Deep-Sea Research I 47 (2000) 655}680
Upper layer cooling and freshening
in the Norwegian Sea in relation to
atmospheric forcing
J. Blindheim *, V. Borovkov, B. Hansen, S.-Aa. Malmberg,
W.R. Turrell, S. "sterhus
Institute of Marine Research, P.O. Box 1870, Nordnes, N-5817 Bergen, Norway
Knipovich Polar Research Institute of Marine Fisheries and Oceanography, Murmansk, Russia
Fisheries Laboratory of the Faroes, Faroe Islands, Denmark
Marine Research Institute, Reykjavik, Iceland
Marine Laboratory, Aberdeen, Scotland, UK
Geophysical Institute, University of Bergen, Bergen, Norway
Received 13 May 1998; received in revised form 31 March 1999; accepted 12 July 1999
Abstract
Several time series in the Norwegian Sea indicate an upper layer decrease in temperature and
salinity since the 1960s. Time series from Weather Station `Ma, from Russian surveys in the
Norwegian Sea, from Icelandic standard sections, and from Scottish and Faroese observations
in the Faroe}Shetland area have similar trends and show that most of the Norwegian Sea is
a!ected. The reason is mainly increased freshwater supply from the East Icelandic Current. As
a result, temperature and salinity in some of the time series were lower in 1996 than during the
Great Salinity Anomaly in the 1970s. There is evidence of strong wind forcing, as the NAO
winter index is highly correlated with the lateral extent of the Norwegian Atlantic Current.
Circulation of Atlantic water into the western Norwegian and Greenland basins seems to be
reduced while circulation of upper layer Arctic and Polar water into the Norwegian Sea has
increased. The water-mass structure is further a!ected in a much wider sense by reduced
deep-water formation and enhanced formation of Arctic intermediate waters. A temperature
rise in the narrowing Norwegian Atlantic Current is strongest in the north. 2000 Elsevier
Science Ltd. All rights reserved.
* Corresponding author. Fax: 0047-5523-8584.
E-mail address: [email protected] (J. Blindheim)
0967-0637/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 7 0 - 9
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J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
1. Introduction
The large-scale surface circulation in the Nordic Seas is dominated by the warm
northward #owing Atlantic in#ow, mainly on the eastern side, and the cold East
Greenland Current (EGC) #owing southward on the western side. The Atlantic in#ow
forms the most northern limb of the North Atlantic current system and carries water
from the North Atlantic to high latitudes. Atlantic water enters the Norwegian Sea
through the Faroe}Shetland Channel, mainly following the Scottish slope, and
between the Faroes and Iceland, where modi"ed North Atlantic Water (MNAW)
feeds the Faroe Current, which #ows eastward north of the ridge. East of the Faroes it
partly recirculates into the Faroe}Shetland Channel, while some of its water continues
toward the northeast, mainly as a western branch of the Norwegian Atlantic Current
(NwAC) (Poulain et al., 1996). Some Atlantic water is also carried into the Iceland Sea
by the North Icelandic branch of the Irminger Current, which #ows eastward along
the North Icelandic coast.
These currents carry large amounts of heat into the Nordic Seas (Hansen et al.,
1998). Besides their crucial importance for the regional climate, they keep the entire
Norwegian Sea, normally the North Icelandic shelf area and large areas of the Barents
Sea ice free and open for biological production. Fluctuations in properties and #uxes
in this current system are also of large ecological importance. Hence, the condition of
the "sh stocks indigenous to the region is normally best when the temperature is
relatively high, as both recruitment success and growth are best during warm periods
(for example Cushing, 1982; Holst, 1996). Improved knowledge of the variability in
this system and its forcing may therefore become useful information in the management of these commercially important "sh stocks.
In its upper layers, the south #owing EGC carries surface water of low salinity,
including ice, from the Arctic Ocean (e.g. Aagaard and Carmack, 1989) while in its
deeper strata there is also a transport of deep water from the Arctic Ocean (Swift and
Koltermann, 1988). This is an important source water mass of the deep water in the
Nordic Seas. A warmer intermediate layer with water of Atlantic origin returns from
the West Spitsbergen Current (WSC) and partly from the Arctic Ocean (e.g. Nansen,
1906; Helland-Hansen and Nansen, 1912; Foldvik et al., 1988; Hopkins, 1991; Mauritzen, 1996). The main branches of the EGC are, "rstly, the Jan Mayen Current, which
brings all three water masses into the cyclonic circulation in the Greenland Basin, and
secondly and further south, the East Icelandic Current (EIC), which carries a somewhat varying combination of the same water masses from the EGC into the Iceland
and Norwegian Seas (Buch et al., 1996). The remaining water in the EGC leaves the
Nordic Seas through the Denmark Strait to supply fresh water to the Subarctic Gyre
in the North Atlantic as well as dense over#ow water, which contributes to the deep
western boundary current in the North Atlantic.
The border zone between the domains of the NwAC and the Arctic waters to the
west is known as the Arctic Front. North of Jan Mayen it is topographically controlled by the mid-ocean ridge and shows only small #uctuations in position. Between
Iceland and Jan Mayen, on the other hand, variations in the volume of Arctic waters
carried by the EIC may result in relatively large shifts of the Arctic Front.
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
657
Fig. 1. Time series of temperature and salinity over the Scottish shelf in the Faroe}Shetland Channel,
1902}1997. Values are presented as anomalies as the seasonal cycle has been removed. The curves are the
result of a 24 month running mean, calculated at 6-month intervals.
Fluctuations in #uxes and water-mass properties in these two major current systems are therefore of decisive importance for the structure and distribution of the water
masses both in the Nordic Seas and, at least to some extent, in the Subarctic Gyre in
the North Atlantic. Hence, the supply of Arctic water carried by the EGC into the
Subarctic Gyre and its mixing with North Atlantic water may be an important, but
not the only, mechanism to create #uctuations in the Atlantic in#ow to the Nordic
Seas. However, the present analysis indicates that the volume and properties of the
Arctic water carried directly into the Norwegian Sea by the EIC play a larger role, in
the creation of variability within the distribution of water masses and their properties
in the Nordic Seas, than previously considered.
