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Atmospheric and marine climate fluctuations in the North Atlantic region
ATMOSPHERIC AND MARINE CLIMATE FI,UCTUATIONS IN THE NORTH ATLANTIC REGION ROBERT DICKSON and ARTHUR LEE Fisheries Laboratory, Lowestoft, England
ONE of the objectives of this symposium is to review the results obtained, so far, from the use of standard oceanographic sections over a long period of time. Such sections have been worked by scientists in the member nations of the International Council for the Exploration of the Sea (ICES) ever since the Council's formation 60 years ago. These ICES standard monitoring sections are nearly all located in the northeastern Atlantic Ocean and its adjacent seas, the very area in which the Intergovernmental Oceanographic Commission is considering working a standard section intensively over a period of one year. The practice of working these sections originated with the Scandinavian oceanographers who founded the Council, but it is perhaps significant that they did not keep it up for very long. The ICES has also been responsible for the working of a number of fixed oceanographic stations in the same area: these are geographical points at which sea surface temperature and salinity observations are made regularly by research vessels, merchant vessels, lightvessels, lighthouses and weather ships, and some of them have been occupied regularly for periods of more than 40 years except for breaks during the two World Wars. During the last two years we have been carrying out an analysis of all the material collected from these standard sections and oceanographical fixed stations in the ICES area. This examination is as yet by no means complete so that this paper is in the nature of a progress report. The long-term trends of temperature in the North Atlantic region have been dealt with by a number of authors, in particular by BJERKNES(1959, 1962, 1963 and 1964). Those in the oceanographical climate as a whole have been dealt with by one of us in BEVERTON and LEE (1965). In the present analysis we have been more concerned with fluctuations in oceanographic conditions lasting over periods of three or four years. It became clear to us quite early that the standard section observations had been made in such an unsystematic manner that any statistical analysis of the data as a long time series was impossible. We therefore decided to examine the conditions in 55
56
ROBERT DICKSON AND ARTHUR LEE
various specific periods of time in which it was thought that marked changes had taken place. As a starting point we took the period 1958-60. RODEWALD (1963) has pointed out that marked changes occurred in the sea surface temperatures at the ocean weather stations in the North Atlantic at that time: there was a positive anomaly of up to 1.1°C in the northeast Atlantic compared with norms for the period 190(040, and in the northwestern Atlantic there was a negative anomaly of up to 0.8 ° C. Further, NAMIAS (1964, 1965) has shown that starting with the autumn of 1958, there was during this period a marked blocking action over Scandinavia and that this had such dramatic consequences as a marked reduction in precipitation in all the autumn seasons, particularly in that of 1960 when Bergen had a precipitation only 44 ~ of normal. The negative anomaly of sea surface temperature in the northwest Atlantic was due to an anomalously strong Iceland low stationed off southern Greenland: the greater windiness and the transport over the northwest Atlantic of cold air masses from the north and west led to an increased extraction of latent and sensible heat from the sea. On the other hand, the positive surface temperature anomaly in the northeast Atlantic was due to this area having an anomalous southerly wind which gave rise to a smaller air-sea temperature difference and to a greater than normal vertical stability in the lower atmosphere, so that there was less loss of sensible and latent heat from the sea. Further, BJERKNES(1963) has shown that in the ocean near the centre of lows there is a thermal doming which brings cooler water towards the surface and that conversely under anticyclones there is horizontal convergence. The atmospheric pressure distribution in 1958-60 was such as to give rise to these two effects which would accentuate the anomalous sea surface temperature distribution. As a consequence of the anomalies in the northwest and northeast Atlantic there was a large anomaly (about 2 ° C) in the west-east sea surface temperature gradient. Furthermore, whereas the sea-surface isotherms normally run from southwest to northeast, in 1958-60 they ran from south to north. Namias claims that this change in direction can be derived from the Ekman transport in the surface layers brought about by the wind system which prevailed in 1958-60. Namias has also sought the reason for the persistence of the blocking action. He argues that the strengthened west-east sea surface temperature gradient increased the flux of heat from sea to atmosphere to above normal in the east and reduced it to below normal in the west. In this way direct atmospheric circulations were encouraged through the thermal wind and these favoured increased frontogenesis, stronger than normal southerly winds, and the associated abnormal motion of the depressions as they were steered northwards along the sea surface isotherms instead of northeastwards. The inertial effect, acting through the barotropic response, was to encourage the development of a stronger ridge of high pressure than is normal over northern
ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
57
Europe and the new pressure distribution resulting from this blocking action in its turn favoured the maintenance of the original anomalous sea surface temperature distribution. Apart from the changed sea-surface temperature pattern, what phenomena can we find which show the response of the ocean to this markedly anomalous condition of the atmospheric pressure distribution ? In an attempt to answer this question we have searched those publications of ICES in which the observations on standard oceanographic sections are reported, particularly the Annales Biologiques. These reports are hard to interpret. We are told that temperature and/or salinity was above average on a certain section in a certain month, but we are not told how the average was derived or where to obtain it. We are shown the distributions of temperature and salinity along a particular section in different years but the scales of the published diagrams change from one year to the next. Further, the positions of the stations making up the sections are not always constant and the depths sampled are not the same in every year. Yet despite these shortcomings it is possible to derive some information from the Annales Biologiques for 1958, 1959 and 1960 