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%. At the —7

time, the data of only one cruise were considered. Data from an a large number of cru ises from 1915 to 1956 are now being con- ire sidered, The T-S cu ?yes (Fig. 6) show some common characteris- tics such as a maximum temperature at approximately 34.6 °/oo. 'a), This point has been d afined earlier as the core of the deep layer.

(a) The Core of the D aelaiaterLLJaer .ss The temperature in the core of the deep water layer varied er through the years (171i 3). The T-S relationships depicting stal warm, average and col i waters in the core of the deep layer have at been used to study the possible mixture of Labrador and Slope the waters as components of the core, The T-S relationship of Slope .er water is taken from McLellan (1957a),and those of the Labrador nt water from the data of the International Ice Patrol. Three possible T-.S relation Ships of Labrador waters were used, repro- senting cold, average , and warm waters in the Newfoundland area. iks It is assumed that the mixing takes place along straight lines on a T-S diagram between waters of equal density or same initial It was found that the core of the deep layer may be formed by a mixture of the two bodies of water in approximately

3rs, the same proportions, if the corresponding temperature conditions, cold, average, and warm, are the same for the Labrador water and le the core of the deep layer. The results are shown in Table I. In other words, if the mixture of Slope and Labrador they waters takes place in the same proportion from year to year, the Lth temperature of the cor e of the deep layer in the Laurentian ae Channel, is related to the temperature of the Labrador water. - 8 -

TABLE I

The constitution of the core of the deep layer of the Laurentian Channel from Slope Water and Labrador Water, C 01 at maximum Proportion Period Initial 6. temperature Labrador water (%) Cold 27.33 27.40 49 Average 27,22 27.32 47

Warm 27,10 23,23 44

11211heDee2LEfer in Cabot Strait

The possible mixing of Labrador and Slope waters, at the

same initial ott , was next considered to produce the deep layer as a whole in the Laurentian Channel, under average and extreme

temperature conditions. The results are shown in Table The proportions of Labrador water involved varied by nearly 30% from lighter to heavier waters. Labrador water pre- dominated in the first case and the Slope water in the second. ( c At all et levels, the proportions of Labra dor waters involved in this type of mixing were greater for the production of cold deep waters than for the production of warm deep waters in Cabot fr Strait. The results obtained by McLellan (1957a), given in F0 Table II, show that he had considered the m ixing at the time the of temperature conditions were average. wa

Ca 9

TABLE II

'The tonstitution of the deep waters of the Laurentian Channel from Slope Water and Labrador Water.

Proportion Labrador Water (%) ( % ) Initial Temperature Conditions ai warm average cold

26070 66 75 76 26.80 60 67 69 67 26.90 55 63 66 63 27000 48 58 63 59 27010 44 52 58 53 •er 27020 41 47 56 48 27.30 38 42 51 43 27.40 35 40 49 39 27 0 50 31 38 48 38

(c) The Dee • La er in the Lower Laurentian Channel and at the Entrance

As shown in Figure 7, the T-S relationships vary greatly ■ot from the entrance to the Laurentian Channel to Cabot Strait.

For the four cases considered, it was found that with the waters he of salinity greater than 34.0 °/oo„ the proportions of Labrador water, generally increase from the entrance to the Channel to Cabot Strait, with most of the increase occurring near the 1 0 entrance. However, at the entrance outsid e the Continental Shelf, the variations of the proportions of Labrador water are erratic as compared with those in Cabot Str ait. Such conditions can be seen from the T®S relationships. These conditions are considered to be the initial steps of mixir g which Echart (1948) defines as stirring.

(d) (3ther Types of Mixing

So far we have considered the mixing at the same initial

61, and it has been shown, that in order to form the deep waters of the Laurentian Channel, the two bodies c f water, Labrador and

Slope, would have to mix in variable propor tions depending on the initial density. It may be assumed that t he mixing can occur in other ways, such as mixing at the same C epth of both compon-

ents or mixing at various depths of constant intervals. It was found that if the Labrador water of depth n d" is mixed with Slope water of depth "d + 50" metres, in proporti ons varying from 60

to 70% of Labrador water, the resultant MiY ture will have T - S relationships very similar to those of the deep waters of the Laurentian Channel. Such mixing has been studied for three types of deep waters in Cabot Strait, cold, average, and warm, as it was done for possible mixing at the ame initial a . The results are given in Table III, NMI

TABLE III

The constitution of the deep waters of the Laurentian Channel from Slope Water and Labrador Water, mixing at depths

d + 50.

Proportions of Labrador Water (%) et Deep Waters of Temperature Conditions Laurentian Channel warm average cold

27,10 27.20 27.30 27.40 27.50

The constancy of mixing ratios or proportions of Labrador water is more remarkable by assuming mixing at different d in both components than at the same initial 07 , The conclusion regarding greater proportions of Labrador waters in colder years as reached previously is confirmed.