Two century-long time series show the irregularity in these variations but also some
parallelism. These time series are obtained from the Scottish standard sections across
the Faroe}Shetland Channel and the Russian Kola section, along 33330E, in the
Barents Sea (e.g. Dooley et al., 1984; Bochkov, 1982; Turrell et al., 1993). While a
broad spectrum of time scales is detected, ranging from seasonal and interannual to
inter decadal (Loeng et al., 1992), both these time series also indicate a more long-term
change. Hence, the salinity trend in the Faroe}Shetland Channel, which is shown in
Fig. 1 (Turrell et al., 1993), indicates low values early in the century and in the second
half of the 1970s, while high salinities were observed in the late 1930s and the 1950s.
Since the late 1950s, salinities generally declined until the minimum in the 1970s,
which, following Dickson et al. (1988), is called the `Great Salinity Anomalya (GSA).
Since this anomaly there has been an increase. Similar #uctuations have occurred in
the Kola Section, and also over wide areas in the northern North Atlantic, in the area
of the Subarctic Gyre. Based on observations of sea surface temperature from ships of
opportunity, Smed (e.g. 1943, 1965) worked out mean surface temperature anomalies
in sub areas in the north Atlantic (503}673N, 03}583W) for the periods 1876}1900,
1901}1925, 1926}1950 and 1951}1961, using the period 1876}1915 as normal.
658
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
Generally, he found the highest temperatures in the period 1951}1961, and compared
with the coldest period, 1901}1925, he found a temperature rise of about 0.53C in the
south, ranging to about 13C in the north. While Smed's analysis of surface temperatures was terminated in 1975 (Smed, 1979), the further temperature trends are
documented by several authors (e.g. Kushnir, 1994; Reverdin et al., 1997) showing
cooling during the 1970s and 1980s. These works also show a relationship between
interannual #uctuations of SST and surface wind conditions. The Nordic Seas have
open connections with the Subarctic Gyre and, indeed, exhibit similar #uctuations.
JoH nsson (1992) has described the importance of the wind stress curl over the Iceland
Sea for the variability in the volume of fresh water in the area and its likely role in the
convective conditions (Meincke et al., 1992). Further, Malmberg and JoH nsson (1997)
compared variations in wind stress curl to the timing of convection in the Iceland and
Greenland Seas and concluded that leakage of freshwater from the EGC is the
primary factor determining the degree of convection.
The present paper focuses on a multi decadal trend in the upper layers of the
Norwegian Sea and its dependence on atmospheric forcing. Related impacts on
deep-water formation in the Greenland Sea and increased Atlantic in#uence in the
Arctic Ocean are also discussed.
2. Data
Positions of sections and stations from which data are obtained are shown in Fig. 2.
These include time series of temperature and salinity at station Ln6 (67355N,
12340W) on the Icelandic Langanes standard section north east of Iceland (e.g.
Malmberg and Kristmannsson, 1992) and data obtained at station Kr6 (93W) on the
Icelandic Krossanes standard section along 653N. Data from May}June are applied
in the time series because this season has the highest observational regularity and
cover the period since 1971 at Ln6 and since 1974 at Kr6. Data from Russian sections
along 63300N (5S) and 65345N (6S) in the Norwegian Sea, occupied during surveys in
May/June since 1959, are also applied (Anon, 1997; Borovkov and Krysov, 1995). The
time series of temperature and salinity from Ocean Weather Station `Ma (OWS `Ma)
at 663N, 023E in the Norwegian Sea (e.g. Gammelsr+d et al., 1992) covers the period
since 1948. Data from a Faroese station in the Faroe Bank Channel are presented as
depth averages between 100 and 300 m. At this station observations have been obtained annually since 1982 while data from previous years are more irregular (Hansen
and Kristiansen, 1994). Data from the Scottish sections across the Faroe}Shetland
Channel, which have been worked since the turn of the century (Turrell et al., 1993),
are used to cover the variability in the Atlantic in#ow. The time series from the upper
Scottish slope (USS) represents the water of highest salinity at stations within the
200 m contour north of the Shetland Islands. Similarly, the time series from the
Faroese side of the channel represents the waters of highest salinity at stations within
the 200 m contour south of the Faroe Islands.
Further to the north, observations have been repeated regularly on three Norwegian standard sections across the Atlantic in#ow: the Svin+y Section between 62322N,
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
659
Fig. 2. Positions where the data were obtained.
05312E and 64344N on the prime meridian, the Gims+y Section between 68324N,
14304E and 70324N, 08312E o! the Lofoten Islands and the zonal S+rkapp Section
o! the southern tip of West Spitsbergen, which has been worked mainly along
76320N across the WSC since 1965. In the Svin+y and Gims+y sections the time series
are based on means of temperature and salinity, averaged between 50 and 200 m
across the core of the in#ow o! the shelf break (e.g. Blindheim and Loeng, 1981; Mork
and Blindheim, in press), while the values in the S+rkapp Section are 50}500 m means,
averaged over two stations at 93E and 113E. The time series from these sections are all
from July/August. Salinity throughout is quoted on the practical salinity scale and
temperature as in situ values.
Meteorological data was obtained from various sources. J.W. Hurrell kindly
provided updated data on the winter index of the North Atlantic Oscillation (NAO).
The NAO is a major atmospheric feature in the North Atlantic sector with large-scale
alternation of atmospheric mass, mainly between the Icelandic Low and the Azores
High. Hurrel's index is based on the gradient of the atmospheric mean sea level
pressure (MSLP) between Lisbon, Portugal, and Stykkisholmur, Iceland, averaged
over December through March (Hurrell, 1995). Data of mean atmospheric sea level
pressure at a number of stations in northwestern Europe have been applied to
evaluate mean winter wind conditions. Most of these stations cover the period
1890}1990 (Frich et al., 1996), while a few go further back in time (Schmith et al.,
660
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
Fig. 3. Time series of temperature and salinity, averaged over the depth intervals 0}50, 50}200 and
200}500 m, at stations Ln6 (67355N, 12340W) and Kr6 (653N, 93W).
1997). Gridded sea level pressure data provided by the Norwegian Meteorological
Institute are also employed (Eide et al., 1985).
3. Results
Fig. 3 shows time series of observations taken in May/June at the two Icelandic
stations Ln6 and Kr6, which are both located within the domain of the EIC (Fig. 2).
Trends of temperature and salinity averages in the depth intervals 0}50, 50}200 and
200}500 m are shown in the "gure.
