SURFACE W ATERS OFF THE CANADIAN PACIFIC COAST 



By J. P. TuLLY and L. A. E. Doe 



Pacific Oceanographic Group, Nanaimo, B.C., Canada 



In 1950 the Pacific Oceanographic Group undertook an explora- 

 tory investigation of the physical oceanographic conditions in the Paci- 

 fic Ocean within some 300 to 650 miles west of the Canadian coast 

 (Figure 1). Four surveys were completed, in August 1950, May 1951, 

 August 1951, and March 1952. These included serial observations of 

 temperature and salinity to depths variously of 900, 1000, and 1200 

 metres on the several cruises. Although the four cruises were observed 

 in different years it is convenient in some cases to regard the data in 

 sequence of seasons. 



Structure and Physical Properties of the Water 



The most conspicuous characteristic of the water structure of this 

 area is the presence of the distinct layers, or zones.* These are con- 

 veniently illustrated by plotting the properties as functions of the loga- 

 rithm of depth as shown in Figure 2 (Tully, 1948, 1953). In this figure 

 the surface zone is defined by the logarithmic salinity-depth relation ex- 

 tending to about 80 metres, the lower zone by the logarithmic relation 

 below 180 metres, and the salicline is regarded as a transition zone be- 

 tween these two principal water masses. A similar but not concurrent 

 structure occurs in the temperature. 



In winter (March 1952) the properties of the upper zone approach 

 vertical homogeneity due to cooling at the surface and vigorous mixing 

 by winter storms. At this time the temperature and salinity structure 

 correspond. As vernal heating progresses (May 1951) the surface waters 

 are warmed and a shallow thermocline develops which gradually deep- 

 ens and becomes more pronounced as summer progresses (August 1950, 

 1951). With the advent of autumn weather and storms in September, 

 the surface waters are cooled and the winds cause mixing to greater and 

 greater depths. In this process the thermocline sinks, becoming less 

 distinct, until late winter (March) it coincides with the salicline. 



During the summer the fresh water from precipitation and land 

 drainage is conserved near the surface in the presence of light winds 



* TuUy, in his discussion of the oceanography of Aiberiii Inlet (1949) has used the term 

 "zone' lo designate the principal layers in the water structure. This term has the advantage 

 of being more specific in its reference than "layer", which is used variously to designate hori- 

 zontal strata of any order. 



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