over the SH by van Loon et al. (1993) and Hurrell
and van Loon (1994); see also Chen and Yen
(1997) and Meehl et al. (1998). All of these studies
show evidence for a change in the atmospheric circulation beginning in the mid to late 1970s, with
SLP in the circumpolar trough generally lower in
the 1980s than in the 1970s. Hurrell and van
Loon (1994) showed that the most pronounced
changes have occurred during the second half of
the year, so that the tropospheric polar vortex
remained strong into November. This was associated with a delayed breakdown in the stratospheric polar vortex and the ozone deficit in the
Antarctic spring.
7.3.2.6 Summary of atmospheric changes
The above results indicate that the state of the
atmosphere during the WOCE period was characterized by pronounced deviations from the longterm mean conditions over both hemispheres.
Over the North Atlantic, the NAO remained in a
highly positive phase, with an amplitude unprecedented in the ϳ170-year observational record. The
result has been stronger than average westerlies
over the middle latitudes during the 1990s, associated with low-pressure anomalies extending from
the Icelandic region across the Arctic and highpressure anomalies over the subtropical Atlantic.
The atmospheric circulation over the North Pacific
has been characterized by a deeper than normal
Aleutian low-pressure system since the mid-1970s,
accompanied by stronger than normal westerly
winds across the central North Pacific and enhanced
southerly flow along the west coast of North
America. The changes over the North Pacific have
been linked to anomalously warm conditions over
the tropical Pacific, including prolonged warm
conditions during much of the 1990s that appear
to be unprecedented in the observational record.
As a result, the tropical Pacific easterlies have
remained anomalously weak during much of the
WOCE period. Over the SH, changes are more difficult to quantify because of the paucity of data.
Nevertheless, the leading mode of atmospheric
variability is characterized by zonally symmetric
out-of-phase fluctuations in atmospheric mass
between high and subtropical latitudes, and the
time series of this mode shows a strong trend
toward lower than average SLPs during the 1990s
at high latitudes with stronger westerlies in middle
latitudes.
7.3.3 The analysis of decadal change in
intermediate water masses of the World
Ocean
The oceans play a central role in defining the
earth’s climate, transporting heat and fresh water
around the globe as part of the thermohaline circulation (e.g. Broecker, 1997). Models of climate
change predict that the thermohaline circulation
will weaken and that the surface waters will
freshen poleward of mid-latitudes, and become
saltier in the extratropical regions. In addition, the
predicted warming of surface waters is latitudedependent, with the Southern Ocean initially tending to warm less than the equivalent latitudes in
the northern hemisphere.
Ocean ventilation will cause these changes in
surface temperature and salinity to effect change in
the ocean interior. However, changes in the temperature and salinity structure of the deep ocean
below 500 m are largely unknown and their study
is strongly hampered by the sparsity of the historical data. This is particularly true of the more poorly
sampled Indian, Pacific and Southern Oceans, yet it
is precisely these observations that are critical to
the verification and validation of the results from
coupled ocean–atmosphere models.
Two later sections, 7.3.4 and 7.3.9, report
decadal changes in temperature and salinity at
intermediate depths in two large areas of the
world ocean – changes in the Atlantic and changes
in the Pacific and Indian Oceans. In both cases
changes along isobars are divided into components
that represent different physical mechanisms of climate change. The Atlantic calculation focuses on
the depth range 1000–2000 dbar and employs a
simpler interpretation because of the uncertain
role of mixing, as discussed below. The Pacific–
Indian calculation focuses on waters above 1500 m
depth where mixing is weaker compared with
advection and allows an interpretation in terms of
changes in surface forcing.
