FISHERIES RESEARCH BOARD 0 F CANADA MANUSCRIPT REPORT SERIES (OCEANOGRAPHIC and LIMNOLOGICAL) No. 5 TITLE THE DEEP WATERS IN THE LAUREN TIAN CHANNEL AUTHORSHIP L. M. Lauzier and R. W. Trites Establishment ATLANTI C OCEANO GRAPHIC GROUP Dated December 9th, 1957 Programmed by THE CANADIAN JOIN T COMMITTEE ON OCEANOGRAPHY The Deep - aters in the Laurentian Channel by L. M. Lauzier and R. V Trites INTRODUCTION The Laurentian Channel is a deep trough that extends from the edge of the Continental Shelf through the Gulf of St. Lawrence and into the estuary of the St. Lawrence (Fig. This channel, which cuts through the Continental Shelf, separates the Grand Banks and the Scotian Shelf. From the edge of the Continental Shelf to Cabot Strait , it has depths ranging approximately from 600 to 400 metres. From Cabot Strait inward for a distance of about 400 miles, it s hallows to 200 metres and terminates abruptly in the vicinity of the Saguenay River. Cabot Strait provides the only opening to the deep waters of the Gulf of St. Lawrence. This Strait is 56 miles wide (104 km.) and has a maximum depth of 480 The cur- metres. Its cross section has an area of 35 sq. km. rents through Cabot Strait have been investigated by Dawson (1913), Sandstrom (1919), and MacGregor (1956). The circulation in Cabot Strait is featured by an outflowing current along the Cape Breton side and an inflowing current along the Newfoundland side. Dynamic calculations show strongest currents in August and least in April and May. The waters of ;he Laurentian Channel are highly stratified (Lauzier and Bailey, 1957) In the summer, a warm surface layer is superimposed on an intermediate cold-water layer over- lying a deep warm la rer. In winter, a single mixed layer of 2 sub-zero temperature overlies the deep warm layer A prelimin- ary study of the water mass characteristics by La uzier and Bailey (1957) showed by means of T-S relationshi ps, that the deep waters of the Gulf of St. Lawrence, includin g Cabot Strait, retain their characteristics geographically and s easonally. OBSERVATIONS Oceanographic observations in the Cabot Str ait area of the Laurentian Channel have been taken for many years The first major contribution was made by the Canadian Fisheries Expedition in 1914-15 (Bjerkan, 1919). It was followed by the work of the C.G.S. "Arleux" in 1923 (Huntsman et al, 1953 ), and the "Cape Agulhas" in 1931-1935 (Thompson and Wilson, 1932-1936). From 1947 to the present, seasonal observations of oceanographic properties have been taken by the Atlantic Herrin g Investigation Committee and by the Atlantic Oceanographic Group of the Fisheries Research Board of Canada. The data co llected during these expeditions and cruises are pertinent to the area, and have been compiled in order to study the properti es of the deep waters of the Laurentian Channels, TEM?ERATURE AND SALINITY VARIATIONS Vertical Structure In this paper y, the deep layer embraces the waters of salinity higher than 34.0°/oo and of temperature generally greater than 4°C. The waters of salinity betwee n 33.0 and -3- 34.0°/oo, with temp erature generally between 1 ° and 4°C. has been defined as a b oundary zone. In the Laurentian Channel, the boundary zone i s located just below the cold-water layer. The temper atu re and salinity gradients in the boundary zone and the deep 1 aver, seem to be independent of the seasons. Figure 2 represents schematically the temperature and salinity structure of the wa ters of the Laurentian Channel in Cabot Strait area for two different years. Exclusive of the cold-water layer between 50 and 100 metres, the main feature in the two sets of data is the maximum temperature within' the deep layer. Irre- spective of its depth and of its actual temperature, the water of maximum temperature has approximately the same salinity, 34.6(3/oo. In some years, the maximum temperature within the deep layer is relatively low as compared with other years. In these years, it occurs at greater depths than when the maximum temperature is high . As defined by its salinity, the deep layer varies in its properties, from thin and cool in some years, to thick and warm in others. From the data accumulated through the years for the C abot Strait area, it is possible to show the time variations of such properties. Term Variation In Figure 3, the temperature of various isohalines, such as 33,0 and 34.0° /o o, are plotted against time, indicating that a general increase in temperature of these waters has occurred during the last thr ee decades. From the early thirties to the late forties, the