and from volume 149 of the Rapports et Procds-Verbaux. In the autumn of 1958, at the end of the International Geophysical Year, there appears to have been a marked inflow of Atlantic water into the Norwegian Sea. This is reported by ALEKSEEV, ZAITSEVand POTAICHUK(1961) and by SAELEN (1961) and was deduced from the extent, both horizontally and vertically, of water with the temperature and salinity characteristics of Atlantic water. The Soviet workers also reported the volume transport through the Faroe-Shetland Channel as being well above normal at this time, but they do not state their norm or give any values for the autumn season in other years. However, TAIT and MARTIN (1961) also report an abnormally powerful impulse of the Atlantic Current through the Faroe-Shetland Channel at the end of 1958. No information is available for the winter and spring of 1958-9, but the areas of Atlantic water in the southern Norwegian Sea are reported as being warmer in June 1959 than in all other years since 1954. These areas comprise the Atlantic Current, which enters through the Faroe-Shetland Channel, and the Irminger Current, which passes to the west of Iceland and enters along the north Icelandic coast: they have been surveyed in early summer for some years past during the international herring investigations carried out under the aegis of ICES by Iceland, Denmark, Norway and the U.S.S.R. According to ALEKSEEVet al. (1961) and STEFANSSON (1961) conditions there were 0.5-3.0 ° C warmer in June 1959 than in 1958 at depths as great as 100 m. Further north in the Greenland Sea, however, the temperature of the West Spitsbergen Current was the same or colder than in 1958. It is also of interest to note that at the same time in the northwest Atlantic, as opposed to the northeast, conditions off Labrador, as
58
ROBERT DICKSON AND ARTHUR LEE
reported by WORTHINGTON0964), were abnormally cold with the slope water displaced some 300 km to the east of the slope. In the summer and autumn of 1959 abnormal conditions prevailed in the North Sea. Hydrographic surveys made by us show that the Atlantic water, which enters from the southeastern Norwegian Sea between Shetland and Norway, extended much further south than usual. Normally this water is confined to the northern and central North Sea, but in the late summer and autumn of 1959 it penetrated into the southern part. Moreover, a Soviet report by ALEKSEEV, ISTOSHIN and PONOMARENKO0961) shows that by the last months of 1959 the warming in the southern part of the Norwegian Sea had reached the northern part as well, with temperatures in the Atlantic Current off the Lofoten Islands 1° C higher than at the same time in 1958 down to 500 m depth. Again no information is available for the winter and spring of 1960, but by the summer of that year the warming had spread to the Greenland and Barents Seas. ALEKSEEV, ISTOSHINand SHMARINA(1962) report the temperature of the West Spitsbergen Current at Bear Island as being 0.4 ° C above normal, and at northern Spitsbergen as being 0.7°C higher than in 1958. They state that "the thermal level of the water masses in the Norwegian and Greenland Seas in the summer of 1960 exceeded on the whole the level of almost all the years during which the observations were carried out". The Barents Sea was also warmer and more saline than usual in the early summer of 1960. According to KISLYAKOV(1962) temperatures between Norway and Bear Island and along the Kola Meridian section were about 0.4-0.7° C above normal and 0.2 ° C higher than in 1959. Further south in the FaroeShetland Channel ALEKSEEV, ISTOSHINand SHMARINA(1962) report the high temperatures of 1959 as being exceeded, and according to STEFANSSON(1962) the temperature down to 100 m depth off northern Iceland was between 0.5 and 2 ° C above normal and the salinity was at the highest level ever observed. Furthermore, warmer water than usual was reported off west Greenland in the other branch of the Irminger Current in July by HERMANN (1962). He states that here the Atlantic component was very well developed and that the temperatures were probably the highest found during the years 1951-60. Further, HARVEY (1962) reports particularly warm water in the region of Cape Farewell and notes that the salinity of the Irminger component of the West Greenland Current there had returned to the high levels found between 1928 and 1948 but not again after the latter year. Harvey also notes that this return seems to have started in 1959 and HERMANN (1960, 1961) reports that in July in both 1958 and 1959 the south-going branch of the Irminger Current to the east of Greenland was better developed than is normal. The course of oceanographic events during the period 1958-60 as outlined above has been obtained from analyses of the temperature and salinity
ATMOSPHERICAND MARINECLIMATEFLUCTUATIONS
59
distributions along various standard sections. The volume transports across most of these sections during the period 1958-60 are not readily available in the literature. In fact, we have been able to find only three sections for which the volume transport has been calculated more than once in any of the three years 1958-60; the West Bear Island Section, which crosses the West Spitsbergen Current in latitude 74°25'N, and the two Faroe-Shetland Channel Sections across the Atlantic Current. The values for the West Bear Island Section are given by one of us (LEE, 1962). Those for the Faroe-Shetland Channel Sections have been very kindly supplied to us by the Marine Laboratory, Aberdeen, together with all the other values collected by that laboratory during the period 1927-64; they are given here as Fig. 1. It can 24 X
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be seen that the five values obtained in 1959 are high relative to the other values. The observations made at the end of 1958 are not distinguished in Fig. 1, but they form the second highest of the September values (18.2 km3/hr) and the lowest of the December values (20.1 km3/hr). Likewise the volume transports for the West Bear Island Section given in Lee (1962) show the values for April-December 1959 to have been high relative to the others for the period 1949-59. It would thus appear at first sight that during the period September 1958-December 1959 the volume transport of the AtlanticWest Spitsbergen Current system was higher than normal, but it is clear from Fig. 1, and also from fig. 3 in LEE (1962), that very little reliance can be put
60
ROBERT DICKSON AND ARTHUR LEE
