The Iceland-Faroe; Ridge International (Ices

38 CHAPTER 3 TEMPERATURE AND SALINITY DISTRIBUTIONS AND WATER-MASSES OF THE REGION By J . B. T a i t Marine Laboratory, Aberdeen A. J. L e e Fisheries Laboratory, Lowestoft U. S t e f a n s s o n Hafrannsöknastofnunin, Reykjavik and F. H e r m a n n Danmarks Fiskeri- og Havundersøgelser, Charlottenlund FOREWORD A general view of these diagrams reveals at once, on While the main objective of the ICES international all three surveys, an increasing complexity of the expedition of 1960 to the Iceland-Faroe Ridge region pattern of temperature and salinity distributions from was the detection, delineation, and, as far as possible, the surface downwards to at least 300 metres, where the measurement, of cold, deep water overflow across the complexity is greatest. It is only a little less so at the Ridge from the Norwegian Sea into the North 400 metres, but at 500 metres, the bathymetrics of Atlantic Ocean basin, the region has for so long been this depth defining the northern and southern “shoul known as one of conjunction and conflict between ders” of the Ridge proper, the intervention of the relatively warm oceanic waters on the one hand, and Ridge physically separates the deep oceanic and cold cold, Arctic waters on the other, that the occasion of northern water-masses, except for small overspills such a unique exercise in concentrated oceanographi from one into the other. The temperature and salinity cal observation should not be overlooked for the distribution patterns therefore at this level are com material it provides for a closer study of these con paratively straightforward and uncomplicated. flicting phenomena than has previously been possible. Figures 3:1, 3:8 and 3:15 represent surface temper An immediate reason, moreover, for such a study, ature and salinity conditions on each of the three from the ICES point of view, resides in the fact of the surveys. On all three occasions the Ridge area was significance of the region from a fisheries standpoint. completely overlaid, on the surface, by oceanic water Here, as is well known, commercial fisheries are which in fact extended northward far beyond the subject to wide and often apparently abrupt fluctu Ridge itself. Only in the northwest corner of the ations from more or less ample abundance to complete region investigated, as well as eastward beyond the absence, and the oceanographical nature of the region Ridge, and farther north between about latitudes suggests that these fluctuations may be more or less 64°30'N and 65° N, was there evidence of the near closely associated with corresponding changes in the vicinity of Arctic type waters of low temperature physico-chemical environmental milieu. ( < 6° C) and salinity of less than 34-90 °/00. High J. B. T a i t salinity oceanic water, in excess of 35-30 °/00, ocurred in patches over the Ridge and, on the first survey, HORIZONTAL TEMPERATURE AND SALINITY DISTRI beyond it. Highest salinity water, of over 35-40 °/00 BUTIONS (J. B. T a i t ) salinity, was found only over the southern end of the Taking, first, horizontal distributions and, in parti Faroe-Shetland Channel and, on the third survey cular, those of temperature and salinity, Figures 3:1- only, some 30-40 miles northwest of the Faroe Bank 3:21 give the dispositions of these parameters at the and a similar distance northwest of the Faroe Islands surface, 100, 300, 400, and 500 metres, and on the respectively. bottom, on each of the three surveys of the expedition. As between the first and second surveys, the relative 39 16° 15° 14° 10° 65° .34-80 I '----- 35-10 <35-00 !35-00_X 35-1: 35-20- > 35-30 ■ 3.5-25 64° 35-25 500m <35-25 10001 35-0! < 9 35-25 ,>35-30 .>35-30 63° 500m 35-25 35-30 e - 35-30 15-35 •M / 7 > 35-30' ■<35-30 1500 [>35-35] '>35-35 FAROE Is. -10 35-30' >35-3! .<35-31 > 35-35 ■35-30 500m 1000m 61° 35-35 500m ' . 35-40 - r 1000m > 35-40 500i 35-40- <35-40 60° 14° 10 ° Figure 3:1. First Survey: surface temperature and salinity. 40 positions of the surface 9° G and 10°C isotherms, and appear on the third survey, perhaps because the the occurrence of the 11° G isotherm just within the narrow region of its occurrence on the second survey region investigated on the second survey, argues an was not included on the third occasion. Likewise, the increasingly progressive surface oceanic influence main 35-30 °/00 isohalines of the two surveys were on northwards towards and over the Ridge, with, of the whole similarly disposed, although the third course, a concomitant regression northwards of the survey would appear to have witnessed a greater Arctic influence tending to bear southwards towards intrusion of higher salinity water, above 35-35 °/00, the Ridge. On the