to play in the processes of global change. Polar
amplification and feedback are recurrent themes in
numerical climate modelling (Manabe and Stouffer, 1993). However, we have hitherto lacked
many of the ocean time series needed to demonstrate the basic features of long-term hydrographic
change within the Arctic Ocean.
This situation improved dramatically during the
1990s. First, there was a major increase in the
ship-based ocean-observing effort, contributed
both by surface ships (Polarstern and Oden in
1987 and 1991, the first US/Canadian TransArctic Section in the summer of 1994 aboard
Polar Sea and Louis St Laurent, and three further
Polarstern cruises in 1993, 1995 and 1996 were
highlights), and by the almost-annual submarine
surveys of the US SCICEX Program (1993–99). Second, the release of a vast military archive of ocean
data supplied the improved ocean ‘climatology’
(Environmental Working Group, 1997) against
which the new data sets might be compared for
evidence of change. And third, whether we use the
newly described Arctic Oscillation (AO; Thompson
and Wallace, 1998) or the North Atlantic Oscillation (NAO; Dickson et al., 2000) as the more
appropriate index of Arctic climate, it seems clear
that winter climatic forcing over the Arctic and its
subjacent seas was at a century-long extreme state
during the 1990s (Walsh et al., 1996). This meant
that when data and climatology were compared,
the changes identified were spectacular and of a
large-enough amplitude to be traceable through
a gappy observing system and stand out against a
still-shaky climatology. Three main changes in
particular have characterized the WOCE period of
the 1990s.
The first was a more intense and more widespread influence of Atlantic water than previously
observed, with a warming and spreading of the
Atlantic-derived sublayer across the Eurasian Basin
(Quadfasel et al., 1991; Carmack et al., 1995,
1997; Aagaard et al., 1996; Swift et al., 1997;
Morison et al., 1998a,b, 2001; see also Kolatschek
et al., 1996; McLaughlan et al., 1996). As the winter NAO index increased from its lowest values on
record (generally) in the 1960s to its highest
recorded values during the early 1990s (see Fig.
7.3.2a), the increasingly anomalous southerly airflow that accompanies such a change over Nordic
seas is held responsible for a progressive warming
and perhaps a strengthening of the two streams of
Atlantic water that enter the Arctic Ocean, across
the Barents Sea shelf and along the Arctic Slope
west of Spitsbergen. By the late 1980s–early
1990s, when the NAO reached its interannual and
interdecadal maximum, the superposition of a
short-term warming event on the long-term warming trend meant that both Atlantic-inflow streams
were running between 1 and 2°C warmer than
normal (Grotefendt et al., 1998; Dickson et al.,
2000). Adlandsvik’s barotropic transport model
(Adlandsvik, 1989; Adlandsvik and Loeng, 1991;
Loeng et al., 1997) suggests that the transport
through the Barents Sea pathway may have
increased by around a quarter at this time, and we
have possible proxy evidence from sea-level
records to suggest that the West Spitsbergen Current was similarly boosted. From an annual standard section across the West Spitsbergen Current at
76°20ЈN, the warming of the inflow through Fram
Strait is seen to be accompanied by a progressive
freshening and decrease in density in the upper
500 m since the 1960s, and the indications are that
freshening also affected the Barents Sea branch of
the Atlantic Current (Dickson et al., 2000). Since
the increasing NAO is associated both with a
major increase in precipitation along the length of
the Norwegian Atlantic Current (⌬PϷ;15 cm per
winter in positive NAO years compared with
negative NAO years), and with decreased winter
production of sea ice (Deser et al., 2000), a broadscale freshening throughout the Atlantic water
domain is not unexpected. Entering the Eurasian
Basin, Morison’s comparison of SCICEX data
with ‘climatology’ (Fig. 7.3.9, from Morison et al.,
2001; see Plate 7.3.9, p. 588) showed that the
Atlantic sublayer had shoaled and warmed by up
to 2°C and extended in distribution by about 20%
(Morison et al., 1998b), so that the mutual front
between waters of Pacific and Atlantic origin had
shifted from the Lomonosov to the Alpha–
Mendeleev Ridge.
Accompanying this change at shallower depths,
Steele and Boyd (1998) reveal that the cold halocline layer, which acts to insulate the sea ice from
the warm Atlantic layer below, had dwindled away
in the Eurasian Basin, with profound effects on the
surface energy and mass balance of sea ice in that
region. Hydrography, tracers and modelling all suggest that this change stemmed from the eastward
diversion of Russian river input in response to the
altered atmospheric circulation (see Maslowski’s
SECTION 7 INSIGHTS FOR THE FUTURE
572
amplification and feedback are recurrent themes in
numerical climate modelling (Manabe and Stouffer, 1993). However, we have hitherto lacked
many of the ocean time series needed to demonstrate the basic features of long-term hydrographic
change within the Arctic Ocean.