on this interpretation since the volume transports across the sections have been obtained so infrequently in certain months of the year that it is impossible to establish a norm from which to calculate deviations. Further, LEE (1962) has shown that there are a number of dynamic considerations which affect the accuracy of the volume transport values for the sections in question. We have also examined the volume transport distributions along the Faroe-Shetland Channel Sections using the transport values between the successive pairs of stations, which constitute the sections, given by TAIT (1957) for the period 1934-52. We find that on the Faroe-Shetland section 5 6 ~ of the mean northeast-going transport passes between two stations, which are about 30 km apart on the slope on the Shetland side of the channel where the bottom shelves from 1200 m to 500 m, and that on the Faroe Bank-Butt of Lewis section, while 18 ~ passes between two stations which are about 30 km apart on the slope on the Scottish side of the channel where the bottom shelves from 940 m to 470 m, 67 ~/o passes between two stations situated 56 km apart and straddling the Wyville Thomson Ridge. Thus, the majority of the northeast-going transport occurs in areas where the bottom topography is such as to make the selection of a reference level particularly difficult and where the usual hydrodynamical method of calculating transports cannot easily be applied. We are therefore presented with a tantalizing picture. It looks as if something dramatic happened in the ocean in 1958-60 in response to the blocking action in the atmosphere over Scandinavia, but we cannot be absolutely sure of this and we certainly cannot express the reaction in terms of numbers. To make matters worse we do not know what was happening in the ocean before the blocking action started. We do know that 1958 was a year in which there was positive anomaly of sea-surface temperature in the area east of Newfoundland. The data given by SMED (1960) show that in the spring it amounted to 2°C. U.S. Coast Guard Bulletin No. 44 shows that the same area was also remarkably free of icebergs in 1958 and that subsurface waters were abnormally warm. Was this positive temperature anomaly propagated downstream along the North Atlantic Drift in 1958 so as to produce by the autumn a sea surface temperature distribution favourable to the establishment of the blocking action ? Did the transfer of anomalously warm water into the Norwegian Sea in 1959 help to make the blocking action in 1960 particularly effective as far as Scandinavia was concerned but not so for the southern British Isles ? From the examination of the period 1958-60 we have passed to a search for earlier periods in which the same phenomena were present in the atmospheric and oceanic circulations. Mr. H. H. L a m b of the Meteorological Office, Bracknell, England, has calculated an index of the meridional flow over the northeast Atlantic for all months since 1873. He has very kindly
61
ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
put these data at our disposal and the index is shown in Figs. 2 and 3 for the periods 1904-21 and 1947-63 respectively. Figure 3 shows the marked southerly flow during the period 1958-60. It also shows similar flows in 1948-50 and 1953-4. Now during both of these periods there are recorded in the literature oceanographic events in the northeastern Atlantic region like those described above for the period 1958-60. For example, 1954 was i
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the warmest year recorded on the standard sections off northern Iceland, in the Barents Sea and in the West Spitsbergen Current, and the fact that anomalously high temperatures were observed in each of the periods 1948-50, 1954 and 1958-60 in the Norwegian and Barents Seas can be seen from the paper by MmTrtm (p.p. 41ft. of this volume) and from Fig. 3, which shows the 0-200 m layer temperature anomaly on the Kola Meridian section in the southeastern Barents Sea as based on data kindly supplied to us by the Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, U.S.S.R. for the period 1921-61.
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ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
63
Figures 2 and 3 show the sea-surface salinity anomaly in the eastern English Channel and eastern North Sea respectively. The anomaly is with respect to the mean for the period 1905-54 given in the ICES atlas "Mean Monthly Temperature and Salinity of the Surface Layer of the North Sea and Adjacent Waters from 1905 to 1954". This atlas is to a large extent based on the data collected at the oceanographical fixed stations described above. From Figs. 2 and 3 it can be seen that at times of southerly air flow the salinity anomaly becomes increasingly positive. We have found that this phenomenon occurs more or less simultaneously in all the seas around the British Isles. The increase in salinity may be due to two factors (a) an increased advection of a more extreme form of North Atlantic Central Water into these seas under the influence of southerly winds (b) a decrease in precipitation and an increase in evaporation with the change from northerly and westerly winds to southerly. Data supplied to us by the Meteorological Office, Bracknell, does indeed show that precipitation over Britain is correlated with the meridional index anomaly. In support of the idea that increased advection of Atlantic water takes place during periods of southerly air now we find that away from the Atlantic there is a lag between the increase in the southerly air flow and the increase in salinity and that this usually gets bigger with increasing distance from the Atlantic. Again, in periods when the salinity increases, exotic marine organisms of southerly origin become more common in the seas around Britain. Also the salinity at depth in the extensions of the Gulf Stream system itself seems to increase. Thus, in the early 1930s when there was a marked increase in salinity in the seas around Britain there was an increase in salinity in the northeast Atlantic at a few hundred metres depth in the Atlantic Current in the Faroe-Shetland Channel: this is said by TAIT (1957) to be due to advection of water from the Gulf of Gibraltar. There were also higher salinity conditions in the northwest Atlantic: this is shown by the values for the Irminger component of the West Greenland Current discussed by HARVEY (1962). It is clear therefore that changes in the atmospheric circulation over the North Atlantic have a dramatic response in the sea itself and that there are possibly feedback effects since the ocean is seen to be actively transporting heat from one place to another and not acting merely as a reservoir. But the standard oceanographic observations made in the past only indicate the sort of things which might be happening: they are inadequate for use in any detailed analysis of the coupling between atmosphere and ocean. For such an analysis the proper design and execution of standard sections, which will reveal the horizontal velocity field through direct current measurements as well as show the temperature and salinity fields, would seem to be essential.