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
661
Table 1
Trends in various depth intervals at stations Ln6 (67355N, 12340W), OWS `Ma (663N, 23E) and 5S
(63300N, 0305E}3330E) during the period 1971}1997, and at the station Kr6 (65300N, 9300W) during the
period 1974}1997. Deviations from the trend line during the salinity anomaly in 1977}1978
Ln6
0}50 m
50}200
200}500
Kr6
0}50 m
50}200
200}500
OWS `Ma
0}50 m
50}200
200}500
5S
0}50 m
0}200
200}500
Trend
temperature, 3C
Salinity
Anomaly
temperature, 3C
1997}1978
Salinity
!0.45
#0.17
!0.05
#0.02
!0.024
!0.023
}
}
}
!0.13
!0.045
}
!1.1
!0.45
!0.24
!0.1
!0.03
!0.05
!0.9
!0.85
}
!0.155
!0.075
}
!0.25
!0.5
!0.6
!0.01
!0.043
!0.046
!0.9
!0.6
}
!0.09
!0.08
!0.06
0
0
!0.7
!0.055
!0.025
!0.075
!0.9
!0.9
}
!0.09
!0.06
!0.075
Ln6 has shown considerable year-to-year temperature variations in the upper
200 m, often near 13C from one year to the next. The trend during the 1971}1997
period varied with depth, and in the upper 200 m layer variabilities in temperature
and salinity were not correlated. Hence, in the 0}50 m interval the temperatures
decreased by 0.453C while the salinity trend was toward higher values during the
period, although not by more than 0.02 units. Also between 50 and 200 m depth the
temperature and salinity trends were of opposite sign, with a temperature increase of
0.173C and salinities decreasing by 0.025. The 200}500 m interval showed a slight
decline in both temperature and salinity. See also Table 1, where the trends at Ln6 are
compiled along with similar data from Kr6, 5S and OWS `Ma. The trends for 5S and
OWS `Ma were in this case also worked out for the period 1971}1997, and at OWS
`Ma only observations taken in June were applied.
At Kr6 both temperature and salinity decreased in all the three depth intervals to
500 m. The temperature trends showed a decline ranging from 1.13C in the 0}50 m
layer to 0.243C in the 200}500 m interval, while the salinity decline also was largest in
the upper 50 m, amounting to 0.1, but at this station the decrease of 0.05 in the
200}500 m interval was somewhat larger than the decrease in the 50}200 m interval.
The two positions in the open Norwegian Sea, OWS `Ma and the Russian 5S
section (Fig. 4A and B), also showed a long-term decrease in both temperature and
salinity in the upper 500 m of the water column. At OWS `Ma the whole observational period since 1948 was characterised by the typical spectrum of interannual and
multiyear variability of varying time scales, but imposed on these there was a more
662
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
Fig. 4. (A) Time series of temperature and salinity, averaged over the depth intervals 50}200 and
200}500 m, at Ocean Weather Station `Ma (663N, 23E). 13-month moving averages are shown. (B) Time
series of temperature and salinity averages obtained from the Russian 5S section along 633N, averaged over
the depth intervals 0}200 and 200}500 m and over "ve stations between 0305E and 3330E. (C) Time series
of temperature and salinity in the central Faroe Bank Channel (FBC), averaged between 100 and 300 m
depth. (D) Time series of temperature and salinity over the upper Faroe slope (UFS). Values are presented
as anamolies as the mean seasonal cycle has been removed. The presented values are the result of a 24
month centered running mean, calculated at 6-month intervals. (E) Time series of temperature and salinity
over the upper Scottish slope (USS). Values are presented as in 4D.
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
663
long-term, multidecadal trend. This was indicated by high values in temperature and
salinity during the 1960s and early 1970s. Within this period the time of maxima was
dependent somewhat on depth, with a considerable delay below the Atlantic water
("sterhus et al., 1996a). In the upper 400 m the maximum occurred in the "rst half of
the 1960s, while it was observed at 500}800 m depth at the beginning of the 1970s.
After this maximum a cooling and freshening occurred at all depths to about 800 m,
and in 1995 both temperature and salinity in the 200}500 m interval were at the lowest
level since the station was established in 1948 (Fig. 4A).
Trends similar to those observed at OWS `Ma also appear in the Russian 5S
section along 633N, which has been observed regularly since 1959 (Borovkov and
Krysov, 1995; Anon, 1997). Time series of temperature and salinity from this section,
vertically averaged over 0}200 m and 200}500 m depth and horizontally between
3330E and 0305E, are shown in Fig. 4B. In the upper 200 m, both temperature and
salinity have generally increased since the late 1970s, but on average for the whole
period there has been a decrease. Other Russian sections in the southern and central
Norwegian Sea show similar trends (Borovkov and Krysov, 1995).
The largest shorter-term anomaly at Ln6 occurred during 1977}1978. This was due
to very low salinities in the surface layer during this period. In January 1977 the
salinities were near 34.45 in the upper 50 m. During the winter this developed into
a mixed layer of temperatures below !13C and salinities close to 34.6 to about 100 m
depth. In February 1978 a similar surface layer was observed. As indicated in Fig. 3,
this appears as a salinity anomaly to about 200 m depth in the time series of the
May}June observations. The 0}50 m salinity average deviated from the trend line in
the time series by 0.13 while the temperature was not similarly a!ected (see Table 1 for
details).
This anomaly also occurred in the upper 200 m at Kr6, actually more distinct than
at Ln6, and in this position it was also associated with a temperature anomaly.
Further east in the central Norwegian Sea the similar and contemporary anomaly,
which was evident at OWS `Ma as well as on the 5S section (Fig. 4, Table 1), is well
known as the GSA (Dickson et al., 1988; Gammelsr+d et al., 1992). The most evident
di!erence between the stations is its occurrence in the 200}500 m interval on the
stations in the Norwegian Sea while it was limited to about the upper 150 m in the
Iceland Sea. In the 200}500 m interval at Ln6, Kr6 and 5S there was a salinity
anomaly of similar magnitude in the early 1980s. Possibly this was the return of the
GSA via the EGC.
Fig. 4C shows temperatures and salinities, averaged between 100 and 300 m depth,
on the Faroese station in the centre of the Faroe Bank Channel. The water masses
on this station are mainly derived from the North Atlantic Current and #ow east
along the southern slope of the Faroe Bank Channel. In this position the GSA
was clearly the most dominant anomaly, and although the data before 1980 are
sparse, it seems clear that its amplitude was greater than its equivalents on both sides
of the Faroe}Shetland Channel somewhat further east (Fig. 4D and E). After
the GSA the salinity increased and generally exceeded 35.25 between 1985 and 1990,
but since then this water has freshened considerably again and become somewhat
cooler.