Average Densi ty in the Lower Laurentian Channel

From the entrance to the Channel to Cabot Strait, the average vertical structure of the water mass changes in such a way that the isopycnals are sloping down towards Cabot Strait. For instance 9 waters of a value of 26.8 and 27.4 were re- spectively at 90 and 230 metres at the entrance to the Channel, 12 and at 140 and 290 metres respectively in Cabot Strait. Refer- ring back to the deep layer in Cabot Strait, the waters of salinity greater than 34 0 0°/oo form a wedge in the lower Laurentian Channel, The 34.0°/oo isohaline tilts downward reaching a maximum depth in Cabot Strait. The two cases shown in Figure 8 illustrate conditions described previously for Cabot Strait only. In 1915, the conditions are those of a cold year, with a deep 34.0 °/oo isohaline, a relative: y small volume of the

deep layer, and a thick boundary zone, 33.0 - 34.0°/oo in most of the Channel. In 1956, the conditions are those of a relative- ly warm year, with a shallower 34.0 °/oo isohaline, a relatively large volume of the deep layer and a thinner boundary zone. It seems then that such fluctuations occurring from cold to warm years are not only characteristic to Cabot Strait but also to most of the lower Laurentian Channel.

Movement er

The scarcity of data makes it difficult to draw precise conclusions regarding the movement of the deep layer in the Laurentian Channel. However, the volume transport data in Cabot

Strait as compiled by MacGregor (1956) can be used to approximate the average conditions of flow of the deep waters in the

Laurentian Channel. Assuming that the net transport through Cabot Strait is zero, it has been estimated that the deep water layer is generally flowing into the Gulf of St. Lawrence. In one case, the flow of deep water was in a E outheasterly direction or out of the Gulf. The net transport of the deep waters in - 13 r- Cabot Strait varied from 60 to 430 thousand cu. m./sec. which represents between 10 and 40% of the total inflowing waters through Cabot Strait, The direction of flow of water within the boundary zone is not always the same as that of the deep water layer °

)ot DISCUSSION tr, the The wedge-like structure of the deep water layer in the lower Laurentian Channel and the general direction of flow of dve- such a layer in Cabot Strait, suggest that the deep waters are

.Y supplied almost constantly to the Gulf of St. Lawrence. This .t is the only supply of high salinity water to the Gulf. High salinity waters are found all along the Laurentian Channel. They retain the same T-S relationships as those of the deep water

layer in Cabot Strail The gradual shallowing of the bottom cuts off in the upper Laurentian Channel some of the higher salinity water observed in Cabot Strait. At the head of the Laurentian Channel, near the Saguenay River, the remaining portion of the deep layer is used for mixing in the production bot of part of the Gaspe Current. ate It is expected that, in this long chain of events, the variations of properties of the Labrador water will be reflected later in variations of properties not only in the deep waters of r the Gulf and the Estuary, but also in the mixed waters that constantly flow out in the Gaspe Current and the Cape Breton ion Current, The intermediate cold-water layer must necessarily 14

show some variations, such as in volume and in minimum temperature.

SUMMARY

1. Properties of the deep waters of the Laurentian Channel in Cabot Strait area have been studied. The deep water layer, of salinity greater than 34.0 °/oo s is part of a three-layer system. It is characterized by a maximum tem. , o perature at a salinity of 34.6 /oo. 2. Long-term variations in the temperature and the volume of the deep layer have been observed. Warming of the deep waters from about 4.0 °C. to 6.0°C. has been observed in the last three decades. This layer showed an increase in volume at about the same time the warm] ng occurred. 3 0 T-S relationships of the deep waters of the Laurentian Channel have been used to study the formation of this layer by a mixture of Labrador Water and Slope Water, Fluctuations in temperature of Labrador Water are reflected by a corresponding change in temperature of the deep layer in the Laurentian Channel. A persistent feature is the constancy of proportion of Labrador Water in the core of the deep layer. The properties of the deep waters at the entrance to the Channel have been compared to those of the waters of the Cabot Strait area. It is suggested that the coastal waters at the entrance to the Channel represent an intermediate step in the formation of the deep waters of the Laurentian -15-

Channel, and that further mixing occurs in the lower Channel, south of Cabot Strait. 5. The vertical structure of the deep water layer in the lower Laurentian Channel and the general direction of flow of such a layer in Cabot Strait, suggest that the deep waters are supplied almost constantly to the Gulf of St. Lawrence.

NOTE

This paper was presented at the meeting of the Association of Physical Oceanography, Inter- national Union of Geodesy and Geophysics, September, 1957, Toronto, Ontario. REFERENCES

Bjerkan, P. 1919. Results of the Hydrographical Observations made by Dr. Johan Hjort in the Canadian Atlantic waters during the year 1915. Canadian Fisheries Expedition, 1914-

1915, Dept, Naval Services, Ottawa, pp. 349-403. Dawson, W. B. 1913. Currents in the Gulf of St. Lawrence. Canada Dept. Naval Services, Ottawa, 46 pp. Ickart 9 Carl, 1948. An Analysis of the Stirring and Mixing Pro- cesses in Incompressible Fluids. J. Mar. Res. 7(3): 265-275.,

Huntsman, A. G., W. B. Bailey, and H. B. Hachey, 1954. The General Oceanography of the Strait of Belle Isle. J. Fish. Res. Bd. Canada, 11: 198-260. Lauzier, L. and W. Be Bailey, 1957. Features of the Deeper Waters of the Gulf of St. Lawrence. Fish. Res. Bd. Canada,

Bull, No, 111, Sec, 13, pp. 213-250.

