shelf during years of higher temperatures. This increase 

 in volume of slope water coincided with a corresponding 

 decrease in volume of inshore cold water and subse- 

 quently resulted in higher temperature indices for the en- 

 tire area. Wright (1976) summarized southern New 

 England temperature data from 1941 to 1972 and 

 observed that the shelf-slope water boundary on the bot- 

 tom, as indicated by the 10°C isotherm, was usually 

 about 5-10 km south of the 100 m isobath in March and 

 April and only a couple of kilometers north or south of 

 this depth contour during October and November. He 

 also observed that when the bottom position of the 10 C C 

 isotherm was farther offshore, the winter water tempera- 

 ture minimum was low. Figure 2 shows that not only was 

 the 10°C isotherm north of the 100 m contour during the 

 spring warming trend of the 1970s, but that the slope 

 water was warmer than average in the last few years. 



The mechanisms which might account for frontal 

 movements are not well understood, but wind stress 

 transport is thought to be significant, although usually 

 of brief duration (Crist and Chamberlin 2 ). Size and fre- 

 quency of Gulf Stream warm core eddies may also cause 

 significant variation in the slope front position by causing a 

 subsurface inshore flow of slope water as a compensation 

 for offshore surface entrainment of shelf water (Morgan 

 and Bishop 1977). 



In addition to water being carried onto the shelf from 

 offshore, there is good evidence that some of the water 

 flowing westward south of New England in the early 

 spring originates in the Gulf of Maine-Georges Bank area 

 (Bigelow 1933; Beardsley et al. 1976). Drifts from the 

 east act as cooling agents and indrafts of slope water as 

 warming agents, but solar warming of the water, first and 

 most rapidly at the surface and next to land, is the out- 

 standing feature in the spring (Bigelow 1933). During 

 autumn, as surface cooling and mixing progresses and 

 the summer thermocline is destroyed, bottom tempera- 

 tures in the deeper strata increase to their seasonal max- 

 ima. 



Another striking feature of the hydrography in this 

 study area is the pool of cold bottom water that forms 

 during winter mixing, becomes isolated by summer 

 stratification, and persists for several months (Bigelow 

 1933; Ketchum and Corwin 1964). This cold core may ex- 

 tend from Cape Cod to the offing of Chesapeake Bay 

 along the shelf edge and persist into September (Whit- 

 comb 1970). Warming of these waters is associated with 

 admixture of higher-salinity offshore waters or lower- 

 salinity surface waters (Ketchum and Corwin 1964). Iso- 

 lated cold cores of bottom water were clearly evident 

 during the autumn cruises in both the Southern and 

 Northern Bights (Fig. 3) and their extent and minimum 

 temperatures are reflected in the observed temperature 

 indices for each subarea. Especially notable is the mini- 

 mum temperature observed in 1970 in the Southern 



Crist, R. W„ and J. L. Chamberlin. 1977. Temperature structure 

 on the Continental Shelf and slope south of New England during 1976. 

 Unpubl. manuscr., 27 p. Atlantic Environmental Group, National 

 Marine Fisheries Service, NOAA, Narragansett, RI 02881. 



Bight which was strongly influenced by a large cold pool 

 with minimum temperatures of <6°C (Fig. 3g). The 

 most pronounced effect of this cold pool in the Northern 

 Bight occurred in 1967 when over half the subarea was 

 covered by water <10°C with a relatively large inner core 

 <8°C (Fig. 3d). During the warmest years (1972-74, 

 1976) and cold pool is either poorly defined, of small size, 

 or made up of water >8°C. 



Summarily, at least six phenomena directly or in- 

 directly contribute to the bottom thermal environment of 

 the continental shelf between Cape Cod and Cape Hat- 

 teras: 1) location of the slope front; 2) temperature of the 

 slope water; 3) water drift from the east; 4) timing and 

 extent of spring warming and autumn cooling of surface 

 water; 5) size and frequency of Gulf Stream warm core 

 eddies; and 6) size and temperature of the winter-formed 

 "cold pool." To monitor and analyze these varied and 

 complex phenomena presents a formidable task which 

 cannot be accomplished solely on a basis of our survey 

 cruises or data base. 



Relationship of Temperatures 

 East and West of Cape Cod 



Although there is a rather abrupt general division be- 

 tween the hydrographic properties of the waters east and 

 west of Cape Cod (Bigelow 1933; Parr 1933) these two 

 regions have exhibited similar trends of temperature 

 variability in recent years. The spring warming trend in 

 the Gulf of Maine and on Georges Bank (Davis 1978) is 

 especially similar to conditions in the Middle Atlantic 

 Bight since 1973. Temperature variability in the Gulf of 

 Maine is less because of the greater depths encountered 

 there, but in many instances, the magnitude of annual 

 changes were quite comparable in the other three areas. 

 Bottom-water temperatures are strongly affected by the 

 position of the shelf-slope boundary and vernal warming 

 of shoal inshore waters and the effects of these 

 phenomena have coincided with similar annual temper- 

 ature trends in both regions during several years. A very 

 rough estimate of average annual bottom temperature 

 conditions (average of adjusted spring and autumn in- 

 dices) since 1968 is depicted in Figure 10, and except for 

 a couple of anomalous years, similar temperature trends 

 are shown for each area. 



Effects of Temperature Variations 



Attempts to show environmental effects on fish year- 

 class fluctuations by correlation techniques have gen- 

 erally failed because of difficulties of estimating both the 

 year-class strength and the environmental factor (Gul- 

 land 1965). There is also difficulty in comparing distri- 

 butional changes as related to changes in environmental 

 conditions because population size contributes to the dis- 

 persive characteristics of many fishes, and there has been 

 an apparent overexploitation of several fish stocks in the 

 study area in recent years (Clark and Brown 1977). 

 Despite these difficulties some evidence of natural bio- 

 logical changes seems apparent during this recent warm- 



11 