664
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
The waters on the upper Faroe slope (UFS, Fig. 4D) derive mainly from the Faroe
Current and re#ect the conditions in this branch of the in#ow. During the period since
1950 there has been an overall decline in both temperature and salinity, although the
salinity decrease of 0.08 is only about half of the decline in the 5S section. This is
mainly due to a larger increase in the salinity of the UFS water since the 1970s than in
5S salinities. The salinity anomaly in 1977 had, however, about the same magnitude as
in the 0}50 m layer in the Norwegian Sea.
The water over the upper Scottish slope (USS, Fig. 4E) derives mainly from the
Rockall Trough and enters the channel south of the Faroe Bank. While the overall
temperature trend during the period since 1950 has been negligible (Fig. 4E), there has
been considerable variability on shorter time scales. The di!erence between the
coldest anomaly in 1966 and the warmest in 1982 was about 1.33C, and the general
trend between 1966 and 1997 indicates a warming close to 0.53C. Also in the salinity of
this water the variability has been considerably larger than the trend since 1950, which
indicates an overall decrease of hardly 0.03. Since the GSA there has been a gradual
overall salinity increase, with the 1997 value approaching those of the 1960s.
In the Faroe}Shetland Channel, Arctic intermediate water is normally found below
the Atlantic water at a depth of 300}500 m, and it #uctuates in phase with the Atlantic
water with close similarity in magnitude and temporal scale (Dooley et al., 1984). In
this type of intermediate water the salinity has decreased by about 0.03 since 1975
(Turrell et al., 1997). In characteristics it agrees with what Stefansson (1962) de"ned as
North Icelandic Winter Water, because he found it was formed during winter by
convective mixing of the water masses on the North Icelandic shelf. As suggested by
Stefansson and later con"rmed by Malmberg (1969) and Meincke (1978), there may be
considerable variations from year to year in its properties. Further, it seems likely that
much of this water does not derive from the North Icelandic shelf but from the EIC o!
the shelf edge. It seems, therefore, adequate to refer to this water as `modi"ed East
Icelandic Watera (MEIW), as proposed by Read and Pollard (1992).
The salinity of Norwegian Sea Arctic Intermediate Water (NSAIW), below the
MEIW (see below), has also decreased by about 0.04 over the same period. A similar
trend in salinity has been observed in the Bottom water ('800 m) in the Channel,
although at approximately half the rate (Turrell et al., 1999).
Time series of averages covering the core of the Atlantic water o! the shelf break in
the Svin+y, Gims+y and S+rkapp Sections are shown in Fig. 5. A dominating feature is
the variability with time scales of a few years, but superimposed on this is a longerterm trend in all three sections, with a decreasing trend in salinity while the overall
temperature trend indicates a rise. The magnitude of the salinity trend is smallest on
the Gims+y Section, while the temperature rise is increasing northwards, and on the
S+rkapp Section, where the time series go back to 1967, the trend line increase is near
13C. In comparison, 5S in particular, but also UFS and OWS `Ma show positive
temperature trends since 1978, although their linear trends over the whole period
since the 1950s are negative. The USS is di!erent and shows no signi"cant temperature trend.
Before 1978, the Svin+y Section was also observed in 1958 and 1968. Fig. 6 shows
temperature/salinity relationships for stations o! the slope in repetitions of the Svin+y
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
665
Fig. 5. Time series of temperature and salinity, observed in July}August, in the core of Atlantic water in the
sections Svin+y } NW and Gims+y } NW, averaged between 50 and 200 m depth, and in the section
S+rkapp } W, averaged between 50 and 500 m. Time series of NAO winter index (December}March). The
curves for the S+rkapp section and the NAO also show 3-year running means of the annual values.
section in 1968, 1978, and 1996. The observations from 1968 (and from 1958) show
a gradual decrease from the relatively high salinity in the in#owing Atlantic water to
the properties of the Norwegian Sea Deep Water (NSDW) at greater depths. Although these sections were observed with reversing water bottles at standard depths,
there were many observations in the temperature range around 03C with salinities in
excess of 34.9. In sections from the 1980s and 1990s temperatures around 03C coincided with a salinity minimum and salinities below 34.9, as demonstrated in the ¹/S
relation from 1996 in Fig. 6. This salinity minimum, which has been observed over
666
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
Fig. 6. Temperature/salinity relationships obtained from stations o! the slope (depth'900 m) in Svin+y
Sections from the years 1968, 1978 and 1996.
most of the Norwegian Sea and in the Faroe}Shetland Channel, indicates the presence
of NSAIW (Blindheim, 1990; Hopkins, 1991; Martin, 1993; Turrell et al., 1999).
The ¹/S relationship from 1978 shows a drastic change from the 1968 section with
a considerable salinity minimum in the temperature range between about 0 and 53C.
This minimum a!ected the water column between 100 and 600 m depth while the
lowest salinities, below 34.8, occurred at 200}400 m depth with corresponding temperatures of 2}43C. These properties can typically be associated with MEIW. In 1978
there was a gradual salinity increase between this intermediate water and the NSDW
making NSAIW di$cult to detect. The ¹/S relationship from 1996 shows the presence
of both MEIW, which is indicated by a salinity minimum around 33C, and NSAIW,
which is clearly shown by salinities below 34.9 at around 03C. The volume of NSAIW
has expanded during the 1980s and 1990s (Blindheim, in prep) while the MEIW, as
also observed in the 5S sections, showed a maximum during 1976}1978.
4. Atmospheric forcing
Generally, the wind conditions seem to be a principal forcing component in this
system. There is a fairly good correlation between the wind stress curl over the
Greenland Sea and the amount of fresh water in the Norwegian Sea. The relationship
between annual values of wind stress curl over the Greenland Sea (Meincke et al.,
1992) and the 200}500 m salinity average in the 5S section 2}3 yr later has at a
correlation coe$cient of 0.66. This may be taken as a clear indication of the role of
prevailing winds as a forcing factor. Further the duration of this freshening trend in
the Norwegian Sea also suggests associations with the NAO and the possibility that
the wind forcing is not only local or regional, but may also have more far"eld forcing.
The intensity of the westerly winds over the North Atlantic is connected to this system
(Hurrell, 1995).