64
ROBERT DICKSON AND ARTHUR LEE REFERENCES
At.~KSEEV, A., ISTOSmN, B., PACHORUKOV,W., MYRLAND,P., EINARSSON,H., HERMANN,F. and S~EANSSON, U. (1961) International investigations on hydrographic conditions in the Norwegian Sea, June 1959. Annls biol., Copenh., 16, 25. AL~KSEeV, A., IsrosnI~, B. and PONO~aU~NKO, L. (1961) Hydrographic conditions in the Norwegian Sea, Novcmber-Deeembcr 1959. Annls biol., Copenh., 16, 26-28. ALEKSEEV,A. P., ZAH'SEV,G. N. and POTAICI-n.~, S. I. 0961) Results of hydrological and hydrochcmical studies in the Norwegian Sea in 1958. Rapp. P.-v. Rdun. Cons. perm. int. Explor. Mer, 149, 53-55. ALEKSEEV,A. P., ISTOSHIN,B. V. and SHMARINA,L. R. (1962) Hydrographic conditions in the Norwegian and Greenland Seas in the summer of 1960. Annls biol., Copenh., 17, 23-24. BEVERTON,R. J. H. and LEE, A. J. (1965) Hydrographic Fluctuations in the North Atlantic Ocean and some Biological Consequences. The Biological Significance oJ" Climatic Changes in Britain, C. G. Johnson and L. P. Smith, editors, Academic Press, London and New York, Syrup. Inst. Biol., No. 14, 79-107. B.m~NES, J. (1959) The recent warming of the North Atlantic. In: The Atmosphere and the Sea in Motion, B. Bolin, editor, The Rockefeller Inst. Press, New York, 65-73. BJERKNES,J. (1962) Synoptic survey of the interaction of sea and atmosphere in the North Atlantic. Geofys. Publn., 24, 115-145. BJERKNES, J. (1963) Climatic change as an ocean-atmosphere problem. In: Changes of Climate, Arid Zone Research, Proc. Rome Symposium, UNESCO WMO, 20, 297-321. BJERKNE8, J. 0964) Atlantic air-sea interaction. Adv. Geophys., 10, 1-82. HARVEY, J. G. (1962) Hydrographic conditions in Greenland waters during August 1960. Annls biok, Copenh., 17, 14-17. HERMANN, F. (1960) Sections from the Faroes to East Greenland and Cape Farewell to Ireland. Annls biok, Copenh., 15, 19-21. HERMANN, F. (1961) Sections from the Faroes to East Greenland and Cape Farewell to Ireland. Annls biol., Copenh., 16, 21. HERMANN, F. (1962) Hydrographic conditions off West Greenland, July 1960. Annls biol., Copenh., 17, 18-19. KISLYAKOV, A. G. (1962) Hydrographic conditions in the Norwegian Current and tile North Cape Current in the spring of 1960. Annls biol., Copenh., 17, 24-25. LEE, A. J. (1962) The effect of the wind on water movements in the Norwegian and Greenland Seas. Proc. Syrup. Math.-Hydrodynam. Meth. Phys. Oceanogr., Hamburg, 1961, pp. 353-373. MIDTTUN, L. 0968) Variability of temperature and salinity at some localities outside the coast of Norway. Progress in Oceanography 5, 41-54. NAMIAS, J. (1964) Seasonal persistence and recurrence of European blocking during 1958-1960. Tellus, 16, 394-407. NAMIAS,J. (1965) Short period climatic fluctuations. Science, 147, 696-706. RODEWALD, M. (1963) Sea-surface temperatures of the North Atlantic Ocean during the decade 1951~50, their anomalies and development in relation to the atmospheric circulation. In Changes of Climate, Arid Zone Research, Proc. Rome Symposium, UNESCO WMO, 20, 97-107. SAELEN, O. H. (1961) Preliminary report on the hydrographic sections made in the Norwegian Sea 1958 by the Geophysical Institute, Bergen. Rapp. P.-t~. R~un. Cons. perm. int. Explor. Mer, 149, 56-59. SrC~D, J. (1960) Monthly anomalies of surface temperature in areas of the northern North Atlantic in 1958. Annls biol., Copenh., 15, 14-15. S~rANSSON, U. (1961) Hydrographic conditions in Icelandic waters in May-June 1959. Annls biol., Copenh., 16, 18-20. S~FANSSON, U. (1962) Hydrographic conditions in Icelandic waters in May-June 1960. Annls biol., Copenh., 17, 20-21.
VARIABILITY OF TEMPERATUREAND SALINITY
65
TAIT, J. B. (1957) Hydrography of the Faroc-Shetland Channel, 1927-1952. Mar. Res., No. 2, 309 pp. TArr, J. B. and MARTIN, J. H. A. (1961) The Atlantic Current and water masses in the Faroc--Shetland Channel and over the Ic¢land-Faro¢ Ridge during the IGY. Rapp. P.-v. Rdun. Cons. perm. int. Explor. Mer, 149, 60-83. WORTHINGTON,L. V. (1964) Anomalous conditions in the slope water area in 1959. J. Fish. Res. Bd., Can., 21, 327-333.
