the South Pacific subtropical gyre and water from
the Southern Ocean and its weak signal is therefore to be expected.
7.3.9.3 Discussion
The apparently large scales of the warming of
SAMW in the South Pacific and Indian oceans,
and the freshening of NPIW and AAIW across the
North and South Pacific and the Indian oceans,
suggest that these changes are not random, but
rather a systematic modification of the shallow
thermocline circulation. The warming of SAMW
is largely consistent with the observed warming of
surface waters over the last thirty years of up to
0.5°C (Bottomley et al., 1990) in its source
regions.
The evidence for freshening of the surface ocean
at high latitudes does not exist in the observational
database, but models of climate change due to
increasing greenhouse gases (Manabe and Stouffer,
1994; Gordon and O’Farrell, 1997) all predict
increased precipitation–evaporation poleward of
50°. The pattern of change suggested by these
models is broadly consistent with the observed
changes in SAMW, AAIW and NPIW properties.
An increase in rainfall at high latitudes would
suggest a need for increased evaporation at lower
latitudes so that the overall salt content of the
oceans is conserved. There is some evidence that
the observed high-latitude freshening is partially
balanced (up to 60%) by increased salinity of shallower central waters on the 24°N, 10°N and 17°S
sections (Wong, 1999; Wong et al., 1999, 2001).
These observations suggest that it may be possible to verify the results from coupled ocean–
atmosphere models and in consequence to begin to
test quantitatively detection and attribution methods for evidence of climate change in the oceans.
7.3.10 Summary and Conclusions
In answering the question ‘How was the World
during WOCE?’ two conclusions were perhaps
inevitable: that we would be unable to form an
opinion for some sea areas and depths that don’t
have much of a past observational record; and that
for those areas that do, we would discover that
something had changed. As Wunsch (1992) points
out:
The ocean is a turbulent fluid in intimate contact
with another turbulent fluid, the atmosphere.
Although I am unaware of any formal theorems
on the subject, experience with turbulent systems suggests that it is very unlikely that any
components of such a complex nonlinear system
can actually remain fully steady; the frequency/wavenumber spectrum of the ocean circulation is almost surely everywhere filled.
However, while change per se would be unsurprising, it is still worth trying to determine the
sense and amplitude of the major changes that
were passing through the ocean–atmosphere system in the 1990s, while the WOCE data set was
being collected, in order to have some assessment,
however limited, of its ‘representativeness’. Within
the scope accessible to us (see Section 7.3.1), the
following seem to be the main currents of change.
1 The interdecadal temperature variability of the
Atlantic Ocean at intermediate depths shows
elements of coherent long-term change over a
broad band of latitudes from 32°S to 36°N.
Comparing hydrographic sections worked in the
1920s, 1950s, 1980s and 1990s, it was found
that the largest statistically significant changes
were generally found between 1000 and 2000
dbar. Over this pressure range, temperatures
have increased by ϳ0.5°C century
91 on surfaces
of constant pressure (isobars). (A cooling of
ϳ3°C century
91 , which occurred between the
1950s and 1980s at 48°N, is the exception.) In
the North Atlantic between the 1950s and
1980s, the warmings are mostly due to downward displacements of isopcynal (neutral) surfaces, while changes on isopycnal surfaces
dominate all but the most recent of the South
Atlantic signals.
2 The WOCE field phase (1990–97) occurred at a
most unusual interval in the climatic history of
the North Atlantic, at the end of a period in
which the principal recurrent mode of atmospheric forcing in this sector – the winter NAO –
had amplified over three or four decades to its
most extreme positive state (generally) in a 176year instrumental record, and possibly much
longer. A varied and extreme response is evident
in sea areas dominated by the NAO – the North
Atlantic, Nordic Seas and Arctic Ocean.
3 Since convective sites are important in carrying
the signal of climate change to intermediate and
greater depths (and from there, by horizontal
spreading, throughout the ocean basin), the
SECTION 7 INSIGHTS FOR THE FUTURE
580
the Southern Ocean and its weak signal is therefore to be expected.
