CHAPTER 6 



temperature records at Atlantic City showed that in 1976 

 one of the coldest Januarys was followed by one of the 

 warmest Februarys of the 30-year record. Sea-surface tem- 

 peratures also followed this general trend (fig. 6-1). Al- 

 though air and sea-surface temperatures in the spring were 

 unusually warm, they were not unique. 



Perhaps more significant for hydrographic conditions 

 than early warming was that the normal spring increase 

 in river discharge in 1976 began about 2 months early. 

 Monthly mean discharge for February 1976 in the Hudson 

 River was greater than for any February of the preceding 

 29 years (fig. 6-2). The discharge rate in February was 

 comparable to the normal peak discharge that typically 

 occurs in April. 



A comparison of historical data and monthly mean dis- 

 charge rates in 1976 indicated a record high discharge in 

 February into Long Island Sound (48 years of record: 

 1929-76) and the second highest of record for the Dela- 

 ware River (36 years of record: 1941-76). On the Dela- 

 ware River, the monthly mean discharge in February 1976 

 (760 mVs) was greater than the long-term monthly mean 

 for the peak discharge month (616 mVs in April). The 

 February 1976 discharge rate into Long Island Sound 

 (1,651 mVs) was almost as large as the average value for 

 peak-month discharge (1,866 mVs in April). In their de- 

 scription of the seasonal cycle of hydrographic conditions 

 in the New York Bight Apex, Bowman and Wunderlich 

 (1976) noted the close connection between the time of 

 decrease in surface salinities and the usual March-April 

 increase in discharge from the Hudson River. Therefore, 

 an early river discharge of magnitude comparable to the 

 normal spring peak value would have led to a 2-month 

 earlier than normal freshening of surface waters over at 

 least the Bight Apex. 



The results of these unusual climatic events during" the 



Dec Jan Feb 



c 

 o 

 E 



1500 



1000 



500 



Ap r Ma y Jun Jul 



1 1 1 



Record For Month 

 19 



1947- 1975 



Source: Data Irom USGS provisional record 

 I I I 1 



FIGURE 6-2— Monthly mean discharge of Hudson River at Green 

 Island, N.Y., in 1976 and long-term means and extremes. 



first months of 1976 would probably have caused 1) de- 

 struction of any remaining stratification in the shelf waters 

 during the strong cooling in January, and 2) a 1- to 2- 

 month earlier than normal reestablishment of stratifica- 

 tion, resulting from the early decrease of surface salinities 

 due to the early occurrence of spring discharge. Chapters 

 2 and 5 indicate that early warming had minimal effect on 

 density structure. In principal, early warming should have 

 worked in unison with river discharge and minimal storm 

 activity to promote early stratification, since the salinity 

 layering would not have been destroyed by overturning 

 from the continued decline in surface water temperatures 

 that is typical for that time of year. Indications of the early 

 onset of thermal stratification in 1976 are described and 

 compared with historical data in chapter 5. 



OXYGEN CYCLE AND STRATIFICATION 



The annual cycle of D.O. in bottom waters over the 

 continental shelf off New Jersey is shown in figure 6-3. 

 For this analysis the D.O. measurement at the greatest 

 sampling depth in excess of 20 m for each station with 

 bottom depths greater than 20 m and less than 60 m was 

 used; values were compiled by cruises. These depth cri- 

 teria were used to limit the analysis to shelf waters below 

 the pycnocline during spring and summer, or comparable 

 depths in months when the waters are unstratified. For 

 each cruise, the bottom-water oxygen observations were 

 averaged and plotted in figure 6-3, along with the range 

 of values on the average date of station occupation, re- 

 gardless of year. Data were used from 28 cruises (82 sta- 

 tions) between 1932 and 1973. Observations were avail- 

 able in all months. Hand-smoothed curves were drawn 

 through the values to derive a mean annual trend. Rep- 

 resentative maximum and minimum curves were drawn 

 to depict the normal range of conditions. 



Stratification data were developed from temperature 

 and salinity observations for the same stations used in the 

 analysis of D.O. For each station, surface ct, values were 

 subtracted from <j, values at the deepest observation level 

 (corresponding to the subpycnocline or bottom water 

 D.O. measurement). These differences were compiled 

 into cruise averages and ranges and plotted in figure 6-3 

 along with the normal annual trend and ranges derived 

 from the distribution of the data. 



For comparison with the "normal" annual cycles of bot- 

 tom oxygen concentrations and stratification, observations 

 collected in December 1975 and during 1976 were com- 

 piled similarly to the historical data (fig. 6-3). Chapters 

 2 and 5 describe these data more fully. The stratification 

 data acquired by the NMFS Sandy Hook Laboratory in 

 March 1976 had no oxygen observations. Those obser- 

 vations made in August after the passage of hurricane 



139 



