of Trenberth and Hurrell, reflecting the areaweighted mean SLP averaged over most of the
extratropical North Pacific Ocean, underwent a
striking change amounting to a regime shift
after about 1976. For most of the years since
then, including the WOCE period, the index has
been negative, reflecting an eastward-shifted
and intensified Aleutian low. At lower latitudes,
the Southern Oscillation has recently exhibited
an unusual amplitude of decadal and longerterm variability (much like the recent behaviour
of the NAO), with a pronounced change toward
more negative values since the mid-1970s, and
prolonged warm conditions into the start of the
WOCE period in 1990–95.
8 To investigate the possibility of a response in the
Kuroshio and its extension, (analagous to the
changes in the North Atlantic Current described
earlier), the longest time series available to us is
a 40-year series (1956–97) of geostrophic transport estimates by the Kobe Marine Observatory,
covering the strongest part of the Kuroshio’s
eastward flow at 135°15ЈE, south of Japan. A
clear signal of interdecadal variability is apparent, with relatively small transports in the
1950s, large in the 1960s, small in the 1970s
and large in the 1980s, thus some evidence of a
fluctuation of ϳ20-year period. The overall pattern of change agrees very well with the shorter
geostrophic transport series by Qiu and Joyce
(1992) at 137ºE, and the interdecadal fluctuation in the transport of the Kuroshio Extension
inferred by Deser et al. (1999). Thus the fluctuation may also be a manifestation of the same
oceanic response of the North Pacific subtropical gyre to the interdecadal fluctuation in wind
forcing. The transport of the Kuroshio in the
1990s is almost as large as in the 1980s.
9 A comparison of hydrographic data from the
1960s and 1970s with WOCE hydrographic
data from the late 1980s and early 1990s reveals
coherent changes in three distinct intermediate
water masses of the Pacific and Indian Oceans:
Sub-Antarctic Mode Water (SAMW), Antarctic
Intermediate Water (AAIW) and North Pacific
Intermediate Water (NPIW). All are key components of the thermohaline circulation in these
regions. Over this ϳ 20-year period the evidence
is of an extensive isopycnal cooling and freshening of the SAMW layer over much of the South
Pacific, consistent with the warming of surface
waters south of Australia and their subsequent
subduction; the ‘pure freshening’ process is of
lesser importance. In the case of NPIW and
AAIW, the results of section comparisons from
47°N to 43°S show a remarkably coherent pattern of change, with significant cooling (and
freshening) on density surfaces on all of the
sections where they occur. The magnitude of
the freshening of intermediate waters on each
section is roughly consistent with the largescale ocean circulation. While the cooling and
freshening of SAMW on density surfaces was
most simply explained by the subduction of
warmed surface waters, the interpretation of
the cooling (and freshening) of AAIW and
NPIW on density surfaces is by the subduction
of fresher surface water. The warming of
SAMW is largely consistent with the observed
warming of surface waters over the last 30
years of up to 0.5°C in its source regions. The
observational database is not adequate to comment on the supposed freshening of the surface
ocean at high latitudes, but such a change
seems consistent with modelled scenarios of
climate change due to increasing greenhouse
gases. The apparently large scales of the warming of SAMW in the South Pacific and the
freshening of intermediate water in both
Pacific and Indian Oceans suggest that these
changes are not random, but rather a systematic modification of the shallow thermohaline
circulation.
10 The state of the southern hemisphere atmosphere during WOCE is 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.
Acknowledgements
Brian Arbic/Brech Owens thank M. Arhan for
providing the data at 7.5°N; G. Siedler and
J. Holfort for the 1994 11°S data; J. Dunworth,
G. Heimerdinger and T. McKee for providing the
rest of the data from archived sources; P. Robbins
SECTION 7 INSIGHTS FOR THE FUTURE
582
extratropical North Pacific Ocean, underwent a
striking change amounting to a regime shift
after about 1976. For most of the years since
then, including the WOCE period, the index has
been negative, reflecting an eastward-shifted
and intensified Aleutian low. At lower latitudes,
the Southern Oscillation has recently exhibited
an unusual amplitude of decadal and longerterm variability (much like the recent behaviour
of the NAO), with a pronounced change toward
more negative values since the mid-1970s, and
prolonged warm conditions into the start of the
WOCE period in 1990–95.
8 To investigate the possibility of a response in the
Kuroshio and its extension, (analagous to the
changes in the North Atlantic Current described
earlier), the longest time series available to us is
a 40-year series (1956–97) of geostrophic transport estimates by the Kobe Marine Observatory,
covering the strongest part of the Kuroshio’s
eastward flow at 135°15ЈE, south of Japan. A
clear signal of interdecadal variability is apparent, with relatively small transports in the
1950s, large in the 1960s, small in the 1970s
and large in the 1980s, thus some evidence of a
fluctuation of ϳ20-year period. The overall pattern of change agrees very well with the shorter
geostrophic transport series by Qiu and Joyce
(1992) at 137ºE, and the interdecadal fluctuation in the transport of the Kuroshio Extension
inferred by Deser et al. (1999). Thus the fluctuation may also be a manifestation of the same
oceanic response of the North Pacific subtropical gyre to the interdecadal fluctuation in wind
forcing. The transport of the Kuroshio in the
1990s is almost as large as in the 1980s.
9 A comparison of hydrographic data from the
1960s and 1970s with WOCE hydrographic
data from the late 1980s and early 1990s reveals
coherent changes in three distinct intermediate
water masses of the Pacific and Indian Oceans:
Sub-Antarctic Mode Water (SAMW), Antarctic
Intermediate Water (AAIW) and North Pacific
Intermediate Water (NPIW). All are key components of the thermohaline circulation in these
regions. Over this ϳ 20-year period the evidence
is of an extensive isopycnal cooling and freshening of the SAMW layer over much of the South
Pacific, consistent with the warming of surface
waters south of Australia and their subsequent
subduction; the ‘pure freshening’ process is of
lesser importance. In the case of NPIW and
AAIW, the results of section comparisons from
47°N to 43°S show a remarkably coherent pattern of change, with significant cooling (and
freshening) on density surfaces on all of the
sections where they occur. The magnitude of
the freshening of intermediate waters on each
section is roughly consistent with the largescale ocean circulation. While the cooling and
freshening of SAMW on density surfaces was
most simply explained by the subduction of
warmed surface waters, the interpretation of
the cooling (and freshening) of AAIW and
NPIW on density surfaces is by the subduction
of fresher surface water. The warming of
SAMW is largely consistent with the observed
warming of surface waters over the last 30
years of up to 0.5°C in its source regions. The
observational database is not adequate to comment on the supposed freshening of the surface
ocean at high latitudes, but such a change
seems consistent with modelled scenarios of
climate change due to increasing greenhouse
gases. The apparently large scales of the warming of SAMW in the South Pacific and the
freshening of intermediate water in both
Pacific and Indian Oceans suggest that these
changes are not random, but rather a systematic modification of the shallow thermohaline
circulation.
10 The state of the southern hemisphere atmosphere during WOCE is 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.
Acknowledgements
Brian Arbic/Brech Owens thank M. Arhan for
providing the data at 7.5°N; G. Siedler and
J. Holfort for the 1994 11°S data; J. Dunworth,
G. Heimerdinger and T. McKee for providing the
rest of the data from archived sources; P. Robbins
SECTION 7 INSIGHTS FOR THE FUTURE
582