As demonstrated in Fig. 7, it also seems to be evident that the oceanographic
structure in the Nordic Seas is closely linked with this predominant wind system. The
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
667
Fig. 7. Upper curve: Three-year running means of winter NAO index (December}March), updated from
Hurrell (1995). Lower curve: Three-year running means of the westward extent of Atlantic water in the
Russian 6S section along 65345N. The extent is represented by the longitude of the maximum westward
extent of water with a salinity of 35 in the sections from the di!erent years. The negative values of the index
(ordinate) represent degrees west longitude.
upper curve in this "gure shows 3-yr running means of Hurrell's (1995) winter index of
the NAO. The bottom curve shows 3-yr running means of the westward extent of the
Atlantic water in the NwAC, represented by salinity in excess of 35.0. It is based on
observations in the Russian 6S standard section along 65345N (Fig. 2), which has
been worked regularly since 1963. The longitudinal position of the 35 isohaline is
related to the maximum salinity at each station, regardless of depth greater than 10 m,
and Fig. 7 shows 3-yr running means of this longitude. The E}W extent of the Atlantic
water #uctuates closely in phase with the winter index of the NAO, although the
former shows a 2}3-yr delay in relation to the NAO. The correlation between the 3-yr
running means since 1963 of the E}W extent of Atlantic water and 3-yr running means
of the winter NAO index, 2 or 3-yr delayed, is 0.86 and 0.84, respectively. Further, if
5-yr running means of the winter NAO index are applied, the correlation is between
0.90 and 0.92 for both a 2 and 3-yr delay. This indicates that by far most of the
variability in E}W extent of the Atlantic water is controlled by the NAO.
As seen in Fig. 7, the curve for the NAO-index showed a maximum in the late 1960s,
while a counterpart in the E}W curve was lacking. The reason for this mismatch is
668
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
found in the current structure. During the 1960s the westward extent of the Atlantic
water was at its largest, and its distribution to 7}83W reached the western slope of the
Norwegian Basin. Strictly, the most westward extending water of salinity in excess of
35 was not Atlantic water in the western branch of the NwAC. It was a water mass that
Read and Pollard (1992) de"ned as `Norwegian North Atlantic Watera (NNAW).
This water, which derives from the western branch of the NwAC, has entered into the
cyclonic basin circulation and is #owing southwards over the slope of the Iceland
Plateau and when following this circulation it has subducted under the upper layer
waters of the EIC. In agreement with this, it was found at 200}300 m depth during
these years, but in the later years, with more easterly distribution of the Atlantic water,
the salinity maximum was found shallower than 100 m. While these upper layer
waters of the NwAC are exposed to the wind forcing, the deeper circulation in the
basin is topographically controlled. The pathway of NNAW will therefore largely
follow the topography, but its salinity will exceed 35 only when the lateral extent of
the NwAC is large and its western branch is well developed.
These shifts in E}W extent are considerable, and the di!erence between the most
western distribution, to 7325W in 1968, and the most eastern, to the prime meridian in
1993, is about 330 km. A similar relation exists also in the 5S section, but this section
does not demonstrate the distribution of the Atlantic water so well, because the whole
section is within the extent of the Atlantic water during the periods of lowest NAO
index.
The e!ects of the NAO, or rather of the regional atmospheric circulation associated
with it, is observed far north. Hence, as seen in Fig. 5, the curve for the temperature
#uctuations in the S+rkapp Section is highly correlated with the curve for the NAO
winter index, with a correlation between their 3-yr running means of 0.8. While the
lateral extent of the NwAC #uctuates in phase with the NAO with a delay of 2}3 yr, it
is worth noting that there seems to be no delay between the variability in the NAO
winter index and the temperature #uctuations in the S+rkapp Section.
5. Discussion
The long-term decline in temperature and salinity in the time series presented above
indicates an ocean climate phenomenon of fairly large scale with considerable impact
on the oceanographic structure of the Nordic Seas, their salinity and heat balance, and
the ocean-atmosphere interactions in the area.
The gradual freshening revealed by the time series in the Norwegian Sea and the
Faroe}Shetland area is di!erent from the conditions in the North Icelandic shelf
waters, where a rather abrupt decrease in temperature and salinity occurred during
1964}1969 (Malmberg, 1969). Since then, conditions have been more variable than
earlier, and on average both temperature and salinity have remained lower than
before 1965 but without any long-term trend, although there has been a slight
tendency toward warmer and saltier conditions since 1970. Most likely, this change in
conditions during the latter half of the 1960s was due to increased transport in the
EGC and contemporary reduced transport of Atlantic water to the North Icelandic
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
669
Fig. 8. Time series of salinity anomalies on the upper Scottish slope (USS, bold curve), the upper Faroe
slope (UFS also entered in Fig. 4D) in the Faroe}Shetland Channel and at Ocean Weather Station `Ma,
50 m depth, from 1950 to 1995 (2-year running means).
shelf from the Irminger Current, both as an e!ect of the strengthened northerly winds
along East Greenland during this period when the high pressure over Greenland
peaked. Hence, most winter distributions (DJFM) of MSLP during the 1960s were in
favour of increased transport from the Arctic Ocean through the Fram Strait (Fig.
9A). Further, observations at OWS `Aa, at 623N, 333W in the Irminger Sea, indicated
a westward shift of the Irminger Current in response to the intensity of the prevailing
NE winds (Blindheim, 1968) and, accordingly, reduced supply of Atlantic water to the
North Icelandic branch of the Irminger Current. While these two e!ects pull in the
same direction on the rather small volume of the North Icelandic shelf waters, it is
reasonable that the response time will be short with rather abrupt changes from
Atlantic conditions to Arctic or Polar dominance in the North Icelandic waters. This
seems, however, to be limited to the shelf waters, since just o! the shelf on the same
section there has been a more gradual cooling while salinities have shown an overall
increase since 1970. This seems to be a rather widespread feature in the Iceland Sea,
since similar trends were seen in the 0}50 m means on Ln6 (Fig. 3). While the declining
temperatures may indicate increased supply of surface waters from the EGC, the
reason for the rise in salinity seems more uncertain, especially as it is in contrast to the
trends at intermediate layers as indicated by the 50}200 m interval on Ln6, which has
gradually become warmer and fresher. The reason for this seems to be the characteristics of the intermediate water, which via the EGC derives from the WSC, where
similar trends were observed (Fig. 5). Here we adopt the name `Iceland Arctic
Intermediate Watera (IArIW) from Hopkins (1991), while it also is known as `Arctic
670
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
671
IWa (Helland-Hansen and Nansen, 1909; Stefansson, 1962) and `lower IWa (Swift and
Aagaard, 1981). The warming and freshening of this intermediate water re#ect the
development of the properties in its source water. Probably its volume is reduced
because the WSC is pushed toward the slope by the prevailing wind conditions. While
the "rst e!ect seems obvious, there is no evidence for any volume reduction of the
IArIW, although it seems likely that it must be so. The cooling and freshening in
deeper layers may be in support of this. Hence, in the 200}500 m layer both on the
Siglunes Section, the Ln6 and the Kr6 the time series show a cooling and freshening.