7.3.9.3 Discussion
The apparently large scales of the warming of
SAMW in the South Pacific and Indian oceans,
and the freshening of NPIW and AAIW across the
North and South Pacific and the Indian oceans,
suggest that these changes are not random, but
rather a systematic modification of the shallow
thermocline circulation. The warming of SAMW
is largely consistent with the observed warming of
surface waters over the last thirty years of up to
0.5°C (Bottomley et al., 1990) in its source
regions.
The evidence for freshening of the surface ocean
at high latitudes does not exist in the observational
database, but models of climate change due to
increasing greenhouse gases (Manabe and Stouffer,
1994; Gordon and O’Farrell, 1997) all predict
increased precipitation–evaporation poleward of
50°. The pattern of change suggested by these
models is broadly consistent with the observed
changes in SAMW, AAIW and NPIW properties.
An increase in rainfall at high latitudes would
suggest a need for increased evaporation at lower
latitudes so that the overall salt content of the
oceans is conserved. There is some evidence that
the observed high-latitude freshening is partially
balanced (up to 60%) by increased salinity of shallower central waters on the 24°N, 10°N and 17°S
sections (Wong, 1999; Wong et al., 1999, 2001).
These observations suggest that it may be possible to verify the results from coupled ocean–
atmosphere models and in consequence to begin to
test quantitatively detection and attribution methods for evidence of climate change in the oceans.
7.3.10 Summary and Conclusions
In answering the question ‘How was the World
during WOCE?’ two conclusions were perhaps
inevitable: that we would be unable to form an
opinion for some sea areas and depths that don’t
have much of a past observational record; and that
for those areas that do, we would discover that
something had changed. As Wunsch (1992) points
out:
The ocean is a turbulent fluid in intimate contact
with another turbulent fluid, the atmosphere.
Although I am unaware of any formal theorems
on the subject, experience with turbulent systems suggests that it is very unlikely that any
components of such a complex nonlinear system
can actually remain fully steady; the frequency/wavenumber spectrum of the ocean circulation is almost surely everywhere filled.
However, while change per se would be unsurprising, it is still worth trying to determine the
sense and amplitude of the major changes that
were passing through the ocean–atmosphere system in the 1990s, while the WOCE data set was
being collected, in order to have some assessment,
however limited, of its ‘representativeness’. Within
the scope accessible to us (see Section 7.3.1), the
following seem to be the main currents of change.
1 The interdecadal temperature variability of the
Atlantic Ocean at intermediate depths shows
elements of coherent long-term change over a
broad band of latitudes from 32°S to 36°N.
Comparing hydrographic sections worked in the
1920s, 1950s, 1980s and 1990s, it was found
that the largest statistically significant changes
were generally found between 1000 and 2000
dbar. Over this pressure range, temperatures
have increased by ϳ0.5°C century
91 on surfaces
of constant pressure (isobars). (A cooling of
ϳ3°C century
91 , which occurred between the
1950s and 1980s at 48°N, is the exception.) In
the North Atlantic between the 1950s and
1980s, the warmings are mostly due to downward displacements of isopcynal (neutral) surfaces, while changes on isopycnal surfaces
dominate all but the most recent of the South
Atlantic signals.
2 The WOCE field phase (1990–97) occurred at a
most unusual interval in the climatic history of
the North Atlantic, at the end of a period in
which the principal recurrent mode of atmospheric forcing in this sector – the winter NAO –
had amplified over three or four decades to its
most extreme positive state (generally) in a 176year instrumental record, and possibly much
longer. A varied and extreme response is evident
in sea areas dominated by the NAO – the North
Atlantic, Nordic Seas and Arctic Ocean.
3 Since convective sites are important in carrying
the signal of climate change to intermediate and
greater depths (and from there, by horizontal
spreading, throughout the ocean basin), the
SECTION 7 INSIGHTS FOR THE FUTURE
580