w arming was more intense for the 34.0 ° /oo water than for 33 0 0° /oo water. In the la st decade, the 33.0 0/00 reached a maximum temperature in 1952 9 whi le the 340 00/0o reached a maximum a year later. As mentioned previously 9 the deep layer of salinity greater 0 than 34 0 0 /oo is featured by a maximum temperature. This section of the water column exhibiting a maximum temperature is called the core of the deep layer. The temperature of the core has been observed to increase considerably over the last three decades from about 4 0 0°C. in the twenties to 6 0 00 C, in the early fifties (Fig. 3). Also the variability of the temperature of the core from one cruise to the next, is much less i han the variability of the temperature of the 33.0 and 34 0 0° /oo iE ohaline. A long-term maximum temperature within the core was reE ched in 1953-54, some- what later than at the 3400 ° /oo level. is interesting to note that the salinity in the core of the deep layer, which is always greater than 34.0° /oo 9 does not show any long-term variations. Such warming is related in some ways to the shallowing of the isohalines, 33.00 /00 and 34.0 ° /oo. It should be noticed in Figure 4 that after 1952 and 1953, the depth of the isohalines did not change appreciably. The changes in depth of isohalines 33.0 and 3400°/oo are indicative of the ch ange in volume of both the boundary zone and the deep layer. Thus, in the last three decades 9 the deep layer showed an increase in volume at about the same time the warming occurred. The variation in the thickness of th e boundary zone is 5 related to the vari ation in volume of the deep layer. This relationship is ill ustrated in Figure 5. The thickness of the boundary zone is relatively small, when the volume of the deep layer is large 9 as shown by the depth of the 34.0 °/oo isohaline. It seems that this zone is lifted in the water column as well as squeezed° Water Mass Characteristics The temperature-salinity relationships for the waters of the Laurentian Channel in Cabot Strait have always shown the same general pattern ° Only part of the data is illustrated in Figure 6 to give representative T-S curves from 1915 to 1956. Each curve represents the average T-S relationship of all the data collected during the crossing of the Strait within the period mentioned„ The T-S relationships are given only for the waters underneath t he cold-water layer, where the seasonal variations are negl tgibleo The three sets of T-S diagrams, Figure 6, show that the maximum temperature occurs at an average salinity of 3400/000 The range of temperature for the maximum, the core of the dee: layer, is better illustrated in Figure 6 than it was previou sly in Figure 3. The consisten t pattern of the T-S curves suggests that these deep waters a re formed outside the Laurentian Channel. To ftllowMcLellans suggestion (1957a), the deep coastal waters maybe formed by the mixture of Labrador water and Slope water. More recently, McLe Ilan (1957b) made a distinction between the waters in Cabot Str ait and the waters at the entrance to the - 6 Laurentian Channel. Few data have been collected in either the lower Laurentian Channel or the entrance to the Channel. Some c f these data are shown on T=S diagrams to exhibit the differences between the three groups of curves: at the entrance to the C hannel (Fig. 7a), the lower Laurentian Channel (Fig. 7b) 9 and the Cabot Strait (Fig. 7c). The variability of the properties o f the water mass at the entrance to the Channel contrasts with th ose of the water mass in Cabot Strait. Such variability suggest s that the coastal waters found between the Continental Shelf and t he Slope water at the entrance to the Channel represent an interme diate step in the formation of the deep waters of the Laurentian C hannel. Further mixing of the deep coastal waters with water of Labrador Current origin therefore takes place in the lower Lauren ti an Channel south of Cabot Strait. A small branch of Labra dor water pene- trates the deep Channel between Newfoundland and the Grand Banks and eventually reaches the Laurentian Channel no rth of St. Pierre Bank. Mixing Ratios As pointed out by McLellan (1957a) the dee p coastal waters, because of their geographical position, are possibly formed by the mixture of Labrador and Slope waters. He assumed that the waters in the Laurentian Channel have more persistent charac- teristics than the waters off the Continental Shelf and that they would be formed by the mixture of Labrador and Slope waters with proportions of Labrador waters varying from 67% to 38%.