whole, the salinity distributions of and indeed, in two places, corresponding very nearly the first two surveys scarcely bear the same implication. to the two loci of maximum salinity surface waters on Indeed, so far as salinity is an index, the difference the first survey (> 35-35 °/00), above 35-40 °/00. These between Figures 3:1 and 3:8 almost points in the circumstances indicate still further progression of the reverse sense, namely, in favour of a slight impediment oceanic influence northwards at the expense of the to the northern progress of oceanic influence over the colder and fresher northern influence southward Ridge by southerly directed Arctic influence. The towards the Ridge. difficulty in assessing the differential significance as On the other hand, the dilutive eddy or vortex over between oceanic and Arctic influences of the surface the Rose Garden area, which was noticed on both the salinity distributions of the first two surveys arises previous surface charts, had evidently intensified in from the increased evidence of vorticity movement the third week of the exercise, reducing its nuclear over the Ridge on the second survey. On the first surface salinity to below 34-95 °/00. Concomitantly, survey there was apparently only one of those entities, surface temperatures which had been between 8°C but, as will appear later, probably a significant one, and 9° C on the first two surveys, were here reduced to in the area over the Rose Garden (Rosengarten) below 7°C. fishing area. Here, on the first survey, the general Passing to a consideration of conditions at 100 salinity of the neighbourhood of over 35-25 °/00 was, metres’ depth (Figures 3:2, 3:9, and 3:12), it may be presumably by influence from the north, reduced to noticed at once that the vortex of the Rose Garden below 35-15 °/00. O n the second survey, in this same area, similarly increasing in intensity from the first, area and in a similar vortex, the surrounding salinity through the second, to the third survey, was again of about 35-25 °/00 was reduced in the centre of the revealed at this level, with a marked change in vortex to a nuclear value of below 35-00 °/oo> indicative position, however, as between the surface and 100 it would appear of increasing influence, in this parti metres on the first survey, and as between the first and cular region at least, of colder fresher water from the the second and the second and third surveys at the north or northeast. latter level. The curious feature is further noticeable Farther south towards the narrowest part of the at the 100 metres level (Figure 3:2) on the first survey Ridge, but still on its northern boundary, a second, that the thermal vortex was apparently entirely less intense vortex appeared on the surface on the separate from the salinity entity. The 100 metres second survey, reducing the salinity there from a diagram of the first survey, however, as distinct from general distribution of over 35-25 °/00 to below the others, contains, in the trends of isotherms and 35-20 o/00. isohalines, a fairly clear indication of the direction A third of these apparently diluting vortices appe from which dilution of the northward-tending oceanic ared on the second survey on the southwestern water-mass proceeded. This was evidently more from shoulder of the Ridge, enclosing a minimum salinity the eastward than the northward, approximately along of less than 35-15 °/00 in an area of 35-25 °/00 to the latitude of 63°45'N. 35-30 °/00 salinity water. On the broad general features of the three 100- Despite these indications, however, by salinity, and metre charts, the relative positions of the main while at the same time accepting the intrusive in 35-30 °/00 isohalines undoubtedly denote at this level fluence which they evidently signify, the total or also a northwards progression of the oceanic water- aggregate interpretation of the surface temperature mass, more marked between the first and second and salinity distributions from the first to the second than between the second and third surveys. A similar survey points to the recession of the northern in inference attaches to the dispositions of the 35-35 °/00 favour of the southern, oceanic influence. and 35-40 °/00 isohalines over the region of the Faroe The difference between the surface distributions of Bank Channel. the second and third surveys is on the whole less It seems rather odd, however, to find, on the third impressive. The 10° C, 9° C, and 8° C isotherms retained survey, (Figure 3:16) an indication of oceanic water much the same positions geographically on the third so far north as almost on latitude 65 N, at longitude s u r v e y , as on the second.

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