flow from the 

 estuary in April was around Sandy Hook and south along 

 the New Jersey coast (fig. 2-2). This dominant flow and 

 the strongly developed halocline disappeared to the east. 

 Virtually no temperature structure is apparent in the April 

 cruise data (fig. 2-3), but a prominent salinity gradient is 

 evident (figs. 2-4 and 2-5). By May. significant stratifi- 

 cation is indicated in both temperature and salinity sec- 

 tions (figs. 2-3. 2-4, 2-6. and 2-7). The effect of the 

 seasonal thermocline is evident in figure 2-2. The density 

 structure, which was localized by the outflow of relatively 

 freshwater from the Hudson-Raritan estuary in April, was 

 under the influence of the thermal structure in May 

 throughout the Bight, as indicated by depth of the ther- 

 mocline. 



The strengthening of the halocline-controlled pycnoc- 

 line by the rapidly developing thermocline waS a regional 

 phenomenon. Locally a tongue of warm, low-salinitv 

 water extended east and south from the mouth of the 

 Hudson-Raritan estuary at the bottom of the pycnocline 

 in May (figs. 2-6 and 2-7). 



Bowman and Wunderlich (1977) examined historical 

 temperature, salinity, and density data for New York 

 Bight. Their findings are presented as monthly mean tem- 

 peratures, seasonal salinity cycles, and seasonal mean den- 

 sity distributions. Relative to these climatological condi- 

 tions, the April 1976 surface waters were about 1° to 2° C 

 warmer than normal and bottom waters temperatures 

 were up to 2.5° C warmer than usual. In May, the surface 

 temperature remained slightly above normal, while the 

 bottom temperature (7.5-9.0° C) was normal to slightly 

 (1° C) below normal. Consequently, the Bight water col- 

 umn was as thermally stratified as usual in May, a con- 

 dition that might have existed earlier. 



Turbulent mixing processes are inhibited by stratifica- 

 tion. Resistance to overturning (i.e., vertical exchange) 

 in the water column was estimated by computing the 

 change of a, within the pycnocline. This stability indicator 



is shown for April in figure 2-8 and for May in figure 2-9. 

 The weak pycnocline offshore and the stronger pycnocline 

 associated with the outflow from the Hudson-Raritan es- 

 tuary along the northern New Jersey coast is evident in 

 the April cruise observations. One month later, stratifi- 

 cation resulting from thermal enhancement of the pyc- 

 nocline is apparent throughout the region, both inshore 

 and offshore. 



By late June, the horizontal temperature gradient at the 

 bottom of the pycnocline increased inshore but did not 

 change significantly offshore (fig. 2-10). However, the 

 surface warmed approximately 7° C (fig. 2-11), and con- 

 sequently a very strong thermocline developed. The sal- 

 inity of the Hudson-Raritan outflow increased from 27'^c 

 to 29%f, whereas no significant change occurred in the 

 salinity distribution at the bottom of the pycnocline (fig. 

 2-12) relative to May (figs. 2-4 and 2-7). The effect of 

 the strong thermocline on the density structure in June 

 (fig. 2-13) is evident when compared to May (fig. 2-2). 

 Consequently, the stability of the Bight-area water column 

 increased significantly in June (fig. 2-14) compared to 

 May (fig. 2-9), except for weakening of the halocline along 

 the northern New Jersey coast associated with the declin- 

 ing Hudson-Raritan outflow (fig. 2-12). Between the hal- 

 ocline-supported, strong stability band along the northern 

 New Jersey coast and the warmer offshore surface water 

 is a relatively less stable zone of water extending south- 

 ward from Long Island over the inner shelf floor (fig. 

 2-14). 



The June 1976 surface and bottom temperatures of 

 20° C and 7.5° C, respectively, were about normal as com- 

 pared to Bowman and Wunderlich (1977). Salinities 

 ranged about 1 .GF/cc above average; densities, while slightly 

 higher than normal, had a typical gradient (Bowman and 

 Wunderlich 1977). By June, the thermocline became the 

 dominant factor in the stratification and, because of its 

 strength (~ 12.0° C change within a layer 20-m thick), 

 isolated the bottom waters from the surface. The June 

 conditions were the most stable observed in 1976. 



The September data probably closely approximate the 

 maximum seasonal development of water column strati- 

 fication before its destruction by autumnal cooling and 

 wind mixing. Water temperatures at the bottom of the 

 pycnocline (fig. 2-15) were at their maximum and more 

 variable than previously observed. The tongue of warm 

 water was still present south of Long Island; and another 

 tongue was present off central New Jersey. Differences 

 in September and June water temperatures can be seen 

 in figure 2-11, particularly for the Long Island section. 

 Bottom waters, which were consistently colder than 8° C 

 in June and earlier, warmed to about 10° C. A strong 

 thermocline was still evident but was deeper in September. 

 However, when the September data are compared with 

 expendable bathythermograph (XBT) data obtained on 



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