The major heat and salt source in this depth layer is IArIW, and the cooling and
freshening may be due to a volume reduction of this water mass.
The salinity anomaly in the upper 100 m on LN6 and Kr6 in 1977/1978 is clearly
indicated also in the time series in the Norwegian Sea, but while it was observed only
in the upper 200 m at the Icelandic Stations, it clearly a!ected also the 200}500 m
layer at the stations in the Norwegian Sea. This is quite possible, since the density
spectrum in the upper layers at Ln6 agree well with the density values in the upper
500 m of 5S and OWS `Ma. By seasonal warming there will also be water above the
seasonal thermocline with density low enough to mix into the Atlantic waters in the
Norwegian Sea.
As demonstrated in Fig. 7, the NAO a!ects the water-mass distribution in the
Norwegian Sea in the manner that the longitudinal extent of the Atlantic water
#uctuates in phase with the variability in the NAO winter index. When the index is
high, the prevailing winds set up a wind stress that brings the Atlantic water toward
the slope and shelf, and the eastward extent of Arctic waters is increased. Accordingly,
the circulation of Atlantic waters into the interior of the deep basins is probably
reduced, and the upper and intermediate layers become colder and fresher, particularly in the western parts of the basins. For example, Rudels and Quadfasel (1991)
have estimated the net in#ow to the Arctic Ocean in the Fram Strait at about 1 Sv and
a recirculation of up to 2 Sv. Although the rate of recirculation further south in the
Norwegian and Lofoten basins is unknown, it may also represent considerable
volumes. An example of varying recirculation into the Norwegian Basin is the
NNAW, which was observed over the western slope of the Norwegian Basin during
the late 1960s and which has not been observed later. It may be argued that these high
salinities so far west, and also the east}west #uctuations in general, are associated with
salinity #uctuations in the in#owing Atlantic water. The correlation between the E}W
extent in the 6S Section and the salinity variations just o! the shelf break at 53E in the
same section is, however, !0.27. This indicates some connection between the overall
narrowing of the NwAC and the gradual salinity decline since the late 1960s, rather
than any correlation between #uctuations of shorter time scales. A result of reduced
recirculation may therefore be that more water of Atlantic origin reaches the Arctic
Ocean even if the Atlantic in#ow to the Nordic Seas remains constant. It is likely that
䉳&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
Fig. 9. Distribution of winter (December}March) mean sea level pressure. (A) Typical for situation with
a low winter NAO index (left side,1968}1969). (B) For a high NAO index situation (1989}1990). (C) For the
winter 1994}1995 (D) For the winter 1995}1996. Data from the Norwegian Meteorological Institute.
672
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
this is the reason for the volume increase and temperature rise in the Atlantic layer in
the Arctic Ocean that has attracted much recent attention (Quadfasel et al., 1991;
Carmack et al., 1995; Swift et al., 1997; Grotefendt et al., 1998; Dickson et al., in press).
It is also worth noting that there possibly was a similar temperature development in
the Atlantic layer during the period with increasing NAO winter index early in the
century (Schokalsky, 1936).
The e!ect of the NAO on the volume transport in the NwAC and the WSC is,
however, not clear. As seen above, an increased winter NAO index will cause a
narrower current, but this will not result in reduced transport if the current velocity is
increased accordingly. Any assumptions about this will remain somewhat speculative
as long as there are no con"rming data, although geostrophic estimates of the volume
transport of Atlantic water through the Svin+y Section in summer #uctuate in phase
with the NAO index (FMA) (Mork and Blindheim, in press) and Grotefendt et al.
(1998) also claim that the in#ow of waters from the NwAC must have increased. Fig.
5 shows, however, that the warming of the intermediate Atlantic layer in the Arctic
Ocean does not need to depend on increased supply of AW, since the NwAC has
become warmer since the late 1960s. The increased heat #ux may therefore be
su$cient to create the temperature rise. This warming is markedly stronger in the
northern part of the Norwegian Sea than in the in#ow area, and the correlation with
the NAO winter index increases northward, showing that the warming depends on the
atmospheric forcing. Simply, the narrowing of the current will result in reduced
cooling, since a smaller surface of Atlantic Water is exposed to the atmosphere. While
the overall warming in the S+rkapp Section since the early 1960s is close to 13C, the
amplitudes of the shorter term variability are about twice as large. The temperature
rise of 13C is about the same as the maximum warming observed in the Atlantic layer.
Furthermore, Swift et al. (1997) have observed temperature variability in the Arctic
Ocean in delayed phase with the variability in the S+rkapp Section.
It seems somewhat like a paradox that the narrowing NwAC has become fresher at
the same time as the salinity in the in#ow of both USS and UFS waters has been
increasing since the late 1970s. One important reason for this may be increased wind
driven supply of relatively fresh surface waters derived from the Arctic domain. A clear
indication of such e!ects is seen in the ¹/S relationship from 1978 in Fig. 6, where the
temperature interval between about #0.5 and 53C, corresponding to 150}600 m
depth, is characterised by unusually low salinities, which most likely derive from the
low upper layer salinities in the EIC as observed at Ln6 and Kr6 in 1977}1978.
Further, increased precipitation over northern Europe and the Nordic Seas, which
has occurred during the high NAO situations, may contribute to the freshening
(Dickson et al., in press; Hansen-Bauer and F+rland, 1998b). This may also be the
reason for the observed stability increase in the upper layers, which will contribute to
a reduction of the winter cooling. The increase of precipitation seems, however, to be
of less importance than the admixture of Arctic waters from the west. Hence the
salinities at "xed stations in the Norwegian Coastal Current, which should be more
a!ected by precipitation and runo! than the NwAC, are more closely correlated with
the salinities at OWS `Ma than with the regional precipitation records as described by
Hansen-Bauer and F+rland (1998a).
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
673
Fig. 10. Normalised gradients of winter MSLP (December}March) between Danmarkshavn, 76346N,
18346W, and Svalbard, 78315N, 15328E (upper curve), and between Scoresbysund, 70329N, 22300W, and
Jan Mayen, 70356N, 08340W (lower curve). Bold curves are 3-yr running means. Year is referred to
January in each winter mean. Data from the Danish and Norwegian Meteorological Institutes.
