include the adjustment of the ocean to changing
forcing through planetary wave propagation and
the eddy field, the seasonal adjustment of the
circulation, and interannual (several years) and
decadal phenomena using both historical and
WOCE data. Relevant WOCE data sets include
especially the XBT, altimetric and Lagrangian data
sets, as well as extensions of hydrographic time
series and current meter statistics.
A question that has been posed since the start of
WOCE is to determine the representativeness of
the WOCE data set. One way to phrase this is to
ask how useful the large-scale fields determined in
WOCE are for understanding the long-term
‘mean’ circulation. One obvious measure is where
the WOCE period fits relative to various known
interannual and decadal climate phenomena and
then to determine the size of the WOCE ‘mean’
signal compared with the extrema of these climate
phenomena. The latter requires detailed examination in every region, although one rule of thumb is
that outside the tropics, temperature variations
due to climate variations are on the order of 1°C
in the upper ocean. These regional studies are
being carried out, most notably in the North
Atlantic and some regions of the southern hemisphere, as attention has shifted from describing the
mean fields to unravelling the climate variability
signal. WOCE provides an extremely good sampling for one period for all of these studies.
Some of the interannual and decadal climate
phenomena that affect ocean properties and that
have received major attention are El Niño, the
Atlantic tropical dipole, the North Atlantic Oscillation, the Pacific North American mode (decadal
Pacific oscillation), the Arctic Oscillation, southern
hemisphere annular modes and the Antarctic Circumpolar Wave. Indian Ocean tropical and subtropical dipoles have also been described recently
(Saji et al., 1999; Behera and Yamagata, 2001).
Knowledge of these phenomena is expanding
rapidly, including their interconnections. It is
likely that other patterns will also be described
and that understanding and hence quantification
of already-known climate phenomena will be
greatly advanced in the near future. This is particularly true for the southern hemisphere, which has
not yet received the full scrutiny given to the tropics
and the northen hemisphere. These interannual and
decadal indices are shown in Figure 7.1.3. Dickson
(Chapter 7.3) provide a complete discussion of the
WOCE period in the context of several of these
well-described climate cycles.
The baroclinic and barotropic planetary wave
fields at seasonal and shorter time scales are evident in any basin-scale general circulation model
(e.g. Semtner and Chervin, 1992; Böning et al.,
1996) and are observed with satellite altimetry (e.g.
Stammer and Wunsch, 1999). The latter found that
seasonal variations in surface slope are maximum
in the tropics and northeastern Atlantic and northeastern Pacific, and are correlated with changes in
wind only in the latter two regions, contradicting
earlier ideas that most of the energy outside the
western boundary currents is due to local wind
effects. There is much geographic variation in surface height and eddy energy, both in amplitude
(e.g. historical studies by Wyrtki, 1975, and more
modern equivalents) and in wave propagation as
demonstrated with altimetry (Chelton and Schlax,
1996). The latter showed that phase speeds depend
on more than Coriolis parameter variation, and
hence must be a function of the ocean’s stratification, circulation and possibly topography (Killworth
et al., 1997; de Szoeke and Chelton, 1999).
Gyre-scale seasonal variations in the thermocline have been shown from the WOCE high-resolution XBT programme in the Pacific. Although
the phasing of the wind field with these variations
has been studied, the dynamical relationship
between them has not yet been unequivocally
established and remains an open question. Morris
et al. (1996) reported an annual cycle in the subtropical gyre in the western South Pacific, with
isotherm slopes steepest and the geostrophic circulation most intense in late spring, 2–3 months after
the maximum Ekman pumping. McCarthy et al.
(2000) showed a puzzling phase relation between
subsurface isotherm depth, Sverdrup transport and
sea surface height on interannual time scales across
both the South and North Pacific subtropical gyres,
with surface height increasing as expected with
increased Sverdrup transport, but the subsurface
isotherms shoaling at the same time.
Variations in meridional heat transport at midlatitudes are also being studied using high-resolution XBT transects. Roemmich et al. (2001) used a
large number of transects at 24°N in the Pacific to
obtain 0.76 PW of northward heat transport, with
a robust error estimate of 0.12 PW; they are the
first to estimate the interannual variations, at
0.3 PW, a substantial fraction of the mean.
SECTION 7 INSIGHTS FOR THE FUTURE
530
forcing through planetary wave propagation and
the eddy field, the seasonal adjustment of the
circulation, and interannual (several years) and
decadal phenomena using both historical and
WOCE data. Relevant WOCE data sets include
especially the XBT, altimetric and Lagrangian data
sets, as well as extensions of hydrographic time
series and current meter statistics.