ONE of the objectives of this symposium is to review the results obtained, so far, from the use of standard oceanographic sections over a long period of time. Such sections have been worked by scientists in the member nations of the International Council for the Exploration of the Sea (ICES) ever since the Council's formation 60 years ago. These ICES standard monitoring sections are nearly all located in the northeastern Atlantic Ocean and its adjacent seas, the very area in which the Intergovernmental Oceanographic Commission is considering working a standard section intensively over a period of one year. The practice of working these sections originated with the Scandinavian oceanographers who founded the Council, but it is perhaps significant that they did not keep it up for very long. The ICES has also been responsible for the working of a number of fixed oceanographic stations in the same area: these are geographical points at which sea surface temperature and salinity observations are made regularly by research vessels, merchant vessels, lightvessels, lighthouses and weather ships, and some of them have been occupied regularly for periods of more than 40 years except for breaks during the two World Wars. During the last two years we have been carrying out an analysis of all the material collected from these standard sections and oceanographical fixed stations in the ICES area. This examination is as yet by no means complete so that this paper is in the nature of a progress report. The long-term trends of temperature in the North Atlantic region have been dealt with by a number of authors, in particular by BJERKNES(1959, 1962, 1963 and 1964). Those in the oceanographical climate as a whole have been dealt with by one of us in BEVERTON and LEE (1965). In the present analysis we have been more concerned with fluctuations in oceanographic conditions lasting over periods of three or four years. It became clear to us quite early that the standard section observations had been made in such an unsystematic manner that any statistical analysis of the data as a long time series was impossible. We therefore decided to examine the conditions in 55
56
ROBERT DICKSON AND ARTHUR LEE
various specific periods of time in which it was thought that marked changes had taken place. As a starting point we took the period 1958-60. RODEWALD (1963) has pointed out that marked changes occurred in the sea surface temperatures at the ocean weather stations in the North Atlantic at that time: there was a positive anomaly of up to 1.1°C in the northeast Atlantic compared with norms for the period 190(040, and in the northwestern Atlantic there was a negative anomaly of up to 0.8 ° C. Further, NAMIAS (1964, 1965) has shown that starting with the autumn of 1958, there was during this period a marked blocking action over Scandinavia and that this had such dramatic consequences as a marked reduction in precipitation in all the autumn seasons, particularly in that of 1960 when Bergen had a precipitation only 44 ~ of normal. The negative anomaly of sea surface temperature in the northwest Atlantic was due to an anomalously strong Iceland low stationed off southern Greenland: the greater windiness and the transport over the northwest Atlantic of cold air masses from the north and west led to an increased extraction of latent and sensible heat from the sea. On the other hand, the positive surface temperature anomaly in the northeast Atlantic was due to this area having an anomalous southerly wind which gave rise to a smaller air-sea temperature difference and to a greater than normal vertical stability in the lower atmosphere, so that there was less loss of sensible and latent heat from the sea. Further, BJERKNES(1963) has shown that in the ocean near the centre of lows there is a thermal doming which brings cooler water towards the surface and that conversely under anticyclones there is horizontal convergence. The atmospheric pressure distribution in 1958-60 was such as to give rise to these two effects which would accentuate the anomalous sea surface temperature distribution. As a consequence of the anomalies in the northwest and northeast Atlantic there was a large anomaly (about 2 ° C) in the west-east sea surface temperature gradient. Furthermore, whereas the sea-surface isotherms normally run from southwest to northeast, in 1958-60 they ran from south to north. Namias claims that this change in direction can be derived from the Ekman transport in the surface layers brought about by the wind system which prevailed in 1958-60. Namias has also sought the reason for the persistence of the blocking action. He argues that the strengthened west-east sea surface temperature gradient increased the flux of heat from sea to atmosphere to above normal in the east and reduced it to below normal in the west. In this way direct atmospheric circulations were encouraged through the thermal wind and these favoured increased frontogenesis, stronger than normal southerly winds, and the associated abnormal motion of the depressions as they were steered northwards along the sea surface isotherms instead of northeastwards. The inertial effect, acting through the barotropic response, was to encourage the development of a stronger ridge of high pressure than is normal over northern
ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
57
Europe and the new pressure distribution resulting from this blocking action in its turn favoured the maintenance of the original anomalous sea surface temperature distribution. Apart from the changed sea-surface temperature pattern, what phenomena can we find which show the response of the ocean to this markedly anomalous condition of the atmospheric pressure distribution ? In an attempt to answer this question we have searched those publications of ICES in which the observations on standard oceanographic sections are reported, particularly the Annales Biologiques. These reports are hard to interpret. We are told that temperature and/or salinity was above average on a certain section in a certain month, but we are not told how the average was derived or where to obtain it. We are shown the distributions of temperature and salinity along a particular section in different years but the scales of the published diagrams change from one year to the next. Further, the positions of the stations making up the sections are not always constant and the depths sampled are not the same in every year. Yet despite these shortcomings it is possible to derive some information from the Annales Biologiques for 1958, 1959 and 1960 and from volume 149 of the Rapports et Procds-Verbaux. In the autumn of 1958, at the end of the International Geophysical Year, there appears to have been a marked inflow of Atlantic water into