7.3.3.1 A kinematic subduction model for
interpretation of water mass changes
The subduction of surface water into the interior of
the ocean has been a subject of considerable
research. The similarity of the –S curve of the winter mixed layers with the –S curves obtained from
vertical profiles led Iselin (1939) to propose that the
main thermocline was formed from surface waters
7.3 The World during WOCE
563
Dickson et al.
and van Loon (1994); see also Chen and Yen
(1997) and Meehl et al. (1998). All of these studies
show evidence for a change in the atmospheric circulation beginning in the mid to late 1970s, with
SLP in the circumpolar trough generally lower in
the 1980s than in the 1970s. Hurrell and van
Loon (1994) showed that the most pronounced
changes have occurred during the second half of
the year, so that the tropospheric polar vortex
remained strong into November. This was associated with a delayed breakdown in the stratospheric polar vortex and the ozone deficit in the
Antarctic spring.
7.3.2.6 Summary of atmospheric changes
The above results indicate that the state of the
atmosphere during the WOCE period was characterized by pronounced deviations from the longterm mean conditions over both hemispheres.
Over the North Atlantic, the NAO remained in a
highly positive phase, with an amplitude unprecedented in the ϳ170-year observational record. The
result has been stronger than average westerlies
over the middle latitudes during the 1990s, associated with low-pressure anomalies extending from
the Icelandic region across the Arctic and highpressure anomalies over the subtropical Atlantic.
The atmospheric circulation over the North Pacific
has been characterized by a deeper than normal
Aleutian low-pressure system since the mid-1970s,
accompanied by stronger than normal westerly
winds across the central North Pacific and enhanced
southerly flow along the west coast of North
America. The changes over the North Pacific have
been linked to anomalously warm conditions over
the tropical Pacific, including prolonged warm
conditions during much of the 1990s that appear
to be unprecedented in the observational record.
As a result, the tropical Pacific easterlies have
remained anomalously weak during much of the
WOCE period. Over the SH, changes are more difficult to quantify because of the paucity of data.
Nevertheless, the leading mode of atmospheric
variability is characterized by zonally symmetric
out-of-phase fluctuations in atmospheric mass
between high and subtropical latitudes, and the
time series of this mode shows a strong trend
toward lower than average SLPs during the 1990s
at high latitudes with stronger westerlies in middle
latitudes.
7.3.3 The analysis of decadal change in
intermediate water masses of the World
Ocean
The oceans play a central role in defining the
earth’s climate, transporting heat and fresh water
around the globe as part of the thermohaline circulation (e.g. Broecker, 1997). Models of climate
change predict that the thermohaline circulation
will weaken and that the surface waters will
freshen poleward of mid-latitudes, and become
saltier in the extratropical regions. In addition, the
predicted warming of surface waters is latitudedependent, with the Southern Ocean initially tending to warm less than the equivalent latitudes in
the northern hemisphere.
Ocean ventilation will cause these changes in
surface temperature and salinity to effect change in
the ocean interior. However, changes in the temperature and salinity structure of the deep ocean
below 500 m are largely unknown and their study
is strongly hampered by the sparsity of the historical data. This is particularly true of the more poorly
sampled Indian, Pacific and Southern Oceans, yet it
is precisely these observations that are critical to
the verification and validation of the results from
coupled ocean–atmosphere models.
Two later sections, 7.3.4 and 7.3.9, report
decadal changes in temperature and salinity at
intermediate depths in two large areas of the
world ocean – changes in the Atlantic and changes
in the Pacific and Indian Oceans. In both cases
changes along isobars are divided into components
that represent different physical mechanisms of climate change. The Atlantic calculation focuses on
the depth range 1000–2000 dbar and employs a
simpler interpretation because of the uncertain
role of mixing, as discussed below. The Pacific–
Indian calculation focuses on waters above 1500 m
depth where mixing is weaker compared with
advection and allows an interpretation in terms of
changes in surface forcing.
7.3.3.1 A kinematic subduction model for
interpretation of water mass changes
The subduction of surface water into the interior of
the ocean has been a subject of considerable
research. The similarity of the –S curve of the winter mixed layers with the –S curves obtained from
vertical profiles led Iselin (1939) to propose that the
main thermocline was formed from surface waters
7.3 The World during WOCE
563
Dickson et al.