This situation improved dramatically during the
1990s. First, there was a major increase in the
ship-based ocean-observing effort, contributed
both by surface ships (Polarstern and Oden in
1987 and 1991, the first US/Canadian TransArctic Section in the summer of 1994 aboard
Polar Sea and Louis St Laurent, and three further
Polarstern cruises in 1993, 1995 and 1996 were
highlights), and by the almost-annual submarine
surveys of the US SCICEX Program (1993–99). Second, the release of a vast military archive of ocean
data supplied the improved ocean ‘climatology’
(Environmental Working Group, 1997) against
which the new data sets might be compared for
evidence of change. And third, whether we use the
newly described Arctic Oscillation (AO; Thompson
and Wallace, 1998) or the North Atlantic Oscillation (NAO; Dickson et al., 2000) as the more
appropriate index of Arctic climate, it seems clear
that winter climatic forcing over the Arctic and its
subjacent seas was at a century-long extreme state
during the 1990s (Walsh et al., 1996). This meant
that when data and climatology were compared,
the changes identified were spectacular and of a
large-enough amplitude to be traceable through
a gappy observing system and stand out against a
still-shaky climatology. Three main changes in
particular have characterized the WOCE period of
the 1990s.
The first was a more intense and more widespread influence of Atlantic water than previously
observed, with a warming and spreading of the
Atlantic-derived sublayer across the Eurasian Basin
(Quadfasel et al., 1991; Carmack et al., 1995,
1997; Aagaard et al., 1996; Swift et al., 1997;
Morison et al., 1998a,b, 2001; see also Kolatschek
et al., 1996; McLaughlan et al., 1996). As the winter NAO index increased from its lowest values on
record (generally) in the 1960s to its highest
recorded values during the early 1990s (see Fig.
7.3.2a), the increasingly anomalous southerly airflow that accompanies such a change over Nordic
seas is held responsible for a progressive warming
and perhaps a strengthening of the two streams of
Atlantic water that enter the Arctic Ocean, across
the Barents Sea shelf and along the Arctic Slope
west of Spitsbergen. By the late 1980s–early
1990s, when the NAO reached its interannual and
interdecadal maximum, the superposition of a
short-term warming event on the long-term warming trend meant that both Atlantic-inflow streams
were running between 1 and 2°C warmer than
normal (Grotefendt et al., 1998; Dickson et al.,
2000). Adlandsvik’s barotropic transport model
(Adlandsvik, 1989; Adlandsvik and Loeng, 1991;
Loeng et al., 1997) suggests that the transport
through the Barents Sea pathway may have
increased by around a quarter at this time, and we
have possible proxy evidence from sea-level
records to suggest that the West Spitsbergen Current was similarly boosted. From an annual standard section across the West Spitsbergen Current at
76°20ЈN, the warming of the inflow through Fram
Strait is seen to be accompanied by a progressive
freshening and decrease in density in the upper
500 m since the 1960s, and the indications are that
freshening also affected the Barents Sea branch of
the Atlantic Current (Dickson et al., 2000). Since
the increasing NAO is associated both with a
major increase in precipitation along the length of
the Norwegian Atlantic Current (⌬PϷ;15 cm per
winter in positive NAO years compared with
negative NAO years), and with decreased winter
production of sea ice (Deser et al., 2000), a broadscale freshening throughout the Atlantic water
domain is not unexpected. Entering the Eurasian
Basin, Morison’s comparison of SCICEX data
with ‘climatology’ (Fig. 7.3.9, from Morison et al.,
2001; see Plate 7.3.9, p. 588) showed that the
Atlantic sublayer had shoaled and warmed by up
to 2°C and extended in distribution by about 20%
(Morison et al., 1998b), so that the mutual front
between waters of Pacific and Atlantic origin had
shifted from the Lomonosov to the Alpha–
Mendeleev Ridge.
Accompanying this change at shallower depths,
Steele and Boyd (1998) reveal that the cold halocline layer, which acts to insulate the sea ice from
the warm Atlantic layer below, had dwindled away
in the Eurasian Basin, with profound effects on the
surface energy and mass balance of sea ice in that
region. Hydrography, tracers and modelling all suggest that this change stemmed from the eastward
diversion of Russian river input in response to the
altered atmospheric circulation (see Maslowski’s
SECTION 7 INSIGHTS FOR THE FUTURE
572