the Norwegian Sea. This is reported by ALEKSEEV, ZAITSEVand POTAICHUK(1961) and by SAELEN (1961) and was deduced from the extent, both horizontally and vertically, of water with the temperature and salinity characteristics of Atlantic water. The Soviet workers also reported the volume transport through the Faroe-Shetland Channel as being well above normal at this time, but they do not state their norm or give any values for the autumn season in other years. However, TAIT and MARTIN (1961) also report an abnormally powerful impulse of the Atlantic Current through the Faroe-Shetland Channel at the end of 1958. No information is available for the winter and spring of 1958-9, but the areas of Atlantic water in the southern Norwegian Sea are reported as being warmer in June 1959 than in all other years since 1954. These areas comprise the Atlantic Current, which enters through the Faroe-Shetland Channel, and the Irminger Current, which passes to the west of Iceland and enters along the north Icelandic coast: they have been surveyed in early summer for some years past during the international herring investigations carried out under the aegis of ICES by Iceland, Denmark, Norway and the U.S.S.R. According to ALEKSEEVet al. (1961) and STEFANSSON (1961) conditions there were 0.5-3.0 ° C warmer in June 1959 than in 1958 at depths as great as 100 m. Further north in the Greenland Sea, however, the temperature of the West Spitsbergen Current was the same or colder than in 1958. It is also of interest to note that at the same time in the northwest Atlantic, as opposed to the northeast, conditions off Labrador, as
58
ROBERT DICKSON AND ARTHUR LEE
reported by WORTHINGTON0964), were abnormally cold with the slope water displaced some 300 km to the east of the slope. In the summer and autumn of 1959 abnormal conditions prevailed in the North Sea. Hydrographic surveys made by us show that the Atlantic water, which enters from the southeastern Norwegian Sea between Shetland and Norway, extended much further south than usual. Normally this water is confined to the northern and central North Sea, but in the late summer and autumn of 1959 it penetrated into the southern part. Moreover, a Soviet report by ALEKSEEV, ISTOSHIN and PONOMARENKO0961) shows that by the last months of 1959 the warming in the southern part of the Norwegian Sea had reached the northern part as well, with temperatures in the Atlantic Current off the Lofoten Islands 1° C higher than at the same time in 1958 down to 500 m depth. Again no information is available for the winter and spring of 1960, but by the summer of that year the warming had spread to the Greenland and Barents Seas. ALEKSEEV, ISTOSHINand SHMARINA(1962) report the temperature of the West Spitsbergen Current at Bear Island as being 0.4 ° C above normal, and at northern Spitsbergen as being 0.7°C higher than in 1958. They state that "the thermal level of the water masses in the Norwegian and Greenland Seas in the summer of 1960 exceeded on the whole the level of almost all the years during which the observations were carried out". The Barents Sea was also warmer and more saline than usual in the early summer of 1960. According to KISLYAKOV(1962) temperatures between Norway and Bear Island and along the Kola Meridian section were about 0.4-0.7° C above normal and 0.2 ° C higher than in 1959. Further south in the FaroeShetland Channel ALEKSEEV, ISTOSHINand SHMARINA(1962) report the high temperatures of 1959 as being exceeded, and according to STEFANSSON(1962) the temperature down to 100 m depth off northern Iceland was between 0.5 and 2 ° C above normal and the salinity was at the highest level ever observed. Furthermore, warmer water than usual was reported off west Greenland in the other branch of the Irminger Current in July by HERMANN (1962). He states that here the Atlantic component was very well developed and that the temperatures were probably the highest found during the years 1951-60. Further, HARVEY (1962) reports particularly warm water in the region of Cape Farewell and notes that the salinity of the Irminger component of the West Greenland Current there had returned to the high levels found between 1928 and 1948 but not again after the latter year. Harvey also notes that this return seems to have started in 1959 and HERMANN (1960, 1961) reports that in July in both 1958 and 1959 the south-going branch of the Irminger Current to the east of Greenland was better developed than is normal. The course of oceanographic events during the period 1958-60 as outlined above has been obtained from analyses of the temperature and salinity
ATMOSPHERICAND MARINECLIMATEFLUCTUATIONS
59
distributions along various standard sections. The volume transports across most of these sections during the period 1958-60 are not readily available in the literature. In fact, we have been able to find only three sections for which the volume transport has been calculated more than once in any of the three years 1958-60; the West Bear Island Section, which crosses the West Spitsbergen Current in latitude 74°25'N, and the two Faroe-Shetland Channel Sections across the Atlantic Current. The values for the West Bear Island Section are given by one of us (LEE, 1962). Those for the Faroe-Shetland Channel Sections have been very kindly supplied to us by the Marine Laboratory, Aberdeen, together with all the other values collected by that laboratory during the period 1927-64; they are given here as Fig. 1. It can 24 X
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be seen that the five values obtained in 1959 are high relative to the other values. The observations made at the end of 1958 are not distinguished in Fig. 1, but they form the second highest of the September values (18.2 km3/hr) and the lowest of the December values (20.1 km3/hr). Likewise the volume transports for the West Bear Island Section given in Lee (1962) show the values for April-December 1959 to have been high relative to the others for the period 1949-59. It would thus appear at first sight that during the period September 1958-December 1959 the volume transport of the AtlanticWest Spitsbergen Current system was higher than normal, but it is clear from Fig. 1, and also from fig. 3 in LEE (1962), that very little reliance can be put
60
ROBERT DICKSON AND ARTHUR LEE