In contrast to the strong signal of Arctic in#uence in 1997}1998, the period with
most severe in#uence of Arctic and Polar water north of Iceland during the late 1960s
was not re#ected in the Svin+y section. In the ¹/S relationship for 1968 there was no
signal of any salinity minima typical for the intermediate waters, as the properties
in the water column changed gradually with depth from the characteristics of the
Atlantic water to those of the NSDW. Similar conditions were observed throughout
the 1960s in the Russian 5S series as well as in the Svin+y section from 1958. By then
the NAO was still in its lowest period, and as seen in Fig. 7, the westward extent of the
Atlantic water along 65345N was at its largest. The wind forcing was, however, in
favour of a strong EGC and increased transport through the Denmark Strait (Fig. 9a).
Some support of this is given by Meincke et al. (1992), who considered wind stress curl
over the Greenland Sea in relation to variability of convective conditions. They
claimed that the increased freshwater transport from the Arctic Ocean during the late
1960s did not immediately a!ect the convective conditions in the Greenland Sea when
passing en route to the Iceland Sea because little freshwater entered the circulation in
the Greenland Basin. The intense and persistent high-pressure cell over Greenland in
the 1960s is also mentioned as a plausible forcing of the GSA (e.g. Rodewald, 1967;
Dickson et al., 1988). The relative strength of this high pressure cell is indicated by the
gradient of winter MSLP between Scoresbysund, East Greenland, and Jan Mayen.
The years 1966}1969 were the longest period with a pressure gradient about 1 standard deviation above the average for all observations since 1924 (Fig. 10).
674
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
During the period when the NAO increased from its minimum in the 1960s, the
in#uence of the Arctic waters increased considerably in the Norwegian Basin. The "rst
strong signal of increasing Arctic in#uence was the supply of relatively light MEIW,
which was associated with the salinity anomaly on Ln6 and Kr6 in 1977}1978. As
already described, this mixed into the upper and intermediate layers in the Norwegian
Sea.
In the Norwegian Sea this anomaly has been associated with the GSA, and the
general similarities in salinity variation between the open Norwegian Sea and the two
Atlantic time series USS and UFS (Fig. 4) indicate that part of the salinity de"cit in the
Norwegian Sea derives from a de"cit in the in#owing Atlantic water. This applies both
to the long-term salinity decrease and to the variability on shorter time scales, not
least the GSA. Support of this is also indicated by the long-term #uctuations in the
upper layers of the North Atlantic, which are well in phase with those observed in the
USS time series since the beginning of the century (e.g. Smed, 1965; Kushnir, 1994).
The advective hypothesis for the propagation of the GSA proposed by Dickson et al.
(1988) is also supported by other authors (Ellett, 1978,1982; Reverdin et al., 1997;
Belkin et al., 1998).
On the other hand, however, variability in the Atlantic water salinity can explain
only part of the observations in the Faroe}Shetland Channel and the Norwegian Sea.
It cannot by itself explain the correlation between the variability of surface water
salinity and similar variability in the intermediate water in the Faroe}Shetland
Channel, which is described by Dooley et al. (1984), and neither can it explain the fact
that the salinity de"cits are larger in the UFS water and in the open Norwegian Sea
than in the in#owing USS water. This is demonstrated in Fig. 8, which shows
24-month running means of salinity in the waters on the USS, UFS (as in Fig. 4D and
E) and at 50 m depth at OWS `Ma. The "gure clearly shows that the amplitudes of the
variability are larger in the UFS water than in the USS water, and further, there is less
di!erence between OWS `Ma and the UFS time series than between the latter and
USS water. Also, the 5S time series (Fig. 4B) shows amplitudes more similar to those
from the UFS and OWS `Ma than to those of the USS water. Therefore, all these
observations indicate that the variability in the open Norwegian Sea as well as around
the Faroes, is re#ecting varying in#uence from the EIC rather than #uctuations in the
Atlantic in#ow. This coincided, however, with the time when the GSA arrived in the
Faroe}Shetland Channel and propagated through the Norwegian Sea. It is therefore
likely that the GSA was reinforced by the increased supply of Arctic water from the
EIC when it arrived in the Faroe}Shetland area.
The correlation between the E}W variability of the Atlantic water in the NwAC and
the NAO is better than the correlation to more local MSLP gradients, for example,
the gradient between Ona lighthouse at 62.873N on the Norwegian coast and OWS
`Ma. This may be interpreted in the way that much of the e!ects of the wind forcing
occurs south of the Nordic Seas or in the southern areas of the Norwegian Sea. The
charts of winter MSLP in Fig. 9 show that when the NAO is high, the strongest
gradients occur in the Faroe}Shetland area and adjacent parts of the northeast
Atlantic toward the southwest. The oceanographic feature that will be most exposed
to this wind forcing is the Faroe Current. This is in support of the suggestion put
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
675
forward by Hansen and Kristiansen (1994) of a mechanism behind a dynamical
anomaly in terms of a changed balance between the EIC and the Faroe Current. They
thought of a structure of wind forcing, which may favour increased transport in the
EIC and at the same time reduce the #ow that feeds the Faroe Current. This is in
agreement with the e!ect of the NAO on the Atlantic In#ow as observed in the
Russian 6S section.
It is therefore quite likely that the reduced Atlantic in#ow north of the Faroes that
has been observed during the later years is associated with the recent high NAO index.
It would also result in a greater admixture of MNAW to the #ow into the Faroe}
Shetland Channel. Also in the northeastern North Atlantic the high NAO index may
have given rise to an eastward shift of the North Atlantic Current, allowing easier
access to the Norwegian Sea of waters from the Sub Arctic Gyre as proposed by
Dooley et al. (1984). While an assessment of the magnitude of such a shift will remain
speculative due to lack of data, it is worth noting that the eastward shift of the Atlantic
Water along 65345N was more than 300 km from 1968 to 1976 (Fig. 7). It is therefore
not unreasonable to conclude that the eastward shift would be larger west of Britain,
where the SLP gradients were stronger. Support of this is also given by Belkin and
Levitus (1996), who described meridional displacements up to 300 km of the `North
Subarctic Fronta near the Charlie}Gibbs Fracture Zone.
