Long-term trends and changes in the hydrography of the Faroe-Shetland Channel region 49 1 



noticed and recognised, after investigation (Tait 1935), to be true records, indicative, 

 it was then empirically concluded, of abnormally powerful oceanic incursion into 

 the region. As illustrated in Figure 1 of this reference, a not insignificant aspect of the 

 geographical incidence of these abnormal northern North Sea salinity observations in 

 adducement of evidence towards their reality, links that particular research with the 

 results of an investigation into the current system of the region (Tait, 1937). 



The dynamic results of Table II, however, in respect of the Faroe-Shetland Channel, 

 belie the earlier conclusion reached in interpreting these abnormal North Sea observa- 

 tions, clue to the origin and meaning of which was found on the Butt of Lewis to 

 Faroe Bank hydrographic section of June 1933 (Fig. 3). Here, on and off the southern 

 slope of the Wyville Thomson Ridge, there occurred in the depths of 840 metres 

 and 700 metres at two adjacent stations the high salinities of 35-73° , and 35-61 1^^ 

 respectively, with related high observations, although of lower order, both above 

 and below the latter especially. The two outstanding values, in conjunction with the 

 simultaneously recorded temperatures at the same depths, furnish densities (a,) 

 of 27-74 and 27-63 respectively, that is, within the bracket which Sverdrup, Johnson, 

 and Fleming (1942, p. 670) cite as characteristic and significant of the North Atlantic 

 Ocean intermediate layer of so-called Mediterranean water effluent. It is clear from 

 Fig. 3 that the above salinity records signify the core of such a body of Mediterranean 

 water impinging upon the southern slope of the Wyville Thomson Ridge, and thereby, 

 by the consequent turbulent motion which the trends of isotherms, and especially 

 of isohalines, almost invariably indicate in this region, becoming disintegrated, to 

 appear in the upper water levels northward of the Ridge. — but only extremely seldom 

 at the surface — as isolated high salinity nuclei within the body of the oceanic water- 

 mass, the maximum saUnity of which here normally lies between 35-35 °/o o and 35-45 % ^ • 

 Such isolated nuclei did in fact occur on the Faroe-Shetland Channel sections of 

 1931 (as Table II indicates, there was no 1932 section), 1933, 1935, 1936, and 1938, 

 being absent, however, from the 1934 and 1937 sections. Despite the increased 

 frequency of traverse of these sections since 1946, high salinity values suggestive of 

 Mediterranean water intrusion into the oceanic water-mass have occurred only spora- 

 dically, as in May 1948, August 1951, and November 1952, and then only as singular 

 records on the southern section over the threshold to the Channel. It may be that, 

 by application of the principle of discarding apparently aberrant observations, 

 previous indications of the presence of Mediterranean water in the Faroe-Shetland 

 Channel, and even within the northern North Sea, have thus been lost. It is safe 

 to say, however, that this is a phenomenon which, substantially, occurs only once in 

 a while, and probably, as in the fourth decade, in groups of years, in other words 

 it is a phenomenon of only long-term recurrence. 



Yet another example, this time with reference to the deep waters of the Faroe- 

 Shetland Channel, may be cited from this investigation as suggestive of probably 

 similar long-term fluctuation. What have hitherto been accepted as the normal deep 

 water temperature and salinity distributions in the Channel, i.e., below the oceanic 

 water-mass, are almost uniform conditions in both characters, namely, thermal 

 registrations at or near zero temperature Celsius, and salinity records of around 

 34-92° I ^^ to 34-94°/^ „. These properties define the origin of the deep Channel water 

 as from deep levels of the Norwegian sea to the northward. Since the eff'ective 

 commencement of observations in the Faroe-Shetland Channel at the beginning of 



