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There is an increasing awareness of how oceanic
teleconnections link phenomena in distant regions to each
other. This was first described for the Pacific Ocean,
where the strong events related to El Niņo/Southern
Oscillation (ENSO) have been shown to generate secondary
anomalies that propagate over thousands of
kilometers. Using results from the Naval Research
Laboratory (NRL) Layered Ocean Model (NLOM) in conjuncture
with observations, such signal propagation has been
detected across the central Pacific [Jacobs et al. 1994]
and along the American Pacific coastline of the North East
Pacific Ocean and the Gulf of Alaska [Melsom et al., 1999,
2000].
Although the Atlantic Ocean lacks a momentous signal such
as El Niņo a considerable variability in the equatorial
Atlantic Ocean has been detected. This has been attributed
to the Gulf of Guinea upwelling (the Benguela Niņo, or the
Atlantic El Niņo) [Adamec and O'Brien, 1978; Zebiak, 1993]
and the tropical Atlantic dipole [Chang et al., 1997]. The
existence of the dipole has, however, been shown to be the
exception rather than the rule [Enfield et al.,
1999]. Penland and Matrosova [1998] have shown that the
influence of the Pacific ENSO enhances the short-term
predictability of the north tropical Atlantic SST for
lead-time up to 18 months. Sutton and Allen [1997]
speculate that the variability in the equatorial Atlantic
Ocean is related to anomalous events in the Gulf Stream
system and the North Atlantic current.
A large number of studies of the North Atlantic Ocean and
the Nordic Seas has been based on the North Atlantic
Oscillation (NAO) index [Hurrell, 1995]. Kushnir [1994]
demonstrated that winter SSTs in the subtropical and
subpolar gyres were warmer during a 15 year period of low
NAO index (1950-1964) than during a subsequent 15 year
period of high index values (1970-84). In relation to
this, there is increasing evidence for the presence of of
teleconnections by an oceanic pathway in the Atlantic
Ocean: Hansen and Bezdek [1996] and Sutton and Allen
[1997] have shown that Kushnir's warm and cold periods
involved oceanic teleconnections by propagating warm and
cold SST anomaly patches. These anomalies migrate across
the North Atlantic Ocean from North America to the
northwest of Scotland, following the strong SST gradients
that are associated with the Gulf Stream and the North
Atlantic Current.
During the last decades, much attention has been focused
on the northward transport of Atlantic Water (AW) into the
Nordic Seas. This is because the heat transport associated
with the AW inflow to the Norwegian Sea and its extension
northward is an important factor for climate and
biological production in Northern Europe. Most of the
calculations of the AW inflow are based on indirect
methods where currents are inferred from hydrographic
observations, sea surface height from satellite altimetry,
numerical models and box models of energy and mass
exchanges.
Hydrographic observations in the Faroe-Shetland Channel
and the Norwegian Sea reveal the AW inflow as a warm and
saline, wedge-shaped current. Based on year-long direct
current measurements on the continental slope north-west
of Shetland, Gould et al. [1985] calculated the mean
inflow to be 7.5 Sv
(1 Sv=106 m3s-1).
They indicated a seasonality with a maximum in
winter. Investigations using satellite altimetry to obtain
estimates of the inflow by Pistek and Johnsen [1992] and
Samuel et al. [1994] showed annual mean transports of
2.9 Sv and 2.7 Sv. They both showed a strong
annual cycle with summer to winter variations of about 100
percent. Monthly geostrophical calculations by Blindheim
[1993] for 1990 ranged from a winter maximum of
7.9 Sv to a summer minimum of 2.9 Sv, with an
annual mean of 5.5 Sv.
From year-long direct current measurements in the Svinøy
section, Orvik and Mork [1996] estimated the annual mean
transport to be 5.3 Sv, but this study did not cover
the whole width of the AW inflow. The estimates showed no
systematic seasonal signal and disputed the widely held
view of an annual cycle. However, the lack of significant
seasonality corresponded well with Dickson et al. [1990],
who from direct current measurements, found little
seasonal and inter-annual variation in the overflow from
the Nordic Seas. In a recent analysis of observations from
a variety of instruments, Orvik et al. [2000] has found
that the AW inflow can be separated into an easterly and a
westerly branch, and that the transport exhibits a high
degree of variability on virtually all time scales.
The main features of the circulation system within the
Nordic Seas are a northward flow of warm water on the
eastern side and a cold current flowing southward on the
western side. North of Jan Mayen the front between these
water masses is topographically controlled with small
fluctuations in its position. Between Iceland and Jan
Mayen variation in the volume of Arctic waters carried by
the East Icelandic Current (EIC) may result in relatively
large shifts of this front. The East Greenland Current
(EGC) forms the cold flow to the west. In its mixed layer,
the EGC carries water of low salinity, and ice, from the
Arctic Ocean.