on this interpretation since the volume transports across the sections have been obtained so infrequently in certain months of the year that it is impossible to establish a norm from which to calculate deviations. Further, LEE (1962) has shown that there are a number of dynamic considerations which affect the accuracy of the volume transport values for the sections in question. We have also examined the volume transport distributions along the Faroe-Shetland Channel Sections using the transport values between the successive pairs of stations, which constitute the sections, given by TAIT (1957) for the period 1934-52. We find that on the Faroe-Shetland section 5 6 ~ of the mean northeast-going transport passes between two stations, which are about 30 km apart on the slope on the Shetland side of the channel where the bottom shelves from 1200 m to 500 m, and that on the Faroe Bank-Butt of Lewis section, while 18 ~ passes between two stations which are about 30 km apart on the slope on the Scottish side of the channel where the bottom shelves from 940 m to 470 m, 67 ~/o passes between two stations situated 56 km apart and straddling the Wyville Thomson Ridge. Thus, the majority of the northeast-going transport occurs in areas where the bottom topography is such as to make the selection of a reference level particularly difficult and where the usual hydrodynamical method of calculating transports cannot easily be applied. We are therefore presented with a tantalizing picture. It looks as if something dramatic happened in the ocean in 1958-60 in response to the blocking action in the atmosphere over Scandinavia, but we cannot be absolutely sure of this and we certainly cannot express the reaction in terms of numbers. To make matters worse we do not know what was happening in the ocean before the blocking action started. We do know that 1958 was a year in which there was positive anomaly of sea-surface temperature in the area east of Newfoundland. The data given by SMED (1960) show that in the spring it amounted to 2°C. U.S. Coast Guard Bulletin No. 44 shows that the same area was also remarkably free of icebergs in 1958 and that subsurface waters were abnormally warm. Was this positive temperature anomaly propagated downstream along the North Atlantic Drift in 1958 so as to produce by the autumn a sea surface temperature distribution favourable to the establishment of the blocking action ? Did the transfer of anomalously warm water into the Norwegian Sea in 1959 help to make the blocking action in 1960 particularly effective as far as Scandinavia was concerned but not so for the southern British Isles ? From the examination of the period 1958-60 we have passed to a search for earlier periods in which the same phenomena were present in the atmospheric and oceanic circulations. Mr. H. H. L a m b of the Meteorological Office, Bracknell, England, has calculated an index of the meridional flow over the northeast Atlantic for all months since 1873. He has very kindly
61
ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
put these data at our disposal and the index is shown in Figs. 2 and 3 for the periods 1904-21 and 1947-63 respectively. Figure 3 shows the marked southerly flow during the period 1958-60. It also shows similar flows in 1948-50 and 1953-4. Now during both of these periods there are recorded in the literature oceanographic events in the northeastern Atlantic region like those described above for the period 1958-60. For example, 1954 was i
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the warmest year recorded on the standard sections off northern Iceland, in the Barents Sea and in the West Spitsbergen Current, and the fact that anomalously high temperatures were observed in each of the periods 1948-50, 1954 and 1958-60 in the Norwegian and Barents Seas can be seen from the paper by MmTrtm (p.p. 41ft. of this volume) and from Fig. 3, which shows the 0-200 m layer temperature anomaly on the Kola Meridian section in the southeastern Barents Sea as based on data kindly supplied to us by the Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, U.S.S.R. for the period 1921-61.
62
ROBERT DICKSON AND ARTHUR LEE 19.~} '
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ATMOSPHERIC AND MARINE CLIMATE FLUCTUATIONS
63
Figures 2 and 3 show the sea-surface salinity anomaly in the eastern English Channel and eastern North Sea respectively. The anomaly is with respect to the mean for the period 1905-54 given in the ICES atlas "Mean Monthly Temperature and Salinity of the Surface Layer of the North Sea and Adjacent Waters from 1905 to 1954". This atlas is to a large extent based on the data collected at the oceanographical fixed stations described above. From Figs. 2 and 3 it can be seen that at times of southerly air flow the salinity anomaly becomes increasingly positive. We have found that this phenomenon occurs more or less simultaneously in all the seas around the British Isles. The increase in salinity may be due to two factors (a) an increased advection of a more extreme form of North Atlantic Central Water into these seas under the influence of southerly winds (b) a decrease in precipitation and an increase in evaporation with the change from northerly and westerly winds to southerly. Data supplied to us by the Meteorological Office, Bracknell, does indeed show that precipitation over Britain is correlated with the meridional index anomaly. In support of the idea that increased advection of Atlantic water takes place during periods of southerly air now we find that away from the Atlantic there is a lag between the increase in the southerly air flow and the increase in salinity and that this usually gets bigger with increasing distance from the Atlantic. Again, in periods when the salinity increases, exotic marine organisms of southerly origin become more common in the seas around Britain. Also the salinity at depth in the extensions of the Gulf Stream system itself seems to increase. Thus, in the early 1930s when there was a marked increase in salinity in the seas around Britain there was an increase in salinity in the northeast Atlantic at a few hundred metres depth in the Atlantic Current in the Faroe-Shetland Channel: this is said by TAIT (1957) to be due to advection of water from the Gulf of Gibraltar. There were also higher salinity conditions in the northwest Atlantic: this is shown by the values for the Irminger component of the West Greenland Current discussed by HARVEY (1962). It is clear therefore that changes in the atmospheric circulation over the North Atlantic have a dramatic response in the sea itself and that there are possibly feedback effects since the ocean is seen to be actively transporting heat from one place to another and not acting merely as a reservoir. But the standard oceanographic observations made in the past only indicate the sort of things which might be happening: they are inadequate for use in any detailed analysis of the coupling between atmosphere and ocean. For such an analysis the proper design and execution of standard sections, which will reveal the horizontal velocity field through direct current measurements as well as show the temperature and salinity fields, would seem to be essential.