A question that has been posed since the start of
WOCE is to determine the representativeness of
the WOCE data set. One way to phrase this is to
ask how useful the large-scale fields determined in
WOCE are for understanding the long-term
‘mean’ circulation. One obvious measure is where
the WOCE period fits relative to various known
interannual and decadal climate phenomena and
then to determine the size of the WOCE ‘mean’
signal compared with the extrema of these climate
phenomena. The latter requires detailed examination in every region, although one rule of thumb is
that outside the tropics, temperature variations
due to climate variations are on the order of 1°C
in the upper ocean. These regional studies are
being carried out, most notably in the North
Atlantic and some regions of the southern hemisphere, as attention has shifted from describing the
mean fields to unravelling the climate variability
signal. WOCE provides an extremely good sampling for one period for all of these studies.
Some of the interannual and decadal climate
phenomena that affect ocean properties and that
have received major attention are El Niño, the
Atlantic tropical dipole, the North Atlantic Oscillation, the Pacific North American mode (decadal
Pacific oscillation), the Arctic Oscillation, southern
hemisphere annular modes and the Antarctic Circumpolar Wave. Indian Ocean tropical and subtropical dipoles have also been described recently
(Saji et al., 1999; Behera and Yamagata, 2001).
Knowledge of these phenomena is expanding
rapidly, including their interconnections. It is
likely that other patterns will also be described
and that understanding and hence quantification
of already-known climate phenomena will be
greatly advanced in the near future. This is particularly true for the southern hemisphere, which has
not yet received the full scrutiny given to the tropics
and the northen hemisphere. These interannual and
decadal indices are shown in Figure 7.1.3. Dickson
(Chapter 7.3) provide a complete discussion of the
WOCE period in the context of several of these
well-described climate cycles.
The baroclinic and barotropic planetary wave
fields at seasonal and shorter time scales are evident in any basin-scale general circulation model
(e.g. Semtner and Chervin, 1992; Böning et al.,
1996) and are observed with satellite altimetry (e.g.
Stammer and Wunsch, 1999). The latter found that
seasonal variations in surface slope are maximum
in the tropics and northeastern Atlantic and northeastern Pacific, and are correlated with changes in
wind only in the latter two regions, contradicting
earlier ideas that most of the energy outside the
western boundary currents is due to local wind
effects. There is much geographic variation in surface height and eddy energy, both in amplitude
(e.g. historical studies by Wyrtki, 1975, and more
modern equivalents) and in wave propagation as
demonstrated with altimetry (Chelton and Schlax,
1996). The latter showed that phase speeds depend
on more than Coriolis parameter variation, and
hence must be a function of the ocean’s stratification, circulation and possibly topography (Killworth
et al., 1997; de Szoeke and Chelton, 1999).
Gyre-scale seasonal variations in the thermocline have been shown from the WOCE high-resolution XBT programme in the Pacific. Although
the phasing of the wind field with these variations
has been studied, the dynamical relationship
between them has not yet been unequivocally
established and remains an open question. Morris
et al. (1996) reported an annual cycle in the subtropical gyre in the western South Pacific, with
isotherm slopes steepest and the geostrophic circulation most intense in late spring, 2–3 months after
the maximum Ekman pumping. McCarthy et al.
(2000) showed a puzzling phase relation between
subsurface isotherm depth, Sverdrup transport and
sea surface height on interannual time scales across
both the South and North Pacific subtropical gyres,
with surface height increasing as expected with
increased Sverdrup transport, but the subsurface
isotherms shoaling at the same time.
Variations in meridional heat transport at midlatitudes are also being studied using high-resolution XBT transects. Roemmich et al. (2001) used a
large number of transects at 24°N in the Pacific to
obtain 0.76 PW of northward heat transport, with
a robust error estimate of 0.12 PW; they are the
first to estimate the interannual variations, at
0.3 PW, a substantial fraction of the mean.
SECTION 7 INSIGHTS FOR THE FUTURE
530