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Waters of an EGC origin may affect the distribution of
water masses in the Norwegian Sea in two ways. This may
either take place by transport of Arctic Water (mainly in
the EIC) directly into the Norwegian Basin, or indirectly
through the Subpolar Gyre in the North Atlantic by Arctic
influence of the Atlantic water which later flows into the
Nordic Seas. The "Great Salinity Anomaly" (GSA) is an
example of the latter case [Dickson et al., 1988]. From
the 1950s to the 1970s the temperature and salinity
decreased in the upper layers in the western and central
Norwegian Sea until the minimum values that define the GSA
were reached. This development was mainly due to increased
fresh water supply from the EIC [Blindheim et al.,
2000]. Since the time of the GSA, the salinity has
rebounded. Similar fluctuations have occured in the Kola
section, and also in the subpolar gyre of the North
Atlantic. Temperature trends that show cooling during the
1970s and 1980s are documented by several authors
[e.g. Kushnir, 1994; Reverdin et al. 1997]. These works
also show a relationship between interannual fluctuations
of SST and surface wind conditions. The Nordic Seas have
open connections with the subpolar gyre and, indeed,
exhibit similar fluctuations.
The wind forcing plays an important role for the
variability of the water mass properties. Blindheim et
al. [2000] found a strong correlation between the winter
NAO index [Hurrell, 1995] and the lateral extent of AW in
the Norwegian Sea. In association with the increased NAO
index during the last 30 years, the western extent of AW
has retreated eastwards in the Nordic Seas. This has
resulted in a reduction of the AW re-circulation within
the Nordic Seas, while re-circulation of Arctic Water and
Polar Water has increased [Blindheim et al., 2000]. This
affects the distribution of water masses in a much wider
sense, by a reduction of deep water formation and an
increased flux of AW into the Arctic Ocean. Also, Mork and
Blindheim [2000] showed that there are large interannual
variations in depth of the 3oC isotherm in the central
Norwegian Sea and that this variability is influenced by
the winter NAO index.
Recent analysis of the ice cover in the Arctic Ocean has
established that significant changes have occurred in the
latter part of the last century. Based on analysis of
upward looking sonar observations from US nuclear
submarines it has been shown that the average ice
thickness has decreased by 1.3 m, from 3.1 m in the
1958-1976 period to 1,8 m in the 1990īs, in average 4 cm
per year, or 40% of the total ice volume [Rothrock et al.,
1999]. Furthermore, analysis of microwave satellite
observations has established that the total area has
decreased by 6% over the last two decades (1978-1998)
[Johannessen et al., 1995; Bjørgo et al., 1997; Cavalieri
et al., 1997], while the multi-year ice area has decreased
14% over the same period [Johannessen et al., 1999].
Comparison of observations (in situ and satellites) since
1900 with trends seen in two coarse resolution global
climate models, forced by observed greenhouse gases and
tropospheric sulphate aerosols correlates very well. This
is "suggesting strongly" [Vinnikov et al., 1999] that the
observed decrease in sea ice extent since 1950 is related
to the antrophogenic global warming. Prediction by these
two coarse resolution global climate models suggests
furthermore a substantial decrease of the ice extent in
this century [Vinnikov et al., 1999]. However, the
strengthened indices of the Arctic Oscillation (AO) and
the NAO indices [Thompson and Wallace, 1998; Hurrell,
1995] (the correlation between these two indices are very
high suggesting that they are parts of the same system)
has pumped warm air and water masses into the Arctic Ocean
from the North Atlantic Ocean through the Norwegian Sea,
causing ice melting as well as a mechanism for exporting
multi-year ice through the Fram Strait [Rothrock et al.,
1999]. Even some effects on the variability of the sea ice
from El Ninõ has been reported [Gloersen et al., 1995].
Extrapolating the ice thickness decrease of 4 cm/year
[Rothrock et al., 1999] indicates that the Arctic Ocean
could be ice free 50 years from now, causing a dramatic
change in the albedo, with significant effects on the
global climate system. However, we should also be aware
that Russian ice thickness estimates based on dispersion
relationships between the damping of swell propagation
into the Arctic Ocean measured from the North Pole
Stations during the period 1972-1991 [Johannessen et al.,
1999; Nagurnyi et al., 1994, 1999] indicates an average of
0.5- 1.0 cm decrease per year. This is 4-8 times less than
the results form the nuclear submarine data.
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