64
ROBERT DICKSON AND ARTHUR LEE REFERENCES
At.~KSEEV, A., ISTOSmN, B., PACHORUKOV,W., MYRLAND,P., EINARSSON,H., HERMANN,F. and S~EANSSON, U. (1961) International investigations on hydrographic conditions in the Norwegian Sea, June 1959. Annls biol., Copenh., 16, 25. AL~KSEeV, A., IsrosnI~, B. and PONO~aU~NKO, L. (1961) Hydrographic conditions in the Norwegian Sea, Novcmber-Deeembcr 1959. Annls biol., Copenh., 16, 26-28. ALEKSEEV,A. P., ZAH'SEV,G. N. and POTAICI-n.~, S. I. 0961) Results of hydrological and hydrochcmical studies in the Norwegian Sea in 1958. Rapp. P.-v. Rdun. Cons. perm. int. Explor. Mer, 149, 53-55. ALEKSEEV,A. P., ISTOSHIN,B. V. and SHMARINA,L. R. (1962) Hydrographic conditions in the Norwegian and Greenland Seas in the summer of 1960. Annls biol., Copenh., 17, 23-24. BEVERTON,R. J. H. and LEE, A. J. (1965) Hydrographic Fluctuations in the North Atlantic Ocean and some Biological Consequences. The Biological Significance oJ" Climatic Changes in Britain, C. G. Johnson and L. P. Smith, editors, Academic Press, London and New York, Syrup. Inst. Biol., No. 14, 79-107. B.m~NES, J. (1959) The recent warming of the North Atlantic. In: The Atmosphere and the Sea in Motion, B. Bolin, editor, The Rockefeller Inst. Press, New York, 65-73. BJERKNES,J. (1962) Synoptic survey of the interaction of sea and atmosphere in the North Atlantic. Geofys. Publn., 24, 115-145. BJERKNES, J. (1963) Climatic change as an ocean-atmosphere problem. In: Changes of Climate, Arid Zone Research, Proc. Rome Symposium, UNESCO WMO, 20, 297-321. BJERKNE8, J. 0964) Atlantic air-sea interaction. Adv. Geophys., 10, 1-82. HARVEY, J. G. (1962) Hydrographic conditions in Greenland waters during August 1960. Annls biok, Copenh., 17, 14-17. HERMANN, F. (1960) Sections from the Faroes to East Greenland and Cape Farewell to Ireland. Annls biok, Copenh., 15, 19-21. HERMANN, F. (1961) Sections from the Faroes to East Greenland and Cape Farewell to Ireland. Annls biol., Copenh., 16, 21. HERMANN, F. (1962) Hydrographic conditions off West Greenland, July 1960. Annls biol., Copenh., 17, 18-19. KISLYAKOV, A. G. (1962) Hydrographic conditions in the Norwegian Current and tile North Cape Current in the spring of 1960. Annls biol., Copenh., 17, 24-25. LEE, A. J. (1962) The effect of the wind on water movements in the Norwegian and Greenland Seas. Proc. Syrup. Math.-Hydrodynam. Meth. Phys. Oceanogr., Hamburg, 1961, pp. 353-373. MIDTTUN, L. 0968) Variability of temperature and salinity at some localities outside the coast of Norway. Progress in Oceanography 5, 41-54. NAMIAS, J. (1964) Seasonal persistence and recurrence of European blocking during 1958-1960. Tellus, 16, 394-407. NAMIAS,J. (1965) Short period climatic fluctuations. Science, 147, 696-706. RODEWALD, M. (1963) Sea-surface temperatures of the North Atlantic Ocean during the decade 1951~50, their anomalies and development in relation to the atmospheric circulation. In Changes of Climate, Arid Zone Research, Proc. Rome Symposium, UNESCO WMO, 20, 97-107. SAELEN, O. H. (1961) Preliminary report on the hydrographic sections made in the Norwegian Sea 1958 by the Geophysical Institute, Bergen. Rapp. P.-t~. R~un. Cons. perm. int. Explor. Mer, 149, 56-59. SrC~D, J. (1960) Monthly anomalies of surface temperature in areas of the northern North Atlantic in 1958. Annls biol., Copenh., 15, 14-15. S~rANSSON, U. (1961) Hydrographic conditions in Icelandic waters in May-June 1959. Annls biol., Copenh., 16, 18-20. S~FANSSON, U. (1962) Hydrographic conditions in Icelandic waters in May-June 1960. Annls biol., Copenh., 17, 20-21.
VARIABILITY OF TEMPERATUREAND SALINITY
65
TAIT, J. B. (1957) Hydrography of the Faroc-Shetland Channel, 1927-1952. Mar. Res., No. 2, 309 pp. TArr, J. B. and MARTIN, J. H. A. (1961) The Atlantic Current and water masses in the Faroc--Shetland Channel and over the Ic¢land-Faro¢ Ridge during the IGY. Rapp. P.-v. Rdun. Cons. perm. int. Explor. Mer, 149, 60-83. WORTHINGTON,L. V. (1964) Anomalous conditions in the slope water area in 1959. J. Fish. Res. Bd., Can., 21, 327-333.