Surface Layer 



ICNAF 68-1 profiles for temperature in the 

 surface layer of the slope water region (figs. 15, 

 16 and 17) show strong horizontal gradients with 

 little apparent mixing. Temperatures in this 

 region ranged approximately from 6°-ll:° C. 



Vertical sections of ICNAF 69-1 temperature 

 data in this region disclose a somewhat different 

 picture. Moderate mixing was evident as por- 

 trayed by the weak gradients of sections 3 and 5 

 (figs. 47 and 49). Temperature data for most 

 stations showed an increase of 2°-3° C. over 

 temperatures observed at the same locations in 

 1968. Inspection of the sea surface temperature 

 chart for 1969 (fig. 10) revealed a band of dense 

 temperature gradients extending roughly along 

 the 1000 fathom (1829 m.) isobath; in 1968 (fig. 

 9) however, this band extended shoreward only 

 as far as the 3000 fathom (5486 m.) isobath. 



A region of extremely warm water was found 

 in the southeastern section of the Gulf of Maine 

 off Georges Bank during ICXAF 69-1 (sections 

 1-3; figs. 45-47). In this region temperatures 

 increased some 15° C. over a horizontal span 

 (southerly direction) of approximately 40 nauti- 

 cal miles. Temperatures as high as 19° C. were 

 observed at the surface as well as to a depth of 

 80 meters. A pocket of very warm water centered 

 around station 20 in section 3 (fig. 47) was also 

 characterized by strong surrounding gradients 

 with temperatures of adjacent stations as mucli 

 as 4°-5° C. lower at the surface and 2°-3° C. 

 lower at 50-75 meter depths. Analysis of these 

 anomalies indicated Gulf Stream influence. 



According to Bigelow (1927), at most times 

 there is no dominant drift of the Gulf Stream 

 across Georges Bank into the Gulf of Maine, but 

 on rare occasions overflows of tropic waters take 

 place at the surface, probably via that route. 

 Small amounts of Gulf Stream water have been 

 known to drift as far west as the coastline 

 bounded by Martha's Vineyard and Narragansett 

 Bay. Although the data in this report do not 

 encompass that immediate area, evidence that 

 such an intrusion occurred in 1969 is apparent 

 by examination of surface contours and T-S dia- 

 grams of the Slope "Water region. 



Intermediate and Bottom Waters 



Warmer temperatures were also evident in the 

 deeper waters off the shelf in 1969. Water tem- 



peratures of the 150 to 300 meter depth range 

 (9°-18° C.) were approximately 2° C. higher 

 than those observed in 1968. 



This increase in temperature at these lower 

 depths was probably attributable to horizontal 

 mixing of coastal water overlying the shelf with 

 Gulf Stream-influenced olTshore waters, and the 

 subsequent sinking of the products of the mix- 

 ing — a process known as caballing. 



^Mixing takes place most efficiently along sur- 

 faces of constant sigma-t where exchange is not 

 inhibited by buoyant forces. When two different 

 water types — coastal and Gulf Stream in this 

 case — each with the same sigma-t value mix, the 

 product, represented by a straight line on a T-S 

 diagram, will be of greater density (sigma-t) 

 than either of the two parent water types. This 

 resultant "heavy water", having a greater density, 

 will thus tend to sink. This is the caballing 

 process. 



Caballing can potentially contribute to vertical 

 circulation. As indicated by relatively weak 

 isopycnals, tlie vertical stability in the Gulf of 

 Maine is quite low as expected during the winter 

 months (Bigelow, 1927). Accordingly, the 

 "heavy water" formed, as outlined above, will 

 sink and flow under ligliter waters. In the shelf 

 region this is probably seen as what McLellan 

 (1953) has called underruni>ing shoreward (fig. 

 4). This process might then help explain the 

 increase in temperature at the greater depths. 



McLellan (1957) discussed the three distinctly 

 different oceanographic regions found oft' the 

 New England and Nova Scotian coasts — Coastal 

 Slope, and Gulf Stream watei-s. These waters 

 are not o'hly separated by sharp geographical 

 boundaries, but are also well defined graphically 

 through T-S plots. Temperature-Salinity rela- 

 tions as presented by McLellan (1957) for the 

 upper 150 meters of water off the Scotian shelf 

 are shown in figure 5. The three groups indi- 

 cated on this T-S plot do not overlap and, in 

 addition, are separated by blank areas into which 

 no observations fall. 



A fourth water mass. Nova Scotian Current 

 AVater, is often included in discussions of the 

 Gulf of Maine region. This cold (2°-8° C), low 

 saline (<32°/„„) water is usually restricted to 

 the coast off Cape Sable. Bigelow (1927) has 

 noted that this Nova Scotian Current Water 

 exerts \ti, chief thermal effect to the eastward of 