MacGregor, D. G. 1956. Currents and Transport in Cabot Strait.

J. Fish. Res. Bd, Canada, 13(3): 435-448.

McLellan, H. J. 1957a, On the Distinctness and Origin of the Slope Water off the Scotian Shelf and its Easterly Flow South of the Grand Banks. J. Fish. Res. Bd. Canada, 14(2): 213-239, McLellan, H. J. MS, 1957b. A Slope Water Survey - July 1954. Fish. Res, Bd, Canada, MS Rept. Biol. Sta., No. 632, 49 pp.

Sandstrbm, J. W. 1919. The Hydrodynamics of Canadian Atlantic

Waters, Canadian Fisheries Expedition, 1914-15, Dept. Naval Services, Ottawa, pp, 221-291.

1 7

Thompson, H. and A. M. Wilson, 1935, 1936° Ann ual Report of the Newfoundland Fisheries Research Laborato ry, Division of Fishery Research, Dept ° of Natural Resources , Newfoundland, St ° John's, Vol ° II, No ° 3 and No ° 4. Wilson, A. M O and H. Thompson, 1932, 1933. Annu al Report of the Newfoundland Fishery Research Commission for the years 1932

and 1933, St. John's, Vol ° II, No ° 1 and No. 2. 50 FATHOMS 75 100

M. M.

Figure 1. Bottom topography of the continental shelf and the Gulf of St. Lawrence. 32 33 34 35 32 33 34 354—S °A3c, 2 6 2 4 6 -4--;TEMP °C

S %o

Figure 2. Schematic vertical distribution of temperature and salinity in Cabot Strait during a cold year (left) and a warm year (right). 1915 111101111 1923 NOM 1932 1933 1934 1935 1947 1948 1949 1950 1951 1952 1953 1954 1955 1957

6 —6

• • . • • J —5 MAXIMUM °C

RE —•*— `MAX IMUM —4

TU • 34 %a RA

PE 3 —3 • --- 34 Va....Jr ■• • • TEM

—2

—1 33%

—0

—1 1950 1951 1952 1 1953 1 1954 1 1955 1 1956 1 1957 1923 Milli 1933 19341 1935f 11947 1 1948 1 1949 1

Figure 3. Temperature variation of isohalines 33.0 0/00 and 34.0ujoo and at the core of the deep layer in Cabot Strait, from 1915 to 1957. 1915 1923 r 11932 11933 1934 11935 1 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957

3 4 °la e • de-

—200

33 Vor" ( —33%o • .

• • • —100

MOM 1923 MOM 1947 1948 1949 1950 1951 1952 1953 1954 1955 1 956 1957

Figure 4. Depth variation of isohalines 33.0 0/00 and 34.0 0/00 in Cabot Strait, from 1915 to 1957. 160 z 0 N 140

0

I 120 0

100

(r) • • z 80- • Y • • • U I • • • • 60

40 1 1 160 180 200 220 240 260 280 METRES DEPTH OF ISOHALINE 34-0Y00.

Figure 5. Relationship between the thickness of the boundary zone and the depth of isohaline 34.0 0/00 in Cabot Strait. / / // 4- 4- / / 3- 3- /. / / 2- 2- / / • ••/. • / / / — 1915 — 1950 I - — 1954 -- 1923 -- 1952 -- 1955 —•— 1934 -35 —•— 1953 — • — 1956 0- •• • • 1947 0- 0-

320 33.0 34.0 35.0 32.0 33.0 34.0 350 32.0 33.0 34.0 35.0 Figure 6. Average T-S relationships of the deep waters in Cabot Strait at various times from 1915 to 1956. t / floo 5 5 15 RE

),400 TU 00 300'5, 1 4 4 400

5 PERA

TEM 3— 3—

2— 2—

I — — 1915 --- NOV. 1951 —•— JULY 1954 SEPT. 1956 0— 0—

ENTRANCE TO CHANNEL LOWER CHANNEL CABOT STRAIT

"T"-- 32.0 33.0 34.0 35.0 32 0 33.0 34.0 35.0 32 0 331.0 34'10 35.0 Figure 7. T-S relationships of the deep waters from the entrance to the Laurentian Channel to Cabot Strait during four cruises.

_1 z Z Z O :cT cD < Z 0 • cf) w I-

0

co 0— CC 100— 100— 26.6 26.8 2 — -34.0 -- 27.0 G7.0 4.0 200— 200— 27.2 27.2 274 0

300— 27.4 300— 27.6

400— 27.6 400—

-1915- -1956 -

Qt S °/00

Figure 8. Vertical density and salinity distribution in the lower Laurentian Channel from Cltbot Strait to the entrarc e of the Channel In 1915 Iv , in 125e