The salinities in the Atlantic in#ow, and in the UFS water also the temperatures,
have been increasing since the 1970s. Similarly, there are increasing salinities both in
the Irminger Current (Malmberg, 1998) and in the West Greenland waters during the
same period (Buch, in press). The reason for this may be reduced supply of fresh water
to the Sub Arctic Gyre through the Denmark Strait since the early 1970s. Instead,
much of the fresh water in the EGC circulated via the EIC and the Jan Mayen Current
into the Nordic Seas and back to the Arctic Ocean. During the late 1950s and 1960s
the situation was di!erent, with a large fresh water #ux through the Denmark
Strait and declining salinities in the Sub Arctic Gyre. This may be suggestive
of a simple model with a large fresh water #ux through the Denmark Strait during
periods with low NAO index and reduced #ux in high index situations. This seems,
however, not to be valid for earlier salinity variability in the Faroe}Shetland
Channel in relation to the NAO. It must further be admitted that the MSLP maps for
high NAO situations, for example Fig. 9B and C, also show pressure gradients across
the EGC that should constrain it toward the Greenland coast. It seems therefore likely
that the mechanism behind the response to the NAO is not fully understood.
Important details may, for instance, be lost in averaging when maps of the winter
situations are prepared. Decadal variations in cyclone counts (Serreze et al., 1997), in
low-pressure tracks and migration speed may also be of importance. Neither is it
readily understood why the variability in the lateral extent of the NwAC is 2}3 yr
delayed in relation to the NAO while there are seasonal variations in the extent of
NSAIW in the southeastern Norwegian Sea (Mork and Blindheim, in press) and the
temperature #uctuations in the S+rkapp Section show no, or at least far less,
delay.
It is further indicated in Fig. 6 that there are two modes of intermediate water,
MEIW and NSAIW. While there was a maximum of MEIW during the second half of
676
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
the 1970s, NSAIW remained an insigni"cant feature until about 1980. A section
across the Norwegian Sea between 663N o! the Norwegian coast and Jan Mayen in
1967 (not shown) revealed no signs of NSAIW, while an almost identical section in
1983 showed a clear salinity minimum in the temperature interval around 03C across
the whole basin. All later observations show this layer of NSAIW. While the dominance of MEIW in the 1970s possibly was a result of the considerable increase in the
NAO winter index during the early 1970s, it seems likely that the occurrence of the
NSAIW was an indirect result of the same wind forcing. Its appearance in quantity
some years after the considerable reduction or even cessation of the deep-water
formation in the Greenland Sea in the beginning of the 1970s (e.g. BoK nisch et al., 1997)
is suggestive of its source in this process. When the product of the winter convection in
the Greenland Sea gyre is no longer dense enough to form deep water, it may form a
water mass that sinks to the top of the older deep water and spreads isopycnally according to its density. This signal is also seen in the upper deep water in the Norwegian
Basin, where the transition layer is deepening ("sterhus et al., 1996b). It indicates that
the top of the NSDW is gradually eroded by the increased intermixing of the Arctic
water from above, which is possibly of somewhat varying density from one year to the
next. It seems further evident that increased supply of NSAIW also causes the
declining salinities in intermediate and deeper layers in the Faroe}Shetland Channel
(Turrell et al., 1999).
As demonstrated in Fig. 9, the wind forcing in the Greenland Sea is di!erent from
the wind conditions further south. There are, however, variations in phase with the
NAO also in this area. Hence, the depth of the winter convection in the Greenland Sea
also seems to depend on the wind forcing. At least this seemed clear during some
years in the 1990s, when the upper layer salinity in the central Greenland Basin
varied in phase with the MSLP gradients Scorsbysund}Jan Mayen and Danmarkshavn}Svalbard. Hence, there was a gradual increase in surface layer salinity in the
central Greenland Sea from about 34.6 in November 1991 to almost 34.9 in November
1995 (unpublished data). With these high surface layer salinities, the Greenland Sea
was poised for new deep-water formation, but during the winter of 1996 (the year of
January) there was a large supply of ice and low-salinity Polar surface water,
which brought surface salinities back to about 34.6 the following summer (F. Rey,
personal communication). Fig. 10 shows that while the MSLP gradient across the
EGC gradually increased during the period 1986}1995, there was a drastic fall
between the winters of 1995 and 1996. It is worth noting that this coincided
with a similar change in the NAO winter index. Vinje et al. (1998) have monitored
the ice #ux through the Fram Strait during the years 1990}1996, and their
annual volume #ux values are in good agreement with the MSLP gradients in Fig. 10.
While the largest #ux was observed in 1995, it fell to a low value in 1996. As
seen in Fig. 9C and D, this was associated with a great change between the two winters
in the MSLP distribution over the Nordic Seas. In contrast to the previous years,
there was no strong pressure gradient across the EGC during the winter of 1996. This
shows that the supply of ice and Polar surface waters from the EGC to the central
Greenland Sea is more dependent on the wind forcing than on the volume #ux in the
EGC.
J. Blindheim et al. / Deep-Sea Research I 47 (2000) 655}680
677
6. Conclusions
1. The structure of the water-mass distribution in the Nordic Seas has gradually
changed since the 1960s. In particular, this is manifested by the development of a
layer of Arctic intermediate waters, deriving from the Greenland and Iceland Seas
and spreading over the entire Norwegian Sea, and a freshening of the Atlantic
waters above. In the Norwegian Basin it has resulted in an eastward shift of the
Arctic front and, accordingly, an upper layer cooling in wide areas due to increased
Arctic in#uence.
2. The forcing behind this shift is the large-scale wind pattern, principally the North
Atlantic Oscillation. Hence, there is a close correlation between the lateral east}
west extent of the Norwegian Atlantic Current and the NAO winter index, the
di!erence between its broadest extent in 1968 and its narrowest in 1993 being more
than 300 km.
3. The wind-induced eastward advection of Arctic waters seems to be the principal
cause of the freshening, even in the main core of Atlantic in#ow near the edge of the
Norwegian shelf as well as in the shelf waters. The considerable regional increase of
precipitation, also associated with the NAO, seems to have some, but far less, e!ect.
Acknowledgements
We are indebted to Karen Gjertsen for preparing the illustrations and to Kjell Arne
Mork, who prepared the winter MSLP maps from the hind cast data, which are
provided by the Norwegian Meteorological Institute, Oslo. NAO data were kindly
supplied by J. Hurrell, National Center for Atmospheric Research, Boulder, CO,
USA. We appreciate the comments of the reviewers, which contributed most usefully
to the "nal revision of the paper.
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